diff mbox series

security: keys: perform capable check only on privileged operations

Message ID 20230511123252.723185-1-cgzones@googlemail.com
State New
Headers
Series security: keys: perform capable check only on privileged operations |

Commit Message

Christian Göttsche May 11, 2023, 12:32 p.m. UTC 
  If the current task fails the check for the queried capability via
`capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
Issuing such denial messages unnecessarily can lead to a policy author
granting more privileges to a subject than needed to silence them.

Reorder CAP_SYS_ADMIN checks after the check whether the operation is
actually privileged.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
---
 security/keys/keyctl.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

Comments

Paul Moore May 19, 2023, 9:07 p.m. UTC | #1 
On Thu, May 11, 2023 at 8:33 AM Christian Göttsche
<cgzones@googlemail.com> wrote:
>
> If the current task fails the check for the queried capability via
> `capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
> Issuing such denial messages unnecessarily can lead to a policy author
> granting more privileges to a subject than needed to silence them.
>
> Reorder CAP_SYS_ADMIN checks after the check whether the operation is
> actually privileged.
>
> Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
> ---
>  security/keys/keyctl.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index d54f73c558f7..19be69fa4d05 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -980,14 +980,19 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
>         ret = -EACCES;
>         down_write(&key->sem);
>
> -       if (!capable(CAP_SYS_ADMIN)) {
> +       {
> +               bool is_privileged_op = false;
> +
>                 /* only the sysadmin can chown a key to some other UID */
>                 if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
> -                       goto error_put;
> +                       is_privileged_op = true;
>
>                 /* only the sysadmin can set the key's GID to a group other
>                  * than one of those that the current process subscribes to */
>                 if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
> +                       is_privileged_op = true;
> +
> +               if (is_privileged_op && !capable(CAP_SYS_ADMIN))
>                         goto error_put;
>         }

Hmm.  Using braces just to create a new scope is a bit hacky; I'll
admit to using it to quickly create new local variables, but I only do
so in debug/test situations, not production code.

What if you move the CAP_SYS_ADMIN check down into the if-conditional
where the code checks to see if CAP_SYS_ADMIN is needed when changing
the UID?  It should be possible to structure the CAP_SYS_ADMIN check
such that it is only executed if needed.  It's a little more
complicated in the GID case, but I believe it should be doable.
Christian Göttsche May 23, 2023, 6:33 p.m. UTC | #2 
On Fri, 19 May 2023 at 23:08, Paul Moore <paul@paul-moore.com> wrote:
>
> On Thu, May 11, 2023 at 8:33 AM Christian Göttsche
> <cgzones@googlemail.com> wrote:
> >
> > If the current task fails the check for the queried capability via
> > `capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
> > Issuing such denial messages unnecessarily can lead to a policy author
> > granting more privileges to a subject than needed to silence them.
> >
> > Reorder CAP_SYS_ADMIN checks after the check whether the operation is
> > actually privileged.
> >
> > Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
> > ---
> >  security/keys/keyctl.c | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> >
> > diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> > index d54f73c558f7..19be69fa4d05 100644
> > --- a/security/keys/keyctl.c
> > +++ b/security/keys/keyctl.c
> > @@ -980,14 +980,19 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
> >         ret = -EACCES;
> >         down_write(&key->sem);
> >
> > -       if (!capable(CAP_SYS_ADMIN)) {
> > +       {
> > +               bool is_privileged_op = false;
> > +
> >                 /* only the sysadmin can chown a key to some other UID */
> >                 if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
> > -                       goto error_put;
> > +                       is_privileged_op = true;
> >
> >                 /* only the sysadmin can set the key's GID to a group other
> >                  * than one of those that the current process subscribes to */
> >                 if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
> > +                       is_privileged_op = true;
> > +
> > +               if (is_privileged_op && !capable(CAP_SYS_ADMIN))
> >                         goto error_put;
> >         }
>
> Hmm.  Using braces just to create a new scope is a bit hacky; I'll
> admit to using it to quickly create new local variables, but I only do
> so in debug/test situations, not production code.
>
> What if you move the CAP_SYS_ADMIN check down into the if-conditional
> where the code checks to see if CAP_SYS_ADMIN is needed when changing
> the UID?  It should be possible to structure the CAP_SYS_ADMIN check
> such that it is only executed if needed.  It's a little more
> complicated in the GID case, but I believe it should be doable.

