[v3,00/10] Add CA enforcement keyring restrictions

  Prior to the introduction of the machine keyring, most distros simply 
allowed all keys contained within the platform keyring to be used
for both kernel and module verification.  This was done by an out of
tree patch.  Some distros took it even further and loaded all these keys
into the secondary trusted keyring.  This also allowed the system owner 
to add their own key for IMA usage.

Each distro contains similar documentation on how to sign kernel modules
and enroll the key into the MOK.  The process is fairly straightforward.
With the introduction of the machine keyring, the process remains
basically the same, without the need for any out of tree patches.

The machine keyring allowed distros to eliminate the out of tree patches
for kernel module signing.  However, it falls short in allowing the end 
user to add their own keys for IMA. Currently the machine keyring can not 
be used as another trust anchor for adding keys to the ima keyring, since 
CA enforcement does not currently exist.  This would expand the current 
integrity gap. The IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY 
Kconfig states that keys may be added to the ima keyrings if the key is 
validly signed by a CA cert in the system built-in or secondary trusted 
keyring.  Currently there is not code that enforces the contents of a
CA cert. Any key in the builtin or secondary keyring can be used. 

To allow IMA to be enabled with the machine keyring, this series introduces
enforcement of key usage in the certificate. This series also applies
this enforcement across all kernel keyrings.

The machine keyring shares  similarities with both the builtin and
secondary keyrings.  Similar to the builtin, no keys may be added to the
machine keyring following boot. The secondary keyring allows user
provided keys to be added following boot; however, a previously enrolled
kernel key must vouch for the key before it may be included. The system
owner may include their own keys into the machine keyring prior to boot.
If the end-user is not the system owner, they may not add their own keys
to the machine keyring.  

The machine keyring is only populated when Secure Boot is enabled.  A
system owner has the ability to control the entire Secure Boot keychain
(PK, KEK, DB, and DBX).  The system owner can also turn Secure Boot off.
With this control, they may use insert-sys-cert to include their own key 
and re-sign their kernel and have it boot.  The system owner also has 
control to include or exclude MOK keys. This series does not try to 
interpret how a system owner has configured their machine.  If the system 
owner has taken the steps to add their own MOK keys, they will be 
included in the machine keyring and used for verification, exactly 
the same way as keys contained in the builtin and secondary keyrings.
Since the system owner has the ability to add keys before booting to
either the machine or builtin keyrings, it is viewed as inconsequential 
if the key originated from one or the other.

This series introduces two different ways to configure the machine keyring.
By default, nothing changes and all MOK keys are loaded into it.  Whenever
a CA cert is found within the machine, builtin, or secondary, a flag 
indicating this is stored in the public key struct.  The other option is 
if the new Kconfig INTEGRITY_CA_MACHINE_KEYRING is enabled, only CA certs 
will be loaded into the machine keyring. All remaining MOK keys will be 
loaded into the platform keyring.

A CA cert shall be defined as any X509 certificate that contains the 
keyCertSign key usage and has the CA bit set to true.

With this series applied, CA enforcement is in place whenever 
IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY is enabled.  Meaning, 
before any key can be included into the ima keyring, it must be
vouched for by a CA key contained within the builtin, secondary, or 
machine keyrings.

IMA allows userspace applications to be signed. The enduser may sign
their own application, however they may also want to use an application
provided by a 3rd party.  The entity building the kernel, may not be the
same entity building the userspace program.  The system owner may also
be a third entity.  If the system owner trusts the entity building the
userspace program, they will include their public key within the MOK.
This key would be used to sign the key added to the ima keyring. Not all
3rd party userspace providers have the capability to properly manage a
root CA.  Some may outsource to a different code signing provider.  Many
code signing providers use Intermediate CA certificates. Therefore, this
series also includes support for Intermediate CA certificates.

This series could be broken up into 3 different parts.  The first two
patches could be taken now.  They solve current issues that will be
triggered by the build robots.  Patches 3-8 add CA enforcement for the
ima keyring.  Patches 9-10 restrict the machine keyring to only load CA
certs into it.  Patches 9-10 require all the previous patches. 

Changelog:

v3:
- Allow Intermediate CA certs to be enrolled through the MOK. The
  Intermediate CA cert must contain keyCertSign key usage and have the 
  CA bit set to true. This was done by removing the self signed
  requirement.


Eric Snowberg (10):
  KEYS: Create static version of public_key_verify_signature
  KEYS: Add missing function documentation
  KEYS: X.509: Parse Basic Constraints for CA
  KEYS: X.509: Parse Key Usage
  KEYS: Introduce a CA endorsed flag
  KEYS: Introduce keyring restriction that validates ca trust
  KEYS: X.509: Flag Intermediate CA certs as endorsed
  integrity: Use root of trust signature restriction
  KEYS: CA link restriction
  integrity: restrict INTEGRITY_KEYRING_MACHINE to restrict_link_by_ca

 certs/system_keyring.c                    | 32 +++++++++-
 crypto/asymmetric_keys/restrict.c         | 76 +++++++++++++++++++++++
 crypto/asymmetric_keys/x509_cert_parser.c | 31 +++++++++
 crypto/asymmetric_keys/x509_parser.h      |  2 +
 crypto/asymmetric_keys/x509_public_key.c  | 16 +++++
 include/crypto/public_key.h               | 30 +++++++++
 include/keys/system_keyring.h             | 12 +++-
 include/linux/ima.h                       | 11 ++++
 include/linux/key-type.h                  |  3 +
 include/linux/key.h                       |  2 +
 security/integrity/Kconfig                | 11 +++-
 security/integrity/digsig.c               | 12 ++--
 security/integrity/ima/Kconfig            |  6 +-
 security/keys/key.c                       | 13 ++++
 14 files changed, 245 insertions(+), 12 deletions(-)


base-commit: 830b3c68c1fb1e9176028d02ef86f3cf76aa2476
  

On Tue, 2022-12-13 at 19:33 -0500, Eric Snowberg wrote:
> Prior to the introduction of the machine keyring, most distros simply 
> allowed all keys contained within the platform keyring to be used
> for both kernel and module verification.  This was done by an out of
> tree patch.  Some distros took it even further and loaded all these keys
> into the secondary trusted keyring.  This also allowed the system owner 
> to add their own key for IMA usage.
> 
> Each distro contains similar documentation on how to sign kernel modules
> and enroll the key into the MOK.  The process is fairly straightforward.
> With the introduction of the machine keyring, the process remains
> basically the same, without the need for any out of tree patches.
> 
> The machine keyring allowed distros to eliminate the out of tree patches
> for kernel module signing.  However, it falls short in allowing the end 
> user to add their own keys for IMA. Currently the machine keyring can not 
> be used as another trust anchor for adding keys to the ima keyring, since 
> CA enforcement does not currently exist.  This would expand the current 
> integrity gap. The IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY 
> Kconfig states that keys may be added to the ima keyrings if the key is 
> validly signed by a CA cert in the system built-in or secondary trusted 
> keyring.  Currently there is not code that enforces the contents of a
> CA cert. Any key in the builtin or secondary keyring can be used. 
> 
> To allow IMA to be enabled with the machine keyring, this series introduces
> enforcement of key usage in the certificate. This series also applies
> this enforcement across all kernel keyrings.
> 
> The machine keyring shares  similarities with both the builtin and
> secondary keyrings.  Similar to the builtin, no keys may be added to the
> machine keyring following boot. The secondary keyring allows user
> provided keys to be added following boot; however, a previously enrolled
> kernel key must vouch for the key before it may be included. The system
> owner may include their own keys into the machine keyring prior to boot.
> If the end-user is not the system owner, they may not add their own keys
> to the machine keyring.  
> 
> The machine keyring is only populated when Secure Boot is enabled.  A
> system owner has the ability to control the entire Secure Boot keychain
> (PK, KEK, DB, and DBX).  The system owner can also turn Secure Boot off.
> With this control, they may use insert-sys-cert to include their own key 
> and re-sign their kernel and have it boot.  The system owner also has 
> control to include or exclude MOK keys. This series does not try to 
> interpret how a system owner has configured their machine.  If the system 
> owner has taken the steps to add their own MOK keys, they will be 
> included in the machine keyring and used for verification, exactly 
> the same way as keys contained in the builtin and secondary keyrings.
> Since the system owner has the ability to add keys before booting to
> either the machine or builtin keyrings, it is viewed as inconsequential 
> if the key originated from one or the other.
> 
> This series introduces two different ways to configure the machine keyring.
> By default, nothing changes and all MOK keys are loaded into it.  Whenever
> a CA cert is found within the machine, builtin, or secondary, a flag 
> indicating this is stored in the public key struct.  The other option is 
> if the new Kconfig INTEGRITY_CA_MACHINE_KEYRING is enabled, only CA certs 
> will be loaded into the machine keyring. All remaining MOK keys will be 
> loaded into the platform keyring.
> 
> A CA cert shall be defined as any X509 certificate that contains the 
> keyCertSign key usage and has the CA bit set to true.

Hi Eric,

Allowing CA certificates with the digitalSignature key usage flag
enabled defeats the purpose of the new Kconfig.  Please update the
above definition to exclude the digitalSignature key usage flag and
modify the code accordingly.

thanks,

Mimi

> With this series applied, CA enforcement is in place whenever 
> IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY is enabled.  Meaning, 
> before any key can be included into the ima keyring, it must be
> vouched for by a CA key contained within the builtin, secondary, or 
> machine keyrings.
> 
> IMA allows userspace applications to be signed. The enduser may sign
> their own application, however they may also want to use an application
> provided by a 3rd party.  The entity building the kernel, may not be the
> same entity building the userspace program.  The system owner may also
> be a third entity.  If the system owner trusts the entity building the
> userspace program, they will include their public key within the MOK.
> This key would be used to sign the key added to the ima keyring. Not all
> 3rd party userspace providers have the capability to properly manage a
> root CA.  Some may outsource to a different code signing provider.  Many
> code signing providers use Intermediate CA certificates. Therefore, this
> series also includes support for Intermediate CA certificates.
> 
> This series could be broken up into 3 different parts.  The first two
> patches could be taken now.  They solve current issues that will be
> triggered by the build robots.  Patches 3-8 add CA enforcement for the
> ima keyring.  Patches 9-10 restrict the machine keyring to only load CA
> certs into it.  Patches 9-10 require all the previous patches. 
> 
> Changelog:
> 
> v3:
> - Allow Intermediate CA certs to be enrolled through the MOK. The
>   Intermediate CA cert must contain keyCertSign key usage and have the 
>   CA bit set to true. This was done by removing the self signed
>   requirement.
> 
> 
> Eric Snowberg (10):
>   KEYS: Create static version of public_key_verify_signature
>   KEYS: Add missing function documentation
>   KEYS: X.509: Parse Basic Constraints for CA
>   KEYS: X.509: Parse Key Usage
>   KEYS: Introduce a CA endorsed flag
>   KEYS: Introduce keyring restriction that validates ca trust
>   KEYS: X.509: Flag Intermediate CA certs as endorsed
>   integrity: Use root of trust signature restriction
>   KEYS: CA link restriction
>   integrity: restrict INTEGRITY_KEYRING_MACHINE to restrict_link_by_ca
> 
>  certs/system_keyring.c                    | 32 +++++++++-
>  crypto/asymmetric_keys/restrict.c         | 76 +++++++++++++++++++++++
>  crypto/asymmetric_keys/x509_cert_parser.c | 31 +++++++++
>  crypto/asymmetric_keys/x509_parser.h      |  2 +
>  crypto/asymmetric_keys/x509_public_key.c  | 16 +++++
>  include/crypto/public_key.h               | 30 +++++++++
>  include/keys/system_keyring.h             | 12 +++-
>  include/linux/ima.h                       | 11 ++++
>  include/linux/key-type.h                  |  3 +
>  include/linux/key.h                       |  2 +
>  security/integrity/Kconfig                | 11 +++-
>  security/integrity/digsig.c               | 12 ++--
>  security/integrity/ima/Kconfig            |  6 +-
>  security/keys/key.c                       | 13 ++++
>  14 files changed, 245 insertions(+), 12 deletions(-)
> 
> 
> base-commit: 830b3c68c1fb1e9176028d02ef86f3cf76aa2476
  

