[v8,1/4] crypto: ccp - Name -1 return value as SEV_RET_NO_FW_CALL

  From: Peter Gonda <pgonda@google.com>

The PSP can return a "firmware error" code of -1 in circumstances where
the PSP is not actually called. To make this protocol unambiguous, we
add a constant naming the return value.

Cc: Thomas Lendacky <Thomas.Lendacky@amd.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Andy Lutomirsky <luto@kernel.org>
Cc: John Allen <john.allen@amd.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>

Signed-off-by: Peter Gonda <pgonda@google.com>
Signed-off-by: Dionna Glaze <dionnaglaze@google.com>
---
 drivers/crypto/ccp/sev-dev.c | 2 +-
 include/uapi/linux/psp-sev.h | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)
  

On 11/4/22 18:00, Dionna Glaze wrote:
> From: Peter Gonda <pgonda@google.com>
> 
> The PSP can return a "firmware error" code of -1 in circumstances where
> the PSP is not actually called. To make this protocol unambiguous, we
> add a constant naming the return value.
> 
> Cc: Thomas Lendacky <Thomas.Lendacky@amd.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Joerg Roedel <jroedel@suse.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Andy Lutomirsky <luto@kernel.org>
> Cc: John Allen <john.allen@amd.com>
> Cc: Herbert Xu <herbert@gondor.apana.org.au>
> Cc: "David S. Miller" <davem@davemloft.net>
> 
> Signed-off-by: Peter Gonda <pgonda@google.com>
> Signed-off-by: Dionna Glaze <dionnaglaze@google.com>

Looks like you missed my Reviewed-by: from an earlier version, so...

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

> ---
>   drivers/crypto/ccp/sev-dev.c | 2 +-
>   include/uapi/linux/psp-sev.h | 7 +++++++
>   2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
> index 06fc7156c04f..97eb3544ab36 100644
> --- a/drivers/crypto/ccp/sev-dev.c
> +++ b/drivers/crypto/ccp/sev-dev.c
> @@ -444,7 +444,7 @@ static int __sev_platform_init_locked(int *error)
>   {
>   	struct psp_device *psp = psp_master;
>   	struct sev_device *sev;
> -	int rc = 0, psp_ret = -1;
> +	int rc = 0, psp_ret = SEV_RET_NO_FW_CALL;
>   	int (*init_function)(int *error);
>   
>   	if (!psp || !psp->sev_data)
> diff --git a/include/uapi/linux/psp-sev.h b/include/uapi/linux/psp-sev.h
> index 91b4c63d5cbf..1ad7f0a7e328 100644
> --- a/include/uapi/linux/psp-sev.h
> +++ b/include/uapi/linux/psp-sev.h
> @@ -36,6 +36,13 @@ enum {
>    * SEV Firmware status code
>    */
>   typedef enum {
> +	/*
> +	 * This error code is not in the SEV spec but is added to convey that
> +	 * there was an error that prevented the SEV Firmware from being called.
> +	 * This is (u32)-1 since the firmware error code is represented as a
> +	 * 32-bit integer.
> +	 */
> +	SEV_RET_NO_FW_CALL = 0xffffffff,
>   	SEV_RET_SUCCESS = 0,
>   	SEV_RET_INVALID_PLATFORM_STATE,
>   	SEV_RET_INVALID_GUEST_STATE,
  

On Fri, Nov 04, 2022 at 11:00:37PM +0000, Dionna Glaze wrote:
> From: Peter Gonda <pgonda@google.com>
> 
> The PSP can return a "firmware error" code of -1 in circumstances where
> the PSP is not actually called. To make this protocol unambiguous, we

Please use passive voice in your commit message: no "we" or "I", etc,
and describe your changes in imperative mood.

> add a constant naming the return value.
> 
> Cc: Thomas Lendacky <Thomas.Lendacky@amd.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Joerg Roedel <jroedel@suse.de>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Andy Lutomirsky <luto@kernel.org>
> Cc: John Allen <john.allen@amd.com>
> Cc: Herbert Xu <herbert@gondor.apana.org.au>
> Cc: "David S. Miller" <davem@davemloft.net>
> 
> Signed-off-by: Peter Gonda <pgonda@google.com>
> Signed-off-by: Dionna Glaze <dionnaglaze@google.com>
> ---
>  drivers/crypto/ccp/sev-dev.c | 2 +-
>  include/uapi/linux/psp-sev.h | 7 +++++++
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
> index 06fc7156c04f..97eb3544ab36 100644
> --- a/drivers/crypto/ccp/sev-dev.c
> +++ b/drivers/crypto/ccp/sev-dev.c
> @@ -444,7 +444,7 @@ static int __sev_platform_init_locked(int *error)
>  {
>  	struct psp_device *psp = psp_master;
>  	struct sev_device *sev;
> -	int rc = 0, psp_ret = -1;
> +	int rc = 0, psp_ret = SEV_RET_NO_FW_CALL;
>  	int (*init_function)(int *error);
>  
>  	if (!psp || !psp->sev_data)

Ok, lemme chase down this flow here:

__sev_platform_init_locked() calls that automatic variable function
pointer ->init_function which already looks funky. See the end of this
mail for a diff removing it and making the code more readable.