This complication I exactly wanted to avoid.  For me the inner scope
encapsulates the all the logic around the capability check just fine
and is quite readable.  An alternative would be to create a new
function performing the checks and call it via

    if (!chown_key_capable(key, user, uid, group, gid))
        got error_put;

A minor inconvenience is the number of needed arguments (and the
actual code after inlining should be the same to the inner scope in
the end).

>
> --
> paul-moore.com
Jarkko Sakkinen May 23, 2023, 7:31 p.m. UTC | #3 
On Thu May 11, 2023 at 3:32 PM EEST, Christian Göttsche wrote:
> If the current task fails the check for the queried capability via
> `capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
> Issuing such denial messages unnecessarily can lead to a policy author
> granting more privileges to a subject than needed to silence them.
>
> Reorder CAP_SYS_ADMIN checks after the check whether the operation is
> actually privileged.
>
> Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
> ---
>  security/keys/keyctl.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> index d54f73c558f7..19be69fa4d05 100644
> --- a/security/keys/keyctl.c
> +++ b/security/keys/keyctl.c
> @@ -980,14 +980,19 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
>  	ret = -EACCES;
>  	down_write(&key->sem);
>  
> -	if (!capable(CAP_SYS_ADMIN)) {
> +	{
> +		bool is_privileged_op = false;
> +
>  		/* only the sysadmin can chown a key to some other UID */
>  		if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
> -			goto error_put;
> +			is_privileged_op = true;
>  
>  		/* only the sysadmin can set the key's GID to a group other
>  		 * than one of those that the current process subscribes to */
>  		if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
> +			is_privileged_op = true;
> +
> +		if (is_privileged_op && !capable(CAP_SYS_ADMIN))
>  			goto error_put;
>  	}
>  
> @@ -1088,7 +1093,7 @@ long keyctl_setperm_key(key_serial_t id, key_perm_t perm)
>  	down_write(&key->sem);
>  
>  	/* if we're not the sysadmin, we can only change a key that we own */
> -	if (capable(CAP_SYS_ADMIN) || uid_eq(key->uid, current_fsuid())) {
> +	if (uid_eq(key->uid, current_fsuid()) || capable(CAP_SYS_ADMIN)) {
>  		key->perm = perm;
>  		notify_key(key, NOTIFY_KEY_SETATTR, 0);
>  		ret = 0;
> -- 
> 2.40.1