> On Dec 15, 2022, at 3:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> 
> On Tue, 2022-12-13 at 19:33 -0500, Eric Snowberg wrote:
>> Prior to the introduction of the machine keyring, most distros simply 
>> allowed all keys contained within the platform keyring to be used
>> for both kernel and module verification.  This was done by an out of
>> tree patch.  Some distros took it even further and loaded all these keys
>> into the secondary trusted keyring.  This also allowed the system owner 
>> to add their own key for IMA usage.
>> 
>> Each distro contains similar documentation on how to sign kernel modules
>> and enroll the key into the MOK.  The process is fairly straightforward.
>> With the introduction of the machine keyring, the process remains
>> basically the same, without the need for any out of tree patches.
>> 
>> The machine keyring allowed distros to eliminate the out of tree patches
>> for kernel module signing.  However, it falls short in allowing the end 
>> user to add their own keys for IMA. Currently the machine keyring can not 
>> be used as another trust anchor for adding keys to the ima keyring, since 
>> CA enforcement does not currently exist.  This would expand the current 
>> integrity gap. The IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY 
>> Kconfig states that keys may be added to the ima keyrings if the key is 
>> validly signed by a CA cert in the system built-in or secondary trusted 
>> keyring.  Currently there is not code that enforces the contents of a
>> CA cert. Any key in the builtin or secondary keyring can be used. 
>> 
>> To allow IMA to be enabled with the machine keyring, this series introduces
>> enforcement of key usage in the certificate. This series also applies
>> this enforcement across all kernel keyrings.
>> 
>> The machine keyring shares  similarities with both the builtin and
>> secondary keyrings.  Similar to the builtin, no keys may be added to the
>> machine keyring following boot. The secondary keyring allows user
>> provided keys to be added following boot; however, a previously enrolled
>> kernel key must vouch for the key before it may be included. The system
>> owner may include their own keys into the machine keyring prior to boot.
>> If the end-user is not the system owner, they may not add their own keys
>> to the machine keyring.  
>> 
>> The machine keyring is only populated when Secure Boot is enabled.  A
>> system owner has the ability to control the entire Secure Boot keychain
>> (PK, KEK, DB, and DBX).  The system owner can also turn Secure Boot off.
>> With this control, they may use insert-sys-cert to include their own key 
>> and re-sign their kernel and have it boot.  The system owner also has 
>> control to include or exclude MOK keys. This series does not try to 
>> interpret how a system owner has configured their machine.  If the system 
>> owner has taken the steps to add their own MOK keys, they will be 
>> included in the machine keyring and used for verification, exactly 
>> the same way as keys contained in the builtin and secondary keyrings.
>> Since the system owner has the ability to add keys before booting to
>> either the machine or builtin keyrings, it is viewed as inconsequential 
>> if the key originated from one or the other.
>> 
>> This series introduces two different ways to configure the machine keyring.
>> By default, nothing changes and all MOK keys are loaded into it.  Whenever
>> a CA cert is found within the machine, builtin, or secondary, a flag 
>> indicating this is stored in the public key struct.  The other option is 
>> if the new Kconfig INTEGRITY_CA_MACHINE_KEYRING is enabled, only CA certs 
>> will be loaded into the machine keyring. All remaining MOK keys will be 
>> loaded into the platform keyring.
>> 
>> A CA cert shall be defined as any X509 certificate that contains the 
>> keyCertSign key usage and has the CA bit set to true.
> 
> Hi Eric,
> 
> Allowing CA certificates with the digitalSignature key usage flag
> enabled defeats the purpose of the new Kconfig.  Please update the
> above definition to exclude the digitalSignature key usage flag and
> modify the code accordingly.

Within v2, the request was made to allow Intermediate CA certificates to be 
loaded directly.  The Intermediate CA referenced was the one used by kernel.org.  
This Intermediate CA contains both digitalSignature and keyCertSign.  If the code 
is changed to exclude this certificate, now the root CA has to be loaded again.  Is that 
the intent?
  

On Thu, 2022-12-15 at 16:26 +0000, Eric Snowberg wrote:
> 
> > On Dec 15, 2022, at 3:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> > 
> > On Tue, 2022-12-13 at 19:33 -0500, Eric Snowberg wrote:
> >> Prior to the introduction of the machine keyring, most distros simply 
> >> allowed all keys contained within the platform keyring to be used
> >> for both kernel and module verification.  This was done by an out of
> >> tree patch.  Some distros took it even further and loaded all these keys
> >> into the secondary trusted keyring.  This also allowed the system owner 
> >> to add their own key for IMA usage.
> >> 
> >> Each distro contains similar documentation on how to sign kernel modules
> >> and enroll the key into the MOK.  The process is fairly straightforward.
> >> With the introduction of the machine keyring, the process remains
> >> basically the same, without the need for any out of tree patches.
> >> 
> >> The machine keyring allowed distros to eliminate the out of tree patches
> >> for kernel module signing.  However, it falls short in allowing the end 
> >> user to add their own keys for IMA. Currently the machine keyring can not 
> >> be used as another trust anchor for adding keys to the ima keyring, since 
> >> CA enforcement does not currently exist.  This would expand the current 
> >> integrity gap. The IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY 
> >> Kconfig states that keys may be added to the ima keyrings if the key is 
> >> validly signed by a CA cert in the system built-in or secondary trusted 
> >> keyring.  Currently there is not code that enforces the contents of a
> >> CA cert. Any key in the builtin or secondary keyring can be used. 
> >> 
> >> To allow IMA to be enabled with the machine keyring, this series introduces
> >> enforcement of key usage in the certificate. This series also applies
> >> this enforcement across all kernel keyrings.
> >> 
> >> The machine keyring shares  similarities with both the builtin and
> >> secondary keyrings.  Similar to the builtin, no keys may be added to the
> >> machine keyring following boot. The secondary keyring allows user
> >> provided keys to be added following boot; however, a previously enrolled
> >> kernel key must vouch for the key before it may be included. The system
> >> owner may include their own keys into the machine keyring prior to boot.
> >> If the end-user is not the system owner, they may not add their own keys
> >> to the machine keyring.  
> >> 
> >> The machine keyring is only populated when Secure Boot is enabled.  A
> >> system owner has the ability to control the entire Secure Boot keychain
> >> (PK, KEK, DB, and DBX).  The system owner can also turn Secure Boot off.
> >> With this control, they may use insert-sys-cert to include their own key 
> >> and re-sign their kernel and have it boot.  The system owner also has 
> >> control to include or exclude MOK keys. This series does not try to 
> >> interpret how a system owner has configured their machine.  If the system 
> >> owner has taken the steps to add their own MOK keys, they will be 
> >> included in the machine keyring and used for verification, exactly 
> >> the same way as keys contained in the builtin and secondary keyrings.
> >> Since the system owner has the ability to add keys before booting to
> >> either the machine or builtin keyrings, it is viewed as inconsequential 
> >> if the key originated from one or the other.
> >> 
> >> This series introduces two different ways to configure the machine keyring.
> >> By default, nothing changes and all MOK keys are loaded into it.  Whenever
> >> a CA cert is found within the machine, builtin, or secondary, a flag 
> >> indicating this is stored in the public key struct.  The other option is 
> >> if the new Kconfig INTEGRITY_CA_MACHINE_KEYRING is enabled, only CA certs 
> >> will be loaded into the machine keyring. All remaining MOK keys will be 
> >> loaded into the platform keyring.
> >> 
> >> A CA cert shall be defined as any X509 certificate that contains the 
> >> keyCertSign key usage and has the CA bit set to true.
> > 
> > Hi Eric,
> > 
> > Allowing CA certificates with the digitalSignature key usage flag
> > enabled defeats the purpose of the new Kconfig.  Please update the
> > above definition to exclude the digitalSignature key usage flag and
> > modify the code accordingly.
> 
> Within v2, the request was made to allow Intermediate CA certificates to be 
> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.  
> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code 
> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that 
> the intent?

That definitely was not the intent.  Nor would it address the issue of
a particular intermediate CA certificate having both keyCertSign and
digitalSignature.

thanks,

Mimi
  

> On Dec 15, 2022, at 12:58 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> 
> On Thu, 2022-12-15 at 16:26 +0000, Eric Snowberg wrote:
>> 
>>> On Dec 15, 2022, at 3:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>>> 
>>> On Tue, 2022-12-13 at 19:33 -0500, Eric Snowberg wrote:
>>>> Prior to the introduction of the machine keyring, most distros simply 
>>>> allowed all keys contained within the platform keyring to be used
>>>> for both kernel and module verification.  This was done by an out of
>>>> tree patch.  Some distros took it even further and loaded all these keys
>>>> into the secondary trusted keyring.  This also allowed the system owner 
>>>> to add their own key for IMA usage.
>>>> 
>>>> Each distro contains similar documentation on how to sign kernel modules
>>>> and enroll the key into the MOK.  The process is fairly straightforward.
>>>> With the introduction of the machine keyring, the process remains
>>>> basically the same, without the need for any out of tree patches.
>>>> 
>>>> The machine keyring allowed distros to eliminate the out of tree patches
>>>> for kernel module signing.  However, it falls short in allowing the end 
>>>> user to add their own keys for IMA. Currently the machine keyring can not 
>>>> be used as another trust anchor for adding keys to the ima keyring, since 
>>>> CA enforcement does not currently exist.  This would expand the current 
>>>> integrity gap. The IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY 
>>>> Kconfig states that keys may be added to the ima keyrings if the key is 
>>>> validly signed by a CA cert in the system built-in or secondary trusted 
>>>> keyring.  Currently there is not code that enforces the contents of a
>>>> CA cert. Any key in the builtin or secondary keyring can be used. 
>>>> 
>>>> To allow IMA to be enabled with the machine keyring, this series introduces
>>>> enforcement of key usage in the certificate. This series also applies
>>>> this enforcement across all kernel keyrings.
>>>> 
>>>> The machine keyring shares  similarities with both the builtin and
>>>> secondary keyrings.  Similar to the builtin, no keys may be added to the
>>>> machine keyring following boot. The secondary keyring allows user
>>>> provided keys to be added following boot; however, a previously enrolled
>>>> kernel key must vouch for the key before it may be included. The system
>>>> owner may include their own keys into the machine keyring prior to boot.
>>>> If the end-user is not the system owner, they may not add their own keys
>>>> to the machine keyring.  
>>>> 
>>>> The machine keyring is only populated when Secure Boot is enabled.  A
>>>> system owner has the ability to control the entire Secure Boot keychain
>>>> (PK, KEK, DB, and DBX).  The system owner can also turn Secure Boot off.
>>>> With this control, they may use insert-sys-cert to include their own key 
>>>> and re-sign their kernel and have it boot.  The system owner also has 
>>>> control to include or exclude MOK keys. This series does not try to 
>>>> interpret how a system owner has configured their machine.  If the system 
>>>> owner has taken the steps to add their own MOK keys, they will be 
>>>> included in the machine keyring and used for verification, exactly 
>>>> the same way as keys contained in the builtin and secondary keyrings.
>>>> Since the system owner has the ability to add keys before booting to
>>>> either the machine or builtin keyrings, it is viewed as inconsequential 
>>>> if the key originated from one or the other.
>>>> 
>>>> This series introduces two different ways to configure the machine keyring.
>>>> By default, nothing changes and all MOK keys are loaded into it.  Whenever
>>>> a CA cert is found within the machine, builtin, or secondary, a flag 
>>>> indicating this is stored in the public key struct.  The other option is 
>>>> if the new Kconfig INTEGRITY_CA_MACHINE_KEYRING is enabled, only CA certs 
>>>> will be loaded into the machine keyring. All remaining MOK keys will be 
>>>> loaded into the platform keyring.
>>>> 
>>>> A CA cert shall be defined as any X509 certificate that contains the 
>>>> keyCertSign key usage and has the CA bit set to true.
>>> 
>>> Hi Eric,
>>> 
>>> Allowing CA certificates with the digitalSignature key usage flag
>>> enabled defeats the purpose of the new Kconfig.  Please update the
>>> above definition to exclude the digitalSignature key usage flag and
>>> modify the code accordingly.
>> 
>> Within v2, the request was made to allow Intermediate CA certificates to be 
>> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.  
>> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code 
>> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that 
>> the intent?
> 
> That definitely was not the intent.  Nor would it address the issue of
> a particular intermediate CA certificate having both keyCertSign and
> digitalSignature.

Sorry, I’m not following.  Why is it an issue that an intermediate CA certificate contains 
both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
CA cert like the one used on kernel.org?
  

On Thu, 2022-12-15 at 20:28 +0000, Eric Snowberg wrote:
> 
> > On Dec 15, 2022, at 12:58 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> > 
> > On Thu, 2022-12-15 at 16:26 +0000, Eric Snowberg wrote:
> >> 
> >>> On Dec 15, 2022, at 3:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> >>> 
> >>> On Tue, 2022-12-13 at 19:33 -0500, Eric Snowberg wrote:
> >>>> Prior to the introduction of the machine keyring, most distros simply 
> >>>> allowed all keys contained within the platform keyring to be used
> >>>> for both kernel and module verification.  This was done by an out of
> >>>> tree patch.  Some distros took it even further and loaded all these keys
> >>>> into the secondary trusted keyring.  This also allowed the system owner 
> >>>> to add their own key for IMA usage.
> >>>> 
> >>>> Each distro contains similar documentation on how to sign kernel modules
> >>>> and enroll the key into the MOK.  The process is fairly straightforward.
> >>>> With the introduction of the machine keyring, the process remains
> >>>> basically the same, without the need for any out of tree patches.
> >>>> 
> >>>> The machine keyring allowed distros to eliminate the out of tree patches
> >>>> for kernel module signing.  However, it falls short in allowing the end 
> >>>> user to add their own keys for IMA. Currently the machine keyring can not 
> >>>> be used as another trust anchor for adding keys to the ima keyring, since 
> >>>> CA enforcement does not currently exist.  This would expand the current 
> >>>> integrity gap. The IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY 
> >>>> Kconfig states that keys may be added to the ima keyrings if the key is 
> >>>> validly signed by a CA cert in the system built-in or secondary trusted 
> >>>> keyring.  Currently there is not code that enforces the contents of a
> >>>> CA cert. Any key in the builtin or secondary keyring can be used. 
> >>>> 
> >>>> To allow IMA to be enabled with the machine keyring, this series introduces
> >>>> enforcement of key usage in the certificate. This series also applies
> >>>> this enforcement across all kernel keyrings.
> >>>> 
> >>>> The machine keyring shares  similarities with both the builtin and
> >>>> secondary keyrings.  Similar to the builtin, no keys may be added to the
> >>>> machine keyring following boot. The secondary keyring allows user
> >>>> provided keys to be added following boot; however, a previously enrolled
> >>>> kernel key must vouch for the key before it may be included. The system
> >>>> owner may include their own keys into the machine keyring prior to boot.
> >>>> If the end-user is not the system owner, they may not add their own keys
> >>>> to the machine keyring.  
> >>>> 
> >>>> The machine keyring is only populated when Secure Boot is enabled.  A
> >>>> system owner has the ability to control the entire Secure Boot keychain
> >>>> (PK, KEK, DB, and DBX).  The system owner can also turn Secure Boot off.
> >>>> With this control, they may use insert-sys-cert to include their own key 
> >>>> and re-sign their kernel and have it boot.  The system owner also has 
> >>>> control to include or exclude MOK keys. This series does not try to 
> >>>> interpret how a system owner has configured their machine.  If the system 
> >>>> owner has taken the steps to add their own MOK keys, they will be 
> >>>> included in the machine keyring and used for verification, exactly 
> >>>> the same way as keys contained in the builtin and secondary keyrings.
> >>>> Since the system owner has the ability to add keys before booting to
> >>>> either the machine or builtin keyrings, it is viewed as inconsequential 
> >>>> if the key originated from one or the other.
> >>>> 
> >>>> This series introduces two different ways to configure the machine keyring.
> >>>> By default, nothing changes and all MOK keys are loaded into it.  Whenever
> >>>> a CA cert is found within the machine, builtin, or secondary, a flag 
> >>>> indicating this is stored in the public key struct.  The other option is 
> >>>> if the new Kconfig INTEGRITY_CA_MACHINE_KEYRING is enabled, only CA certs 
> >>>> will be loaded into the machine keyring. All remaining MOK keys will be 
> >>>> loaded into the platform keyring.
> >>>> 
> >>>> A CA cert shall be defined as any X509 certificate that contains the 
> >>>> keyCertSign key usage and has the CA bit set to true.
> >>> 
> >>> Hi Eric,
> >>> 
> >>> Allowing CA certificates with the digitalSignature key usage flag
> >>> enabled defeats the purpose of the new Kconfig.  Please update the
> >>> above definition to exclude the digitalSignature key usage flag and
> >>> modify the code accordingly.
> >> 
> >> Within v2, the request was made to allow Intermediate CA certificates to be 
> >> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.  
> >> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code 
> >> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that 
> >> the intent?
> > 
> > That definitely was not the intent.  Nor would it address the issue of
> > a particular intermediate CA certificate having both keyCertSign and
> > digitalSignature.
> 
> Sorry, I’m not following.  Why is it an issue that an intermediate CA certificate contains 
> both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
> CA cert like the one used on kernel.org?

I must be missing something.  Isn't the purpose of "keyUsage" to
minimize how a certificate may be used?   Why would we want the same
certificate to be used for both certificate signing and code signing?

thanks,

Mimi
  

> On Dec 15, 2022, at 2:03 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> 
> On Thu, 2022-12-15 at 20:28 +0000, Eric Snowberg wrote:
>> 
>>> On Dec 15, 2022, at 12:58 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>>> 
>>> On Thu, 2022-12-15 at 16:26 +0000, Eric Snowberg wrote:
>>>> 
>>>>> On Dec 15, 2022, at 3:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>>>>> 
>>>>> On Tue, 2022-12-13 at 19:33 -0500, Eric Snowberg wrote:
>>>>>> Prior to the introduction of the machine keyring, most distros simply 
>>>>>> allowed all keys contained within the platform keyring to be used
>>>>>> for both kernel and module verification.  This was done by an out of
>>>>>> tree patch.  Some distros took it even further and loaded all these keys
>>>>>> into the secondary trusted keyring.  This also allowed the system owner 
>>>>>> to add their own key for IMA usage.
>>>>>> 
>>>>>> Each distro contains similar documentation on how to sign kernel modules
>>>>>> and enroll the key into the MOK.  The process is fairly straightforward.
>>>>>> With the introduction of the machine keyring, the process remains
>>>>>> basically the same, without the need for any out of tree patches.
>>>>>> 
>>>>>> The machine keyring allowed distros to eliminate the out of tree patches
>>>>>> for kernel module signing.  However, it falls short in allowing the end 
>>>>>> user to add their own keys for IMA. Currently the machine keyring can not 
>>>>>> be used as another trust anchor for adding keys to the ima keyring, since 
>>>>>> CA enforcement does not currently exist.  This would expand the current 
>>>>>> integrity gap. The IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY 
>>>>>> Kconfig states that keys may be added to the ima keyrings if the key is 
>>>>>> validly signed by a CA cert in the system built-in or secondary trusted 
>>>>>> keyring.  Currently there is not code that enforces the contents of a
>>>>>> CA cert. Any key in the builtin or secondary keyring can be used. 
>>>>>> 
>>>>>> To allow IMA to be enabled with the machine keyring, this series introduces
>>>>>> enforcement of key usage in the certificate. This series also applies
>>>>>> this enforcement across all kernel keyrings.
>>>>>> 
>>>>>> The machine keyring shares  similarities with both the builtin and
>>>>>> secondary keyrings.  Similar to the builtin, no keys may be added to the
>>>>>> machine keyring following boot. The secondary keyring allows user
>>>>>> provided keys to be added following boot; however, a previously enrolled
>>>>>> kernel key must vouch for the key before it may be included. The system
>>>>>> owner may include their own keys into the machine keyring prior to boot.
>>>>>> If the end-user is not the system owner, they may not add their own keys
>>>>>> to the machine keyring.  
>>>>>> 
>>>>>> The machine keyring is only populated when Secure Boot is enabled.  A
>>>>>> system owner has the ability to control the entire Secure Boot keychain
>>>>>> (PK, KEK, DB, and DBX).  The system owner can also turn Secure Boot off.
>>>>>> With this control, they may use insert-sys-cert to include their own key 
>>>>>> and re-sign their kernel and have it boot.  The system owner also has 
>>>>>> control to include or exclude MOK keys. This series does not try to 
>>>>>> interpret how a system owner has configured their machine.  If the system 
>>>>>> owner has taken the steps to add their own MOK keys, they will be 
>>>>>> included in the machine keyring and used for verification, exactly 
>>>>>> the same way as keys contained in the builtin and secondary keyrings.
>>>>>> Since the system owner has the ability to add keys before booting to
>>>>>> either the machine or builtin keyrings, it is viewed as inconsequential 
>>>>>> if the key originated from one or the other.
>>>>>> 
>>>>>> This series introduces two different ways to configure the machine keyring.
>>>>>> By default, nothing changes and all MOK keys are loaded into it.  Whenever
>>>>>> a CA cert is found within the machine, builtin, or secondary, a flag 
>>>>>> indicating this is stored in the public key struct.  The other option is 
>>>>>> if the new Kconfig INTEGRITY_CA_MACHINE_KEYRING is enabled, only CA certs 
>>>>>> will be loaded into the machine keyring. All remaining MOK keys will be 
>>>>>> loaded into the platform keyring.
>>>>>> 
>>>>>> A CA cert shall be defined as any X509 certificate that contains the 
>>>>>> keyCertSign key usage and has the CA bit set to true.
>>>>> 
>>>>> Hi Eric,
>>>>> 
>>>>> Allowing CA certificates with the digitalSignature key usage flag
>>>>> enabled defeats the purpose of the new Kconfig.  Please update the
>>>>> above definition to exclude the digitalSignature key usage flag and
>>>>> modify the code accordingly.
>>>> 
>>>> Within v2, the request was made to allow Intermediate CA certificates to be 
>>>> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.  
>>>> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code 
>>>> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that 
>>>> the intent?
>>> 
>>> That definitely was not the intent.  Nor would it address the issue of
>>> a particular intermediate CA certificate having both keyCertSign and
>>> digitalSignature.
>> 
>> Sorry, I’m not following.  Why is it an issue that an intermediate CA certificate contains 
>> both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
>> CA cert like the one used on kernel.org?
> 
> I must be missing something.  Isn't the purpose of "keyUsage" to
> minimize how a certificate may be used?   Why would we want the same
> certificate to be used for both certificate signing and code signing?

Every 3rd party intermediate CA I have looked at so far contains both set. Most have CRLSign set.  
Typically the root CA contains keyCertSign and CRLSign, but some also have digitalSignature 
set.  Finding a 3rd party Intermediate CA without digitalSignature set is probably going to be 
challenging and will severely limit usage.
  

Hi Eric and Mimi,

On Thu, Dec 15, 2022 at 09:45:37PM +0000, Eric Snowberg wrote:
>
>
>>>>>>> A CA cert shall be defined as any X509 certificate that contains the
>>>>>>> keyCertSign key usage and has the CA bit set to true.
>>>>>>
>>>>>> Hi Eric,
>>>>>>
>>>>>> Allowing CA certificates with the digitalSignature key usage flag
>>>>>> enabled defeats the purpose of the new Kconfig.  Please update the
>>>>>> above definition to exclude the digitalSignature key usage flag and
>>>>>> modify the code accordingly.
>>>>>
>>>>> Within v2, the request was made to allow Intermediate CA certificates to be
>>>>> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.
>>>>> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code
>>>>> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that
>>>>> the intent?
>>>>
>>>> That definitely was not the intent.  Nor would it address the issue of
>>>> a particular intermediate CA certificate having both keyCertSign and
>>>> digitalSignature.
>>>
>>> Sorry, I’m not following.  Why is it an issue that an intermediate CA certificate contains
>>> both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
>>> CA cert like the one used on kernel.org?
>>
>> I must be missing something.  Isn't the purpose of "keyUsage" to
>> minimize how a certificate may be used?   Why would we want the same
>> certificate to be used for both certificate signing and code signing?
>
>Every 3rd party intermediate CA I have looked at so far contains both set. Most have CRLSign set.
>Typically the root CA contains keyCertSign and CRLSign, but some also have digitalSignature
>set.  Finding a 3rd party Intermediate CA without digitalSignature set is probably going to be
>challenging and will severely limit usage.

How about allowing both keyCertSign and digitalSignature asserted but
issuing a warning for this case?

Here's my rationale for this proposal.

I assume we should conform to some X.509 specifications. So I checked
"RFC 5280: Internet X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile" [1] and ITU-T X.509 (2012-10)
[2].

[1] states in 4.2.1.3. Key Usage,
    "If the keyUsage extension is present, then the subject public key
    MUST NOT be used to verify signatures on certificates or CRLs unless
    the corresponding keyCertSign or cRLSign bit is set.  If the subject
    public key is only to be used for verifying signatures on
    certificates and/or CRLs, then the digitalSignature and
    nonRepudiation bits SHOULD NOT be set.  However, the digitalSignature
    and/or nonRepudiation bits MAY be set in addition to the keyCertSign
    and/or cRLSign bits if the subject public key is to be used to verify
    signatures on certificates and/or CRLs as well as other objects."

and [2] states in 8.2.2.3 Key usage extension that,
   "More than one bit may be set in an instance of the keyUsage extension.
   The setting of multiple bits shall not change the meaning of each
   individual bit but shall indicate that the certificate may be used for
   all of the purposes indicated by the set bits. There may be risks
   incurred when setting multiple bits. A review of those risks is
   documented in Annex I."