Acked-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko
Paul Moore May 25, 2023, 9:25 p.m. UTC | #4 
On Tue, May 23, 2023 at 2:33 PM Christian Göttsche
<cgzones@googlemail.com> wrote:
> On Fri, 19 May 2023 at 23:08, Paul Moore <paul@paul-moore.com> wrote:
> > On Thu, May 11, 2023 at 8:33 AM Christian Göttsche
> > <cgzones@googlemail.com> wrote:
> > >
> > > If the current task fails the check for the queried capability via
> > > `capable(CAP_SYS_ADMIN)` LSMs like SELinux generate a denial message.
> > > Issuing such denial messages unnecessarily can lead to a policy author
> > > granting more privileges to a subject than needed to silence them.
> > >
> > > Reorder CAP_SYS_ADMIN checks after the check whether the operation is
> > > actually privileged.
> > >
> > > Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
> > > ---
> > >  security/keys/keyctl.c | 11 ++++++++---
> > >  1 file changed, 8 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
> > > index d54f73c558f7..19be69fa4d05 100644
> > > --- a/security/keys/keyctl.c
> > > +++ b/security/keys/keyctl.c
> > > @@ -980,14 +980,19 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
> > >         ret = -EACCES;
> > >         down_write(&key->sem);
> > >
> > > -       if (!capable(CAP_SYS_ADMIN)) {
> > > +       {
> > > +               bool is_privileged_op = false;
> > > +
> > >                 /* only the sysadmin can chown a key to some other UID */
> > >                 if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
> > > -                       goto error_put;
> > > +                       is_privileged_op = true;
> > >
> > >                 /* only the sysadmin can set the key's GID to a group other
> > >                  * than one of those that the current process subscribes to */
> > >                 if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
> > > +                       is_privileged_op = true;
> > > +
> > > +               if (is_privileged_op && !capable(CAP_SYS_ADMIN))
> > >                         goto error_put;
> > >         }
> >
> > Hmm.  Using braces just to create a new scope is a bit hacky; I'll
> > admit to using it to quickly create new local variables, but I only do
> > so in debug/test situations, not production code.
> >
> > What if you move the CAP_SYS_ADMIN check down into the if-conditional
> > where the code checks to see if CAP_SYS_ADMIN is needed when changing
> > the UID?  It should be possible to structure the CAP_SYS_ADMIN check
> > such that it is only executed if needed.  It's a little more
> > complicated in the GID case, but I believe it should be doable.
>
> This complication I exactly wanted to avoid.  For me the inner scope
> encapsulates the all the logic around the capability check just fine
> and is quite readable.  An alternative would be to create a new
> function performing the checks and call it via
>
>     if (!chown_key_capable(key, user, uid, group, gid))
>         got error_put;
>
> A minor inconvenience is the number of needed arguments (and the
> actual code after inlining should be the same to the inner scope in
> the end).

Well, lucky for you, Jarkko and David maintain the keys code, not me,
and Jarkko seems to like your patch just fine :)

Jarkko, I assume you'll be taking this via the keys tree?
Jarkko Sakkinen May 30, 2023, 11:04 p.m. UTC | #5 
On Thu, 2023-05-25 at 17:25 -0400, Paul Moore wrote:
> > A minor inconvenience is the number of needed arguments (and the
> > actual code after inlining should be the same to the inner scope in
> > the end).
> 
> Well, lucky for you, Jarkko and David maintain the keys code, not me,
> and Jarkko seems to like your patch just fine :)
> 
> Jarkko, I assume you'll be taking this via the keys tree?

I just picked it and mirrored to linux-next.

I think it is super important change because it tones down the human
error (a little bit at least). You could say improves user experience
kind of I guess :-)

BR, Jarkko
diff mbox series

Patch

diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index d54f73c558f7..19be69fa4d05 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -980,14 +980,19 @@  long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
 	ret = -EACCES;
 	down_write(&key->sem);
 
-	if (!capable(CAP_SYS_ADMIN)) {
+	{
+		bool is_privileged_op = false;
+
 		/* only the sysadmin can chown a key to some other UID */
 		if (user != (uid_t) -1 && !uid_eq(key->uid, uid))
-			goto error_put;
+			is_privileged_op = true;
 
 		/* only the sysadmin can set the key's GID to a group other
 		 * than one of those that the current process subscribes to */
 		if (group != (gid_t) -1 && !gid_eq(gid, key->gid) && !in_group_p(gid))
+			is_privileged_op = true;
+
+		if (is_privileged_op && !capable(CAP_SYS_ADMIN))
 			goto error_put;
 	}
 
@@ -1088,7 +1093,7 @@  long keyctl_setperm_key(key_serial_t id, key_perm_t perm)
 	down_write(&key->sem);
 
 	/* if we're not the sysadmin, we can only change a key that we own */
-	if (capable(CAP_SYS_ADMIN) || uid_eq(key->uid, current_fsuid())) {
+	if (uid_eq(key->uid, current_fsuid()) || capable(CAP_SYS_ADMIN)) {
 		key->perm = perm;
 		notify_key(key, NOTIFY_KEY_SETATTR, 0);
 		ret = 0;