I interpret the above texts as we should allow both keyCertSign and
digitalSignature. However [2] warns about the risks of setting multiple
bits. Quoting Annex I,

   "Combining the contentCommitment bit in the keyUsage certificate
   extension with other keyUsage bits may have security implications
   depending on the security environment in which the certificate is to be
   used. If the subject's environment can be fully controlled and trusted,
   then there are no specific security implications. For example, in cases
   where the subject is fully confident about exactly which data is signed
   or cases where the subject is fully confident about the security
   characteristics of the authentication protocol being used. If the
   subject's environment is not fully controlled or not fully trusted, then
   unintentional signing of commitments is possible. Examples include the
   use of badly formed authentication exchanges and the use of a rogue
   software component. If untrusted environments are used by a subject,
   these security implications can be limited through use of the following
   measures:   
    – to not combine the contentCommitment key usage setting in
      certificates with any other key usage setting and to use the
      corresponding private key only with this certificate;   
      
    – to limit the use of private keys associated with certificates that
      have the contentCommitment key usage bit set, to environments which
      are considered adequately controlled and trustworthy"

So maybe it's useful to add a warning if both keyCertSign and
digitalSignature are asserted.
  

On Fri, 2022-12-16 at 22:06 +0800, Coiby Xu wrote:
> Hi Eric and Mimi,
> 
> On Thu, Dec 15, 2022 at 09:45:37PM +0000, Eric Snowberg wrote:
> >
> >
> >>>>>>> A CA cert shall be defined as any X509 certificate that contains the
> >>>>>>> keyCertSign key usage and has the CA bit set to true.
> >>>>>>
> >>>>>> Hi Eric,
> >>>>>>
> >>>>>> Allowing CA certificates with the digitalSignature key usage flag
> >>>>>> enabled defeats the purpose of the new Kconfig.  Please update the
> >>>>>> above definition to exclude the digitalSignature key usage flag and
> >>>>>> modify the code accordingly.
> >>>>>
> >>>>> Within v2, the request was made to allow Intermediate CA certificates to be
> >>>>> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.
> >>>>> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code
> >>>>> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that
> >>>>> the intent?
> >>>>
> >>>> That definitely was not the intent.  Nor would it address the issue of
> >>>> a particular intermediate CA certificate having both keyCertSign and
> >>>> digitalSignature.
> >>>
> >>> Sorry, I’m not following.  Why is it an issue that an intermediate CA certificate contains
> >>> both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
> >>> CA cert like the one used on kernel.org?
> >>
> >> I must be missing something.  Isn't the purpose of "keyUsage" to
> >> minimize how a certificate may be used?   Why would we want the same
> >> certificate to be used for both certificate signing and code signing?
> >
> >Every 3rd party intermediate CA I have looked at so far contains both set. Most have CRLSign set.
> >Typically the root CA contains keyCertSign and CRLSign, but some also have digitalSignature
> >set.  Finding a 3rd party Intermediate CA without digitalSignature set is probably going to be
> >challenging and will severely limit usage.
> 
> How about allowing both keyCertSign and digitalSignature asserted but
> issuing a warning for this case?
> 
> Here's my rationale for this proposal.
> 
> I assume we should conform to some X.509 specifications. So I checked
> "RFC 5280: Internet X.509 Public Key Infrastructure Certificate and
> Certificate Revocation List (CRL) Profile" [1] and ITU-T X.509 (2012-10)
> [2].
> 
> [1] states in 4.2.1.3. Key Usage,
>     "If the keyUsage extension is present, then the subject public key
>     MUST NOT be used to verify signatures on certificates or CRLs unless
>     the corresponding keyCertSign or cRLSign bit is set.  If the subject
>     public key is only to be used for verifying signatures on
>     certificates and/or CRLs, then the digitalSignature and
>     nonRepudiation bits SHOULD NOT be set.  However, the digitalSignature
>     and/or nonRepudiation bits MAY be set in addition to the keyCertSign
>     and/or cRLSign bits if the subject public key is to be used to verify
>     signatures on certificates and/or CRLs as well as other objects."
> 
> and [2] states in 8.2.2.3 Key usage extension that,
>    "More than one bit may be set in an instance of the keyUsage extension.
>    The setting of multiple bits shall not change the meaning of each
>    individual bit but shall indicate that the certificate may be used for
>    all of the purposes indicated by the set bits. There may be risks
>    incurred when setting multiple bits. A review of those risks is
>    documented in Annex I."
> 
> I interpret the above texts as we should allow both keyCertSign and
> digitalSignature. However [2] warns about the risks of setting multiple
> bits. Quoting Annex I,
> 
>    "Combining the contentCommitment bit in the keyUsage certificate
>    extension with other keyUsage bits may have security implications
>    depending on the security environment in which the certificate is to be
>    used. If the subject's environment can be fully controlled and trusted,
>    then there are no specific security implications. For example, in cases
>    where the subject is fully confident about exactly which data is signed
>    or cases where the subject is fully confident about the security
>    characteristics of the authentication protocol being used. If the
>    subject's environment is not fully controlled or not fully trusted, then
>    unintentional signing of commitments is possible. Examples include the
>    use of badly formed authentication exchanges and the use of a rogue
>    software component. If untrusted environments are used by a subject,
>    these security implications can be limited through use of the following
>    measures:   
>     – to not combine the contentCommitment key usage setting in
>       certificates with any other key usage setting and to use the
>       corresponding private key only with this certificate;   
>       
>     – to limit the use of private keys associated with certificates that
>       have the contentCommitment key usage bit set, to environments which
>       are considered adequately controlled and trustworthy"
> 
> So maybe it's useful to add a warning if both keyCertSign and
> digitalSignature are asserted.

Coiby, thank you for adding these details.  I was hoping others would
chime in as well.  I agree at minimum there should be a warning.

Perhaps instead of making INTEGRITY_CA_MACHINE_KEYRING dependent on
INTEGRITY_MACHINE_KEYRING, make them a Kconfig "choice" to support the
more restrictive certificate use case requirements:  all certificates,
CA certificate signing and digital signature, only CA certificate
signing.
  

> On Dec 18, 2022, at 5:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> 
> On Fri, 2022-12-16 at 22:06 +0800, Coiby Xu wrote:
>> Hi Eric and Mimi,
>> 
>> On Thu, Dec 15, 2022 at 09:45:37PM +0000, Eric Snowberg wrote:
>>> 
>>> 
>>>>>>>>> A CA cert shall be defined as any X509 certificate that contains the
>>>>>>>>> keyCertSign key usage and has the CA bit set to true.
>>>>>>>> 
>>>>>>>> Hi Eric,
>>>>>>>> 
>>>>>>>> Allowing CA certificates with the digitalSignature key usage flag
>>>>>>>> enabled defeats the purpose of the new Kconfig.  Please update the
>>>>>>>> above definition to exclude the digitalSignature key usage flag and
>>>>>>>> modify the code accordingly.
>>>>>>> 
>>>>>>> Within v2, the request was made to allow Intermediate CA certificates to be
>>>>>>> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.
>>>>>>> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code
>>>>>>> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that
>>>>>>> the intent?
>>>>>> 
>>>>>> That definitely was not the intent.  Nor would it address the issue of
>>>>>> a particular intermediate CA certificate having both keyCertSign and
>>>>>> digitalSignature.
>>>>> 
>>>>> Sorry, I’m not following.  Why is it an issue that an intermediate CA certificate contains
>>>>> both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
>>>>> CA cert like the one used on kernel.org?
>>>> 
>>>> I must be missing something.  Isn't the purpose of "keyUsage" to
>>>> minimize how a certificate may be used?   Why would we want the same
>>>> certificate to be used for both certificate signing and code signing?
>>> 
>>> Every 3rd party intermediate CA I have looked at so far contains both set. Most have CRLSign set.
>>> Typically the root CA contains keyCertSign and CRLSign, but some also have digitalSignature
>>> set.  Finding a 3rd party Intermediate CA without digitalSignature set is probably going to be
>>> challenging and will severely limit usage.
>> 
>> How about allowing both keyCertSign and digitalSignature asserted but
>> issuing a warning for this case?
>> 
>> Here's my rationale for this proposal.
>> 
>> I assume we should conform to some X.509 specifications. So I checked
>> "RFC 5280: Internet X.509 Public Key Infrastructure Certificate and
>> Certificate Revocation List (CRL) Profile" [1] and ITU-T X.509 (2012-10)
>> [2].
>> 
>> [1] states in 4.2.1.3. Key Usage,
>>    "If the keyUsage extension is present, then the subject public key
>>    MUST NOT be used to verify signatures on certificates or CRLs unless
>>    the corresponding keyCertSign or cRLSign bit is set.  If the subject
>>    public key is only to be used for verifying signatures on
>>    certificates and/or CRLs, then the digitalSignature and
>>    nonRepudiation bits SHOULD NOT be set.  However, the digitalSignature
>>    and/or nonRepudiation bits MAY be set in addition to the keyCertSign
>>    and/or cRLSign bits if the subject public key is to be used to verify
>>    signatures on certificates and/or CRLs as well as other objects."
>> 
>> and [2] states in 8.2.2.3 Key usage extension that,
>>   "More than one bit may be set in an instance of the keyUsage extension.
>>   The setting of multiple bits shall not change the meaning of each
>>   individual bit but shall indicate that the certificate may be used for
>>   all of the purposes indicated by the set bits. There may be risks
>>   incurred when setting multiple bits. A review of those risks is
>>   documented in Annex I."
>> 
>> I interpret the above texts as we should allow both keyCertSign and
>> digitalSignature. However [2] warns about the risks of setting multiple
>> bits. Quoting Annex I,
>> 
>>   "Combining the contentCommitment bit in the keyUsage certificate
>>   extension with other keyUsage bits may have security implications
>>   depending on the security environment in which the certificate is to be
>>   used. If the subject's environment can be fully controlled and trusted,
>>   then there are no specific security implications. For example, in cases
>>   where the subject is fully confident about exactly which data is signed
>>   or cases where the subject is fully confident about the security
>>   characteristics of the authentication protocol being used. If the
>>   subject's environment is not fully controlled or not fully trusted, then
>>   unintentional signing of commitments is possible. Examples include the
>>   use of badly formed authentication exchanges and the use of a rogue
>>   software component. If untrusted environments are used by a subject,
>>   these security implications can be limited through use of the following
>>   measures:   
>>    – to not combine the contentCommitment key usage setting in
>>      certificates with any other key usage setting and to use the
>>      corresponding private key only with this certificate;   
>> 
>>    – to limit the use of private keys associated with certificates that
>>      have the contentCommitment key usage bit set, to environments which
>>      are considered adequately controlled and trustworthy"
>> 
>> So maybe it's useful to add a warning if both keyCertSign and
>> digitalSignature are asserted.
> 
> Coiby, thank you for adding these details.  I was hoping others would
> chime in as well.  I agree at minimum there should be a warning.

A warning could be added.

> Perhaps instead of making INTEGRITY_CA_MACHINE_KEYRING dependent on
> INTEGRITY_MACHINE_KEYRING, make them a Kconfig "choice" to support the
> more restrictive certificate use case requirements:  all certificates,
> CA certificate signing and digital signature, only CA certificate
> signing.

As could support for additional restrictions.

Would these additions be required within this series? What is missing from this 
discussion is why would these additions be necessary?  Why should the kernel 
enforce a restriction that is beyond the scope of the X.509 spec?  If a warning was 
to be added, what would be the justification for adding this additional code?  From 
my research every single 3rd party code signing intermediate CA would be flagged 
with the warning.  Isn’t this just going to cause confusion?  Or is there a benefit that 
I am missing that needs to be stated?
  

On Wed, 2022-12-21 at 18:27 +0000, Eric Snowberg wrote:
> 
> > On Dec 18, 2022, at 5:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> > 
> > On Fri, 2022-12-16 at 22:06 +0800, Coiby Xu wrote:
> >> Hi Eric and Mimi,
> >> 
> >> On Thu, Dec 15, 2022 at 09:45:37PM +0000, Eric Snowberg wrote:
> >>> 
> >>> 
> >>>>>>>>> A CA cert shall be defined as any X509 certificate that contains the
> >>>>>>>>> keyCertSign key usage and has the CA bit set to true.
> >>>>>>>> 
> >>>>>>>> Hi Eric,
> >>>>>>>> 
> >>>>>>>> Allowing CA certificates with the digitalSignature key usage flag
> >>>>>>>> enabled defeats the purpose of the new Kconfig.  Please update the
> >>>>>>>> above definition to exclude the digitalSignature key usage flag and
> >>>>>>>> modify the code accordingly.
> >>>>>>> 
> >>>>>>> Within v2, the request was made to allow Intermediate CA certificates to be
> >>>>>>> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.
> >>>>>>> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code
> >>>>>>> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that
> >>>>>>> the intent?
> >>>>>> 
> >>>>>> That definitely was not the intent.  Nor would it address the issue of
> >>>>>> a particular intermediate CA certificate having both keyCertSign and
> >>>>>> digitalSignature.
> >>>>> 
> >>>>> Sorry, I’m not following.  Why is it an issue that an intermediate CA certificate contains
> >>>>> both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
> >>>>> CA cert like the one used on kernel.org?
> >>>> 
> >>>> I must be missing something.  Isn't the purpose of "keyUsage" to
> >>>> minimize how a certificate may be used?   Why would we want the same
> >>>> certificate to be used for both certificate signing and code signing?
> >>> 
> >>> Every 3rd party intermediate CA I have looked at so far contains both set. Most have CRLSign set.
> >>> Typically the root CA contains keyCertSign and CRLSign, but some also have digitalSignature
> >>> set.  Finding a 3rd party Intermediate CA without digitalSignature set is probably going to be
> >>> challenging and will severely limit usage.
> >> 
> >> How about allowing both keyCertSign and digitalSignature asserted but
> >> issuing a warning for this case?
> >> 
> >> Here's my rationale for this proposal.
> >> 
> >> I assume we should conform to some X.509 specifications. So I checked
> >> "RFC 5280: Internet X.509 Public Key Infrastructure Certificate and
> >> Certificate Revocation List (CRL) Profile" [1] and ITU-T X.509 (2012-10)
> >> [2].
> >> 
> >> [1] states in 4.2.1.3. Key Usage,
> >>    "If the keyUsage extension is present, then the subject public key
> >>    MUST NOT be used to verify signatures on certificates or CRLs unless
> >>    the corresponding keyCertSign or cRLSign bit is set.  If the subject
> >>    public key is only to be used for verifying signatures on
> >>    certificates and/or CRLs, then the digitalSignature and
> >>    nonRepudiation bits SHOULD NOT be set.  However, the digitalSignature
> >>    and/or nonRepudiation bits MAY be set in addition to the keyCertSign
> >>    and/or cRLSign bits if the subject public key is to be used to verify
> >>    signatures on certificates and/or CRLs as well as other objects."
> >> 
> >> and [2] states in 8.2.2.3 Key usage extension that,
> >>   "More than one bit may be set in an instance of the keyUsage extension.
> >>   The setting of multiple bits shall not change the meaning of each
> >>   individual bit but shall indicate that the certificate may be used for
> >>   all of the purposes indicated by the set bits. There may be risks
> >>   incurred when setting multiple bits. A review of those risks is
> >>   documented in Annex I."
> >> 
> >> I interpret the above texts as we should allow both keyCertSign and
> >> digitalSignature. However [2] warns about the risks of setting multiple
> >> bits. Quoting Annex I,
> >> 
> >>   "Combining the contentCommitment bit in the keyUsage certificate
> >>   extension with other keyUsage bits may have security implications
> >>   depending on the security environment in which the certificate is to be
> >>   used. If the subject's environment can be fully controlled and trusted,
> >>   then there are no specific security implications. For example, in cases
> >>   where the subject is fully confident about exactly which data is signed
> >>   or cases where the subject is fully confident about the security
> >>   characteristics of the authentication protocol being used. If the
> >>   subject's environment is not fully controlled or not fully trusted, then
> >>   unintentional signing of commitments is possible. Examples include the
> >>   use of badly formed authentication exchanges and the use of a rogue
> >>   software component. If untrusted environments are used by a subject,
> >>   these security implications can be limited through use of the following
> >>   measures:   
> >>    – to not combine the contentCommitment key usage setting in
> >>      certificates with any other key usage setting and to use the
> >>      corresponding private key only with this certificate;   
> >> 
> >>    – to limit the use of private keys associated with certificates that
> >>      have the contentCommitment key usage bit set, to environments which
> >>      are considered adequately controlled and trustworthy"
> >> 
> >> So maybe it's useful to add a warning if both keyCertSign and
> >> digitalSignature are asserted.
> > 
> > Coiby, thank you for adding these details.  I was hoping others would
> > chime in as well.  I agree at minimum there should be a warning.
> 
> A warning could be added.
> 
> > Perhaps instead of making INTEGRITY_CA_MACHINE_KEYRING dependent on
> > INTEGRITY_MACHINE_KEYRING, make them a Kconfig "choice" to support the
> > more restrictive certificate use case requirements:  all certificates,
> > CA certificate signing and digital signature, only CA certificate
> > signing.
> 
> As could support for additional restrictions.
> 
> Would these additions be required within this series? What is missing from this 
> discussion is why would these additions be necessary?  Why should the kernel 
> enforce a restriction that is beyond the scope of the X.509 spec?  If a warning was 
> to be added, what would be the justification for adding this additional code?  From 
> my research every single 3rd party code signing intermediate CA would be flagged 
> with the warning.  Isn’t this just going to cause confusion?  Or is there a benefit that 
> I am missing that needs to be stated?

You're focusing on third party kernel modules and forgetting about the
simple use case of allowing an end user (or business) to sign their own
code.  True they could use the less restrictive CA certificates, but it
is unnecessary.
  

> On Dec 21, 2022, at 12:01 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> 
> On Wed, 2022-12-21 at 18:27 +0000, Eric Snowberg wrote:
>> 
>>> On Dec 18, 2022, at 5:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>>> 
>>> On Fri, 2022-12-16 at 22:06 +0800, Coiby Xu wrote:
>>>> Hi Eric and Mimi,
>>>> 
>>>> On Thu, Dec 15, 2022 at 09:45:37PM +0000, Eric Snowberg wrote:
>>>>> 
>>>>> 
>>>>>>>>>>> A CA cert shall be defined as any X509 certificate that contains the
>>>>>>>>>>> keyCertSign key usage and has the CA bit set to true.
>>>>>>>>>> 
>>>>>>>>>> Hi Eric,
>>>>>>>>>> 
>>>>>>>>>> Allowing CA certificates with the digitalSignature key usage flag
>>>>>>>>>> enabled defeats the purpose of the new Kconfig.  Please update the
>>>>>>>>>> above definition to exclude the digitalSignature key usage flag and
>>>>>>>>>> modify the code accordingly.
>>>>>>>>> 
>>>>>>>>> Within v2, the request was made to allow Intermediate CA certificates to be
>>>>>>>>> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.
>>>>>>>>> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code
>>>>>>>>> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that
>>>>>>>>> the intent?
>>>>>>>> 
>>>>>>>> That definitely was not the intent.  Nor would it address the issue of
>>>>>>>> a particular intermediate CA certificate having both keyCertSign and
>>>>>>>> digitalSignature.
>>>>>>> 
>>>>>>> Sorry, I’m not following.  Why is it an issue that an intermediate CA certificate contains
>>>>>>> both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
>>>>>>> CA cert like the one used on kernel.org?
>>>>>> 
>>>>>> I must be missing something.  Isn't the purpose of "keyUsage" to
>>>>>> minimize how a certificate may be used?   Why would we want the same
>>>>>> certificate to be used for both certificate signing and code signing?
>>>>> 
>>>>> Every 3rd party intermediate CA I have looked at so far contains both set. Most have CRLSign set.
>>>>> Typically the root CA contains keyCertSign and CRLSign, but some also have digitalSignature
>>>>> set.  Finding a 3rd party Intermediate CA without digitalSignature set is probably going to be
>>>>> challenging and will severely limit usage.
>>>> 
>>>> How about allowing both keyCertSign and digitalSignature asserted but
>>>> issuing a warning for this case?
>>>> 
>>>> Here's my rationale for this proposal.
>>>> 
>>>> I assume we should conform to some X.509 specifications. So I checked
>>>> "RFC 5280: Internet X.509 Public Key Infrastructure Certificate and
>>>> Certificate Revocation List (CRL) Profile" [1] and ITU-T X.509 (2012-10)
>>>> [2].
>>>> 
>>>> [1] states in 4.2.1.3. Key Usage,
>>>>   "If the keyUsage extension is present, then the subject public key
>>>>   MUST NOT be used to verify signatures on certificates or CRLs unless
>>>>   the corresponding keyCertSign or cRLSign bit is set.  If the subject
>>>>   public key is only to be used for verifying signatures on
>>>>   certificates and/or CRLs, then the digitalSignature and
>>>>   nonRepudiation bits SHOULD NOT be set.  However, the digitalSignature
>>>>   and/or nonRepudiation bits MAY be set in addition to the keyCertSign
>>>>   and/or cRLSign bits if the subject public key is to be used to verify
>>>>   signatures on certificates and/or CRLs as well as other objects."
>>>> 
>>>> and [2] states in 8.2.2.3 Key usage extension that,
>>>>  "More than one bit may be set in an instance of the keyUsage extension.
>>>>  The setting of multiple bits shall not change the meaning of each
>>>>  individual bit but shall indicate that the certificate may be used for
>>>>  all of the purposes indicated by the set bits. There may be risks
>>>>  incurred when setting multiple bits. A review of those risks is
>>>>  documented in Annex I."
>>>> 
>>>> I interpret the above texts as we should allow both keyCertSign and
>>>> digitalSignature. However [2] warns about the risks of setting multiple
>>>> bits. Quoting Annex I,
>>>> 
>>>>  "Combining the contentCommitment bit in the keyUsage certificate
>>>>  extension with other keyUsage bits may have security implications
>>>>  depending on the security environment in which the certificate is to be
>>>>  used. If the subject's environment can be fully controlled and trusted,
>>>>  then there are no specific security implications. For example, in cases
>>>>  where the subject is fully confident about exactly which data is signed
>>>>  or cases where the subject is fully confident about the security
>>>>  characteristics of the authentication protocol being used. If the
>>>>  subject's environment is not fully controlled or not fully trusted, then
>>>>  unintentional signing of commitments is possible. Examples include the
>>>>  use of badly formed authentication exchanges and the use of a rogue
>>>>  software component. If untrusted environments are used by a subject,
>>>>  these security implications can be limited through use of the following
>>>>  measures:   
>>>>   – to not combine the contentCommitment key usage setting in
>>>>     certificates with any other key usage setting and to use the
>>>>     corresponding private key only with this certificate;   
>>>> 
>>>>   – to limit the use of private keys associated with certificates that
>>>>     have the contentCommitment key usage bit set, to environments which
>>>>     are considered adequately controlled and trustworthy"
>>>> 
>>>> So maybe it's useful to add a warning if both keyCertSign and
>>>> digitalSignature are asserted.
>>> 
>>> Coiby, thank you for adding these details.  I was hoping others would
>>> chime in as well.  I agree at minimum there should be a warning.
>> 
>> A warning could be added.
>> 
>>> Perhaps instead of making INTEGRITY_CA_MACHINE_KEYRING dependent on
>>> INTEGRITY_MACHINE_KEYRING, make them a Kconfig "choice" to support the
>>> more restrictive certificate use case requirements:  all certificates,
>>> CA certificate signing and digital signature, only CA certificate
>>> signing.
>> 
>> As could support for additional restrictions.
>> 
>> Would these additions be required within this series? What is missing from this 
>> discussion is why would these additions be necessary?  Why should the kernel 
>> enforce a restriction that is beyond the scope of the X.509 spec?  If a warning was 
>> to be added, what would be the justification for adding this additional code?  From 
>> my research every single 3rd party code signing intermediate CA would be flagged 
>> with the warning.  Isn’t this just going to cause confusion?  Or is there a benefit that 
>> I am missing that needs to be stated?
> 
> You're focusing on third party kernel modules and forgetting about the
> simple use case of allowing an end user (or business) to sign their own
> code.  True they could use the less restrictive CA certificates, but it
> is unnecessary.

My focus is on signing user-space applications, as outlined in the cover letter.  This 
series has nothing to do with kernel modules.  Most end-users and businesses rely on 
a third party to deal with code signing.  All third party code signing services I have 
found use an intermediate CA containing more than just the keyCertSign usage set.  
It seems to be an industry accepted practice that does not violate the spec. Before writing
new code to either warn or exclude a third party intermediate CA,  I would like to understand 
the motivation behind this request.
  

On Thu, 2022-12-22 at 15:15 +0000, Eric Snowberg wrote:
> 
> > On Dec 21, 2022, at 12:01 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> > 
> > On Wed, 2022-12-21 at 18:27 +0000, Eric Snowberg wrote:
> >> 
> >>> On Dec 18, 2022, at 5:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> >>> 
> >>> On Fri, 2022-12-16 at 22:06 +0800, Coiby Xu wrote:
> >>>> Hi Eric and Mimi,
> >>>> 
> >>>> On Thu, Dec 15, 2022 at 09:45:37PM +0000, Eric Snowberg wrote:
> >>>>> 
> >>>>> 
> >>>>>>>>>>> A CA cert shall be defined as any X509 certificate that contains the
> >>>>>>>>>>> keyCertSign key usage and has the CA bit set to true.
> >>>>>>>>>> 
> >>>>>>>>>> Hi Eric,
> >>>>>>>>>> 
> >>>>>>>>>> Allowing CA certificates with the digitalSignature key usage flag
> >>>>>>>>>> enabled defeats the purpose of the new Kconfig.  Please update the
> >>>>>>>>>> above definition to exclude the digitalSignature key usage flag and
> >>>>>>>>>> modify the code accordingly.
> >>>>>>>>> 
> >>>>>>>>> Within v2, the request was made to allow Intermediate CA certificates to be
> >>>>>>>>> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.
> >>>>>>>>> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code
> >>>>>>>>> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that
> >>>>>>>>> the intent?
> >>>>>>>> 
> >>>>>>>> That definitely was not the intent.  Nor would it address the issue of
> >>>>>>>> a particular intermediate CA certificate having both keyCertSign and
> >>>>>>>> digitalSignature.
> >>>>>>> 
> >>>>>>> Sorry, I’m not following.  Why is it an issue that an intermediate CA certificate contains
> >>>>>>> both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
> >>>>>>> CA cert like the one used on kernel.org?
> >>>>>> 
> >>>>>> I must be missing something.  Isn't the purpose of "keyUsage" to
> >>>>>> minimize how a certificate may be used?   Why would we want the same
> >>>>>> certificate to be used for both certificate signing and code signing?
> >>>>> 
> >>>>> Every 3rd party intermediate CA I have looked at so far contains both set. Most have CRLSign set.
> >>>>> Typically the root CA contains keyCertSign and CRLSign, but some also have digitalSignature
> >>>>> set.  Finding a 3rd party Intermediate CA without digitalSignature set is probably going to be
> >>>>> challenging and will severely limit usage.
> >>>> 
> >>>> How about allowing both keyCertSign and digitalSignature asserted but
> >>>> issuing a warning for this case?
> >>>> 
> >>>> Here's my rationale for this proposal.
> >>>> 
> >>>> I assume we should conform to some X.509 specifications. So I checked
> >>>> "RFC 5280: Internet X.509 Public Key Infrastructure Certificate and
> >>>> Certificate Revocation List (CRL) Profile" [1] and ITU-T X.509 (2012-10)
> >>>> [2].
> >>>> 
> >>>> [1] states in 4.2.1.3. Key Usage,
> >>>>   "If the keyUsage extension is present, then the subject public key
> >>>>   MUST NOT be used to verify signatures on certificates or CRLs unless
> >>>>   the corresponding keyCertSign or cRLSign bit is set.  If the subject
> >>>>   public key is only to be used for verifying signatures on
> >>>>   certificates and/or CRLs, then the digitalSignature and
> >>>>   nonRepudiation bits SHOULD NOT be set.  However, the digitalSignature
> >>>>   and/or nonRepudiation bits MAY be set in addition to the keyCertSign
> >>>>   and/or cRLSign bits if the subject public key is to be used to verify
> >>>>   signatures on certificates and/or CRLs as well as other objects."
> >>>> 
> >>>> and [2] states in 8.2.2.3 Key usage extension that,
> >>>>  "More than one bit may be set in an instance of the keyUsage extension.
> >>>>  The setting of multiple bits shall not change the meaning of each
> >>>>  individual bit but shall indicate that the certificate may be used for
> >>>>  all of the purposes indicated by the set bits. There may be risks
> >>>>  incurred when setting multiple bits. A review of those risks is
> >>>>  documented in Annex I."
> >>>> 
> >>>> I interpret the above texts as we should allow both keyCertSign and
> >>>> digitalSignature. However [2] warns about the risks of setting multiple
> >>>> bits. Quoting Annex I,
> >>>> 
> >>>>  "Combining the contentCommitment bit in the keyUsage certificate
> >>>>  extension with other keyUsage bits may have security implications
> >>>>  depending on the security environment in which the certificate is to be
> >>>>  used. If the subject's environment can be fully controlled and trusted,
> >>>>  then there are no specific security implications. For example, in cases
> >>>>  where the subject is fully confident about exactly which data is signed
> >>>>  or cases where the subject is fully confident about the security
> >>>>  characteristics of the authentication protocol being used. If the
> >>>>  subject's environment is not fully controlled or not fully trusted, then
> >>>>  unintentional signing of commitments is possible. Examples include the
> >>>>  use of badly formed authentication exchanges and the use of a rogue
> >>>>  software component. If untrusted environments are used by a subject,
> >>>>  these security implications can be limited through use of the following
> >>>>  measures:   
> >>>>   – to not combine the contentCommitment key usage setting in
> >>>>     certificates with any other key usage setting and to use the
> >>>>     corresponding private key only with this certificate;   
> >>>> 
> >>>>   – to limit the use of private keys associated with certificates that
> >>>>     have the contentCommitment key usage bit set, to environments which
> >>>>     are considered adequately controlled and trustworthy"
> >>>> 
> >>>> So maybe it's useful to add a warning if both keyCertSign and
> >>>> digitalSignature are asserted.
> >>> 
> >>> Coiby, thank you for adding these details.  I was hoping others would
> >>> chime in as well.  I agree at minimum there should be a warning.
> >> 
> >> A warning could be added.
> >> 
> >>> Perhaps instead of making INTEGRITY_CA_MACHINE_KEYRING dependent on
> >>> INTEGRITY_MACHINE_KEYRING, make them a Kconfig "choice" to support the
> >>> more restrictive certificate use case requirements:  all certificates,
> >>> CA certificate signing and digital signature, only CA certificate
> >>> signing.
> >> 
> >> As could support for additional restrictions.
> >> 
> >> Would these additions be required within this series? What is missing from this 
> >> discussion is why would these additions be necessary?  Why should the kernel 
> >> enforce a restriction that is beyond the scope of the X.509 spec?  If a warning was 
> >> to be added, what would be the justification for adding this additional code?  From 
> >> my research every single 3rd party code signing intermediate CA would be flagged 
> >> with the warning.  Isn’t this just going to cause confusion?  Or is there a benefit that 
> >> I am missing that needs to be stated?
> > 
> > You're focusing on third party kernel modules and forgetting about the
> > simple use case of allowing an end user (or business) to sign their own
> > code.  True they could use the less restrictive CA certificates, but it
> > is unnecessary.
> 
> My focus is on signing user-space applications, as outlined in the cover letter.  This 
> series has nothing to do with kernel modules.  Most end-users and businesses rely on 
> a third party to deal with code signing.  All third party code signing services I have 
> found use an intermediate CA containing more than just the keyCertSign usage set.  
> It seems to be an industry accepted practice that does not violate the spec. Before writing
> new code to either warn or exclude a third party intermediate CA,  I would like to understand 
> the motivation behind this request.

In older discussions there are comments like, "Any CA certificate, no
matter if it's a root or an intermediate, must have the keyCertSign
extension. If you want to sign a revocation list (CRL) with the CA
certificate as well (you usually do want that), than you have to add
cRLSign as well. Any other keyUsages can and should be avoided for CA
certificates."

The question as to "why" this changed to include "digitalSignature" was
posed here [2] with the response being for "OCSP".   It also includes a
link to Entrusts root and intermediate CAs with just keyCertSign and
cRLSign keyUsages.

The matchine keyring is a means of establishing a new root of trust. 
The motivation for further restricting CA certificates to just
keyCertSign and cRLSign keyUsages is to limit how the CA certificates
may be used.  They should not be used for code signing.

thanks,

Mimi

[1] https://superuser.com/questions/738612/openssl-ca-keyusage-extension
[2] https://security.stackexchange.com/questions/231133/keyusage-extensions-on-a-certificate-authority
  

> On Dec 22, 2022, at 8:41 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> 
> On Thu, 2022-12-22 at 15:15 +0000, Eric Snowberg wrote:
>> 
>>> On Dec 21, 2022, at 12:01 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>>> 
>>> On Wed, 2022-12-21 at 18:27 +0000, Eric Snowberg wrote:
>>>> 
>>>>> On Dec 18, 2022, at 5:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>>>>> 
>>>>> On Fri, 2022-12-16 at 22:06 +0800, Coiby Xu wrote:
>>>>>> Hi Eric and Mimi,
>>>>>> 
>>>>>> On Thu, Dec 15, 2022 at 09:45:37PM +0000, Eric Snowberg wrote:
>>>>>>> 
>>>>>>> 
>>>>>>>>>>>>> A CA cert shall be defined as any X509 certificate that contains the
>>>>>>>>>>>>> keyCertSign key usage and has the CA bit set to true.
>>>>>>>>>>>> 
>>>>>>>>>>>> Hi Eric,
>>>>>>>>>>>> 
>>>>>>>>>>>> Allowing CA certificates with the digitalSignature key usage flag
>>>>>>>>>>>> enabled defeats the purpose of the new Kconfig.  Please update the
>>>>>>>>>>>> above definition to exclude the digitalSignature key usage flag and
>>>>>>>>>>>> modify the code accordingly.
>>>>>>>>>>> 
>>>>>>>>>>> Within v2, the request was made to allow Intermediate CA certificates to be
>>>>>>>>>>> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.
>>>>>>>>>>> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code
>>>>>>>>>>> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that
>>>>>>>>>>> the intent?
>>>>>>>>>> 
>>>>>>>>>> That definitely was not the intent.  Nor would it address the issue of
>>>>>>>>>> a particular intermediate CA certificate having both keyCertSign and
>>>>>>>>>> digitalSignature.
>>>>>>>>> 
>>>>>>>>> Sorry, I’m not following.  Why is it an issue that an intermediate CA certificate contains
>>>>>>>>> both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
>>>>>>>>> CA cert like the one used on kernel.org?
>>>>>>>> 
>>>>>>>> I must be missing something.  Isn't the purpose of "keyUsage" to
>>>>>>>> minimize how a certificate may be used?   Why would we want the same
>>>>>>>> certificate to be used for both certificate signing and code signing?
>>>>>>> 
>>>>>>> Every 3rd party intermediate CA I have looked at so far contains both set. Most have CRLSign set.
>>>>>>> Typically the root CA contains keyCertSign and CRLSign, but some also have digitalSignature
>>>>>>> set.  Finding a 3rd party Intermediate CA without digitalSignature set is probably going to be
>>>>>>> challenging and will severely limit usage.
>>>>>> 
>>>>>> How about allowing both keyCertSign and digitalSignature asserted but
>>>>>> issuing a warning for this case?
>>>>>> 
>>>>>> Here's my rationale for this proposal.
>>>>>> 
>>>>>> I assume we should conform to some X.509 specifications. So I checked
>>>>>> "RFC 5280: Internet X.509 Public Key Infrastructure Certificate and
>>>>>> Certificate Revocation List (CRL) Profile" [1] and ITU-T X.509 (2012-10)
>>>>>> [2].
>>>>>> 
>>>>>> [1] states in 4.2.1.3. Key Usage,
>>>>>>  "If the keyUsage extension is present, then the subject public key
>>>>>>  MUST NOT be used to verify signatures on certificates or CRLs unless
>>>>>>  the corresponding keyCertSign or cRLSign bit is set.  If the subject
>>>>>>  public key is only to be used for verifying signatures on
>>>>>>  certificates and/or CRLs, then the digitalSignature and
>>>>>>  nonRepudiation bits SHOULD NOT be set.  However, the digitalSignature
>>>>>>  and/or nonRepudiation bits MAY be set in addition to the keyCertSign
>>>>>>  and/or cRLSign bits if the subject public key is to be used to verify
>>>>>>  signatures on certificates and/or CRLs as well as other objects."
>>>>>> 
>>>>>> and [2] states in 8.2.2.3 Key usage extension that,
>>>>>> "More than one bit may be set in an instance of the keyUsage extension.
>>>>>> The setting of multiple bits shall not change the meaning of each
>>>>>> individual bit but shall indicate that the certificate may be used for
>>>>>> all of the purposes indicated by the set bits. There may be risks
>>>>>> incurred when setting multiple bits. A review of those risks is
>>>>>> documented in Annex I."
>>>>>> 
>>>>>> I interpret the above texts as we should allow both keyCertSign and
>>>>>> digitalSignature. However [2] warns about the risks of setting multiple
>>>>>> bits. Quoting Annex I,
>>>>>> 
>>>>>> "Combining the contentCommitment bit in the keyUsage certificate
>>>>>> extension with other keyUsage bits may have security implications
>>>>>> depending on the security environment in which the certificate is to be
>>>>>> used. If the subject's environment can be fully controlled and trusted,
>>>>>> then there are no specific security implications. For example, in cases
>>>>>> where the subject is fully confident about exactly which data is signed
>>>>>> or cases where the subject is fully confident about the security
>>>>>> characteristics of the authentication protocol being used. If the
>>>>>> subject's environment is not fully controlled or not fully trusted, then
>>>>>> unintentional signing of commitments is possible. Examples include the
>>>>>> use of badly formed authentication exchanges and the use of a rogue
>>>>>> software component. If untrusted environments are used by a subject,
>>>>>> these security implications can be limited through use of the following
>>>>>> measures:   
>>>>>>  – to not combine the contentCommitment key usage setting in
>>>>>>    certificates with any other key usage setting and to use the
>>>>>>    corresponding private key only with this certificate;   
>>>>>> 
>>>>>>  – to limit the use of private keys associated with certificates that
>>>>>>    have the contentCommitment key usage bit set, to environments which
>>>>>>    are considered adequately controlled and trustworthy"
>>>>>> 
>>>>>> So maybe it's useful to add a warning if both keyCertSign and
>>>>>> digitalSignature are asserted.
>>>>> 
>>>>> Coiby, thank you for adding these details.  I was hoping others would
>>>>> chime in as well.  I agree at minimum there should be a warning.
>>>> 
>>>> A warning could be added.
>>>> 
>>>>> Perhaps instead of making INTEGRITY_CA_MACHINE_KEYRING dependent on
>>>>> INTEGRITY_MACHINE_KEYRING, make them a Kconfig "choice" to support the
>>>>> more restrictive certificate use case requirements:  all certificates,
>>>>> CA certificate signing and digital signature, only CA certificate
>>>>> signing.
>>>> 
>>>> As could support for additional restrictions.
>>>> 
>>>> Would these additions be required within this series? What is missing from this 
>>>> discussion is why would these additions be necessary?  Why should the kernel 
>>>> enforce a restriction that is beyond the scope of the X.509 spec?  If a warning was 
>>>> to be added, what would be the justification for adding this additional code?  From 
>>>> my research every single 3rd party code signing intermediate CA would be flagged 
>>>> with the warning.  Isn’t this just going to cause confusion?  Or is there a benefit that 
>>>> I am missing that needs to be stated?
>>> 
>>> You're focusing on third party kernel modules and forgetting about the
>>> simple use case of allowing an end user (or business) to sign their own
>>> code.  True they could use the less restrictive CA certificates, but it
>>> is unnecessary.
>> 
>> My focus is on signing user-space applications, as outlined in the cover letter.  This 
>> series has nothing to do with kernel modules.  Most end-users and businesses rely on 
>> a third party to deal with code signing.  All third party code signing services I have 
>> found use an intermediate CA containing more than just the keyCertSign usage set.  
>> It seems to be an industry accepted practice that does not violate the spec. Before writing
>> new code to either warn or exclude a third party intermediate CA,  I would like to understand 
>> the motivation behind this request.
> 
> In older discussions there are comments like, "Any CA certificate, no
> matter if it's a root or an intermediate, must have the keyCertSign
> extension. If you want to sign a revocation list (CRL) with the CA
> certificate as well (you usually do want that), than you have to add
> cRLSign as well. Any other keyUsages can and should be avoided for CA
> certificates."
> 
> The question as to "why" this changed to include "digitalSignature" was
> posed here [2] with the response being for "OCSP".   It also includes a
> link to Entrusts root and intermediate CAs with just keyCertSign and
> cRLSign keyUsages.
> 
> The matchine keyring is a means of establishing a new root of trust. 
> The motivation for further restricting CA certificates to just
> keyCertSign and cRLSign keyUsages is to limit how the CA certificates
> may be used.  They should not be used for code signing.

Fair enough.  If this will be viewed as justification for adding the additional 
code, I can work on adding it.  Above you mentioned a warning would be needed 
at a minimum and a restriction could be placed behind a Kconfig.  How about for 
the default case I add the warning and when compiling with 
INTEGRITY_CA_MACHINE_KEYRING the restriction will be enforced.
  

On Fri, 2022-12-23 at 16:13 +0000, Eric Snowberg wrote:
> 
> > On Dec 22, 2022, at 8:41 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> > 
> > On Thu, 2022-12-22 at 15:15 +0000, Eric Snowberg wrote:
> >> 
> >>> On Dec 21, 2022, at 12:01 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> >>> 
> >>> On Wed, 2022-12-21 at 18:27 +0000, Eric Snowberg wrote:
> >>>> 
> >>>>> On Dec 18, 2022, at 5:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> >>>>> 
> >>>>> On Fri, 2022-12-16 at 22:06 +0800, Coiby Xu wrote:
> >>>>>> Hi Eric and Mimi,
> >>>>>> 
> >>>>>> On Thu, Dec 15, 2022 at 09:45:37PM +0000, Eric Snowberg wrote:
> >>>>>>> 
> >>>>>>> 
> >>>>>>>>>>>>> A CA cert shall be defined as any X509 certificate that contains the
> >>>>>>>>>>>>> keyCertSign key usage and has the CA bit set to true.
> >>>>>>>>>>>> 
> >>>>>>>>>>>> Hi Eric,
> >>>>>>>>>>>> 
> >>>>>>>>>>>> Allowing CA certificates with the digitalSignature key usage flag
> >>>>>>>>>>>> enabled defeats the purpose of the new Kconfig.  Please update the
> >>>>>>>>>>>> above definition to exclude the digitalSignature key usage flag and
> >>>>>>>>>>>> modify the code accordingly.
> >>>>>>>>>>> 
> >>>>>>>>>>> Within v2, the request was made to allow Intermediate CA certificates to be
> >>>>>>>>>>> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.
> >>>>>>>>>>> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code
> >>>>>>>>>>> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that
> >>>>>>>>>>> the intent?
> >>>>>>>>>> 
> >>>>>>>>>> That definitely was not the intent.  Nor would it address the issue of
> >>>>>>>>>> a particular intermediate CA certificate having both keyCertSign and
> >>>>>>>>>> digitalSignature.
> >>>>>>>>> 
> >>>>>>>>> Sorry, I’m not following.  Why is it an issue that an intermediate CA certificate contains
> >>>>>>>>> both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
> >>>>>>>>> CA cert like the one used on kernel.org?
> >>>>>>>> 
> >>>>>>>> I must be missing something.  Isn't the purpose of "keyUsage" to
> >>>>>>>> minimize how a certificate may be used?   Why would we want the same
> >>>>>>>> certificate to be used for both certificate signing and code signing?
> >>>>>>> 
> >>>>>>> Every 3rd party intermediate CA I have looked at so far contains both set. Most have CRLSign set.
> >>>>>>> Typically the root CA contains keyCertSign and CRLSign, but some also have digitalSignature
> >>>>>>> set.  Finding a 3rd party Intermediate CA without digitalSignature set is probably going to be
> >>>>>>> challenging and will severely limit usage.
> >>>>>> 
> >>>>>> How about allowing both keyCertSign and digitalSignature asserted but
> >>>>>> issuing a warning for this case?
> >>>>>> 
> >>>>>> Here's my rationale for this proposal.
> >>>>>> 
> >>>>>> I assume we should conform to some X.509 specifications. So I checked
> >>>>>> "RFC 5280: Internet X.509 Public Key Infrastructure Certificate and
> >>>>>> Certificate Revocation List (CRL) Profile" [1] and ITU-T X.509 (2012-10)
> >>>>>> [2].
> >>>>>> 
> >>>>>> [1] states in 4.2.1.3. Key Usage,
> >>>>>>  "If the keyUsage extension is present, then the subject public key
> >>>>>>  MUST NOT be used to verify signatures on certificates or CRLs unless
> >>>>>>  the corresponding keyCertSign or cRLSign bit is set.  If the subject
> >>>>>>  public key is only to be used for verifying signatures on
> >>>>>>  certificates and/or CRLs, then the digitalSignature and
> >>>>>>  nonRepudiation bits SHOULD NOT be set.  However, the digitalSignature
> >>>>>>  and/or nonRepudiation bits MAY be set in addition to the keyCertSign
> >>>>>>  and/or cRLSign bits if the subject public key is to be used to verify
> >>>>>>  signatures on certificates and/or CRLs as well as other objects."
> >>>>>> 
> >>>>>> and [2] states in 8.2.2.3 Key usage extension that,
> >>>>>> "More than one bit may be set in an instance of the keyUsage extension.
> >>>>>> The setting of multiple bits shall not change the meaning of each
> >>>>>> individual bit but shall indicate that the certificate may be used for
> >>>>>> all of the purposes indicated by the set bits. There may be risks
> >>>>>> incurred when setting multiple bits. A review of those risks is
> >>>>>> documented in Annex I."
> >>>>>> 
> >>>>>> I interpret the above texts as we should allow both keyCertSign and
> >>>>>> digitalSignature. However [2] warns about the risks of setting multiple
> >>>>>> bits. Quoting Annex I,
> >>>>>> 
> >>>>>> "Combining the contentCommitment bit in the keyUsage certificate
> >>>>>> extension with other keyUsage bits may have security implications
> >>>>>> depending on the security environment in which the certificate is to be
> >>>>>> used. If the subject's environment can be fully controlled and trusted,
> >>>>>> then there are no specific security implications. For example, in cases
> >>>>>> where the subject is fully confident about exactly which data is signed
> >>>>>> or cases where the subject is fully confident about the security
> >>>>>> characteristics of the authentication protocol being used. If the
> >>>>>> subject's environment is not fully controlled or not fully trusted, then
> >>>>>> unintentional signing of commitments is possible. Examples include the
> >>>>>> use of badly formed authentication exchanges and the use of a rogue
> >>>>>> software component. If untrusted environments are used by a subject,
> >>>>>> these security implications can be limited through use of the following
> >>>>>> measures:   
> >>>>>>  – to not combine the contentCommitment key usage setting in
> >>>>>>    certificates with any other key usage setting and to use the
> >>>>>>    corresponding private key only with this certificate;   
> >>>>>> 
> >>>>>>  – to limit the use of private keys associated with certificates that
> >>>>>>    have the contentCommitment key usage bit set, to environments which
> >>>>>>    are considered adequately controlled and trustworthy"
> >>>>>> 
> >>>>>> So maybe it's useful to add a warning if both keyCertSign and
> >>>>>> digitalSignature are asserted.
> >>>>> 
> >>>>> Coiby, thank you for adding these details.  I was hoping others would
> >>>>> chime in as well.  I agree at minimum there should be a warning.
> >>>> 
> >>>> A warning could be added.
> >>>> 
> >>>>> Perhaps instead of making INTEGRITY_CA_MACHINE_KEYRING dependent on
> >>>>> INTEGRITY_MACHINE_KEYRING, make them a Kconfig "choice" to support the
> >>>>> more restrictive certificate use case requirements:  all certificates,
> >>>>> CA certificate signing and digital signature, only CA certificate
> >>>>> signing.
> >>>> 
> >>>> As could support for additional restrictions.
> >>>> 
> >>>> Would these additions be required within this series? What is missing from this 
> >>>> discussion is why would these additions be necessary?  Why should the kernel 
> >>>> enforce a restriction that is beyond the scope of the X.509 spec?  If a warning was 
> >>>> to be added, what would be the justification for adding this additional code?  From 
> >>>> my research every single 3rd party code signing intermediate CA would be flagged 
> >>>> with the warning.  Isn’t this just going to cause confusion?  Or is there a benefit that 
> >>>> I am missing that needs to be stated?
> >>> 
> >>> You're focusing on third party kernel modules and forgetting about the
> >>> simple use case of allowing an end user (or business) to sign their own
> >>> code.  True they could use the less restrictive CA certificates, but it
> >>> is unnecessary.
> >> 
> >> My focus is on signing user-space applications, as outlined in the cover letter.  This 
> >> series has nothing to do with kernel modules.  Most end-users and businesses rely on 
> >> a third party to deal with code signing.  All third party code signing services I have 
> >> found use an intermediate CA containing more than just the keyCertSign usage set.  
> >> It seems to be an industry accepted practice that does not violate the spec. Before writing
> >> new code to either warn or exclude a third party intermediate CA,  I would like to understand 
> >> the motivation behind this request.
> > 
> > In older discussions there are comments like, "Any CA certificate, no
> > matter if it's a root or an intermediate, must have the keyCertSign
> > extension. If you want to sign a revocation list (CRL) with the CA
> > certificate as well (you usually do want that), than you have to add
> > cRLSign as well. Any other keyUsages can and should be avoided for CA
> > certificates."
> > 
> > The question as to "why" this changed to include "digitalSignature" was
> > posed here [2] with the response being for "OCSP".   It also includes a
> > link to Entrusts root and intermediate CAs with just keyCertSign and
> > cRLSign keyUsages.
> > 
> > The matchine keyring is a means of establishing a new root of trust. 
> > The motivation for further restricting CA certificates to just
> > keyCertSign and cRLSign keyUsages is to limit how the CA certificates
> > may be used.  They should not be used for code signing.
> 
> Fair enough.  If this will be viewed as justification for adding the additional 
> code, I can work on adding it.  Above you mentioned a warning would be needed 
> at a minimum and a restriction could be placed behind a Kconfig.  How about for 
> the default case I add the warning and when compiling with 
> INTEGRITY_CA_MACHINE_KEYRING the restriction will be enforced.

Sounds good to me.  To avoid misunderstandings, will there be a Kconfig
menu with 3 options?   There were a couple of other comments having to
do with variable names.  Will you address them as well?
  

> On Dec 23, 2022, at 9:34 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
> 
> On Fri, 2022-12-23 at 16:13 +0000, Eric Snowberg wrote:
>> 
>>> On Dec 22, 2022, at 8:41 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>>> 
>>> On Thu, 2022-12-22 at 15:15 +0000, Eric Snowberg wrote:
>>>> 
>>>>> On Dec 21, 2022, at 12:01 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>>>>> 
>>>>> On Wed, 2022-12-21 at 18:27 +0000, Eric Snowberg wrote:
>>>>>> 
>>>>>>> On Dec 18, 2022, at 5:21 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>>>>>>> 
>>>>>>> On Fri, 2022-12-16 at 22:06 +0800, Coiby Xu wrote:
>>>>>>>> Hi Eric and Mimi,
>>>>>>>> 
>>>>>>>> On Thu, Dec 15, 2022 at 09:45:37PM +0000, Eric Snowberg wrote:
>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>>>>>>>> A CA cert shall be defined as any X509 certificate that contains the
>>>>>>>>>>>>>>> keyCertSign key usage and has the CA bit set to true.
>>>>>>>>>>>>>> 
>>>>>>>>>>>>>> Hi Eric,
>>>>>>>>>>>>>> 
>>>>>>>>>>>>>> Allowing CA certificates with the digitalSignature key usage flag
>>>>>>>>>>>>>> enabled defeats the purpose of the new Kconfig.  Please update the
>>>>>>>>>>>>>> above definition to exclude the digitalSignature key usage flag and
>>>>>>>>>>>>>> modify the code accordingly.
>>>>>>>>>>>>> 
>>>>>>>>>>>>> Within v2, the request was made to allow Intermediate CA certificates to be
>>>>>>>>>>>>> loaded directly.  The Intermediate CA referenced was the one used by kernel.org.
>>>>>>>>>>>>> This Intermediate CA contains both digitalSignature and keyCertSign.  If the code
>>>>>>>>>>>>> is changed to exclude this certificate, now the root CA has to be loaded again.  Is that
>>>>>>>>>>>>> the intent?
>>>>>>>>>>>> 
>>>>>>>>>>>> That definitely was not the intent.  Nor would it address the issue of
>>>>>>>>>>>> a particular intermediate CA certificate having both keyCertSign and
>>>>>>>>>>>> digitalSignature.
>>>>>>>>>>> 
>>>>>>>>>>> Sorry, I’m not following.  Why is it an issue that an intermediate CA certificate contains
>>>>>>>>>>> both keyCertSign and digitalSignature? Why would we want to exclude an Intermediate
>>>>>>>>>>> CA cert like the one used on kernel.org?
>>>>>>>>>> 
>>>>>>>>>> I must be missing something.  Isn't the purpose of "keyUsage" to
>>>>>>>>>> minimize how a certificate may be used?   Why would we want the same
>>>>>>>>>> certificate to be used for both certificate signing and code signing?
>>>>>>>>> 
>>>>>>>>> Every 3rd party intermediate CA I have looked at so far contains both set. Most have CRLSign set.
>>>>>>>>> Typically the root CA contains keyCertSign and CRLSign, but some also have digitalSignature
>>>>>>>>> set.  Finding a 3rd party Intermediate CA without digitalSignature set is probably going to be
>>>>>>>>> challenging and will severely limit usage.
>>>>>>>> 
>>>>>>>> How about allowing both keyCertSign and digitalSignature asserted but
>>>>>>>> issuing a warning for this case?
>>>>>>>> 
>>>>>>>> Here's my rationale for this proposal.
>>>>>>>> 
>>>>>>>> I assume we should conform to some X.509 specifications. So I checked
>>>>>>>> "RFC 5280: Internet X.509 Public Key Infrastructure Certificate and
>>>>>>>> Certificate Revocation List (CRL) Profile" [1] and ITU-T X.509 (2012-10)
>>>>>>>> [2].
>>>>>>>> 
>>>>>>>> [1] states in 4.2.1.3. Key Usage,
>>>>>>>> "If the keyUsage extension is present, then the subject public key
>>>>>>>> MUST NOT be used to verify signatures on certificates or CRLs unless
>>>>>>>> the corresponding keyCertSign or cRLSign bit is set.  If the subject
>>>>>>>> public key is only to be used for verifying signatures on
>>>>>>>> certificates and/or CRLs, then the digitalSignature and
>>>>>>>> nonRepudiation bits SHOULD NOT be set.  However, the digitalSignature
>>>>>>>> and/or nonRepudiation bits MAY be set in addition to the keyCertSign
>>>>>>>> and/or cRLSign bits if the subject public key is to be used to verify
>>>>>>>> signatures on certificates and/or CRLs as well as other objects."
>>>>>>>> 
>>>>>>>> and [2] states in 8.2.2.3 Key usage extension that,
>>>>>>>> "More than one bit may be set in an instance of the keyUsage extension.
>>>>>>>> The setting of multiple bits shall not change the meaning of each
>>>>>>>> individual bit but shall indicate that the certificate may be used for
>>>>>>>> all of the purposes indicated by the set bits. There may be risks
>>>>>>>> incurred when setting multiple bits. A review of those risks is
>>>>>>>> documented in Annex I."
>>>>>>>> 
>>>>>>>> I interpret the above texts as we should allow both keyCertSign and
>>>>>>>> digitalSignature. However [2] warns about the risks of setting multiple
>>>>>>>> bits. Quoting Annex I,
>>>>>>>> 
>>>>>>>> "Combining the contentCommitment bit in the keyUsage certificate
>>>>>>>> extension with other keyUsage bits may have security implications
>>>>>>>> depending on the security environment in which the certificate is to be
>>>>>>>> used. If the subject's environment can be fully controlled and trusted,
>>>>>>>> then there are no specific security implications. For example, in cases
>>>>>>>> where the subject is fully confident about exactly which data is signed
>>>>>>>> or cases where the subject is fully confident about the security
>>>>>>>> characteristics of the authentication protocol being used. If the
>>>>>>>> subject's environment is not fully controlled or not fully trusted, then
>>>>>>>> unintentional signing of commitments is possible. Examples include the
>>>>>>>> use of badly formed authentication exchanges and the use of a rogue
>>>>>>>> software component. If untrusted environments are used by a subject,
>>>>>>>> these security implications can be limited through use of the following
>>>>>>>> measures:   
>>>>>>>> – to not combine the contentCommitment key usage setting in
>>>>>>>>   certificates with any other key usage setting and to use the
>>>>>>>>   corresponding private key only with this certificate;   
>>>>>>>> 
>>>>>>>> – to limit the use of private keys associated with certificates that
>>>>>>>>   have the contentCommitment key usage bit set, to environments which
>>>>>>>>   are considered adequately controlled and trustworthy"
>>>>>>>> 
>>>>>>>> So maybe it's useful to add a warning if both keyCertSign and
>>>>>>>> digitalSignature are asserted.
>>>>>>> 
>>>>>>> Coiby, thank you for adding these details.  I was hoping others would
>>>>>>> chime in as well.  I agree at minimum there should be a warning.
>>>>>> 
>>>>>> A warning could be added.
>>>>>> 
>>>>>>> Perhaps instead of making INTEGRITY_CA_MACHINE_KEYRING dependent on
>>>>>>> INTEGRITY_MACHINE_KEYRING, make them a Kconfig "choice" to support the
>>>>>>> more restrictive certificate use case requirements:  all certificates,
>>>>>>> CA certificate signing and digital signature, only CA certificate
>>>>>>> signing.
>>>>>> 
>>>>>> As could support for additional restrictions.
>>>>>> 
>>>>>> Would these additions be required within this series? What is missing from this 
>>>>>> discussion is why would these additions be necessary?  Why should the kernel 
>>>>>> enforce a restriction that is beyond the scope of the X.509 spec?  If a warning was 
>>>>>> to be added, what would be the justification for adding this additional code?  From 
>>>>>> my research every single 3rd party code signing intermediate CA would be flagged 
>>>>>> with the warning.  Isn’t this just going to cause confusion?  Or is there a benefit that 
>>>>>> I am missing that needs to be stated?
>>>>> 
>>>>> You're focusing on third party kernel modules and forgetting about the
>>>>> simple use case of allowing an end user (or business) to sign their own
>>>>> code.  True they could use the less restrictive CA certificates, but it
>>>>> is unnecessary.
>>>> 
>>>> My focus is on signing user-space applications, as outlined in the cover letter.  This 
>>>> series has nothing to do with kernel modules.  Most end-users and businesses rely on 
>>>> a third party to deal with code signing.  All third party code signing services I have 
>>>> found use an intermediate CA containing more than just the keyCertSign usage set.  
>>>> It seems to be an industry accepted practice that does not violate the spec. Before writing
>>>> new code to either warn or exclude a third party intermediate CA,  I would like to understand 
>>>> the motivation behind this request.
>>> 
>>> In older discussions there are comments like, "Any CA certificate, no
>>> matter if it's a root or an intermediate, must have the keyCertSign
>>> extension. If you want to sign a revocation list (CRL) with the CA
>>> certificate as well (you usually do want that), than you have to add
>>> cRLSign as well. Any other keyUsages can and should be avoided for CA
>>> certificates."
>>> 
>>> The question as to "why" this changed to include "digitalSignature" was
>>> posed here [2] with the response being for "OCSP".   It also includes a
>>> link to Entrusts root and intermediate CAs with just keyCertSign and
>>> cRLSign keyUsages.
>>> 
>>> The matchine keyring is a means of establishing a new root of trust. 
>>> The motivation for further restricting CA certificates to just
>>> keyCertSign and cRLSign keyUsages is to limit how the CA certificates
>>> may be used.  They should not be used for code signing.
>> 
>> Fair enough.  If this will be viewed as justification for adding the additional 
>> code, I can work on adding it.  Above you mentioned a warning would be needed 
>> at a minimum and a restriction could be placed behind a Kconfig.  How about for 
>> the default case I add the warning and when compiling with 
>> INTEGRITY_CA_MACHINE_KEYRING the restriction will be enforced.
> 
> Sounds good to me.  To avoid misunderstandings, will there be a Kconfig
> menu with 3 options?  

I will add the three options in the next round.

> There were a couple of other comments having to
> do with variable names.  Will you address them as well?

And take care of the variable name changes.  I won’t get back to this until January.
  

On Fri, 2022-12-23 at 18:17 +0000, Eric Snowberg wrote:
> >> Fair enough.  If this will be viewed as justification for adding the additional 
> >> code, I can work on adding it.  Above you mentioned a warning would be needed 
> >> at a minimum and a restriction could be placed behind a Kconfig.  How about for 
> >> the default case I add the warning and when compiling with 
> >> INTEGRITY_CA_MACHINE_KEYRING the restriction will be enforced.
> > 
> > Sounds good to me.  To avoid misunderstandings, will there be a Kconfig
> > menu with 3 options?  
> 
> I will add the three options in the next round.
> 
> > There were a couple of other comments having to
> > do with variable names.  Will you address them as well?
> 
> And take care of the variable name changes.  I won’t get back to this until January.

Enjoy your vacation and the holidays.  Looking forward to the next
version.