The called function can be one of two and both get the pointer to
psp_ret as its only argument.

1. __sev_init_ex_locked() calls __sev_do_cmd_locked() and passes down
*psp_ret.

or

2. __sev_init_locked(). Ditto.

Now, __sev_do_cmd_locked() will overwrite psp_ret when
sev_wait_cmd_ioc() fails. So far so good.

In the case __sev_do_cmd_locked() succeeds, it'll put there something
else:

        if (psp_ret)
                *psp_ret = reg & PSP_CMDRESP_ERR_MASK;

So no caller will ever see SEV_RET_NO_FW_CALL, as far as I can tell.

And looking further through the rest of the set, nothing tests
SEV_RET_NO_FW_CALL - it only gets assigned.

So why are we even bothering with this?

You can hand in *psp_ret uninitialized and you'll get a value in all
cases. Unless I'm missing an angle.

> diff --git a/include/uapi/linux/psp-sev.h b/include/uapi/linux/psp-sev.h
> index 91b4c63d5cbf..1ad7f0a7e328 100644
> --- a/include/uapi/linux/psp-sev.h
> +++ b/include/uapi/linux/psp-sev.h
> @@ -36,6 +36,13 @@ enum {
>   * SEV Firmware status code
>   */
>  typedef enum {
> +	/*
> +	 * This error code is not in the SEV spec but is added to convey that
> +	 * there was an error that prevented the SEV Firmware from being called.
> +	 * This is (u32)-1 since the firmware error code is represented as a
> +	 * 32-bit integer.
> +	 */
> +	SEV_RET_NO_FW_CALL = 0xffffffff,

What's wrong with having -1 here?

>  	SEV_RET_SUCCESS = 0,
>  	SEV_RET_INVALID_PLATFORM_STATE,
>  	SEV_RET_INVALID_GUEST_STATE,
> -- 

diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
index 97eb3544ab36..8bc4209b338b 100644
--- a/drivers/crypto/ccp/sev-dev.c
+++ b/drivers/crypto/ccp/sev-dev.c
@@ -440,12 +440,20 @@ static int __sev_init_ex_locked(int *error)
 	return __sev_do_cmd_locked(SEV_CMD_INIT_EX, &data, error);
 }
 
+static inline int __sev_do_init_locked(int *psp_ret)
+{
+	if (sev_init_ex_buffer)
+		return __sev_init_ex_locked(psp_ret);
+	else
+
+		return __sev_init_locked(psp_ret);
+}
+
 static int __sev_platform_init_locked(int *error)
 {
 	struct psp_device *psp = psp_master;
 	struct sev_device *sev;
-	int rc = 0, psp_ret = SEV_RET_NO_FW_CALL;
-	int (*init_function)(int *error);
+	int rc = 0, psp_ret;
 
 	if (!psp || !psp->sev_data)
 		return -ENODEV;
@@ -456,15 +464,12 @@ static int __sev_platform_init_locked(int *error)
 		return 0;
 
 	if (sev_init_ex_buffer) {
-		init_function = __sev_init_ex_locked;
 		rc = sev_read_init_ex_file();
 		if (rc)
 			return rc;
-	} else {
-		init_function = __sev_init_locked;
 	}
 
-	rc = init_function(&psp_ret);
+	rc = __sev_do_init_locked(&psp_ret);
 	if (rc && psp_ret == SEV_RET_SECURE_DATA_INVALID) {
 		/*
 		 * Initialization command returned an integrity check failure
@@ -473,9 +478,12 @@ static int __sev_platform_init_locked(int *error)
 		 * initialization function should succeed by replacing the state
 		 * with a reset state.
 		 */
-		dev_err(sev->dev, "SEV: retrying INIT command because of SECURE_DATA_INVALID error. Retrying once to reset PSP SEV state.");
-		rc = init_function(&psp_ret);
+		dev_err(sev->dev,
+"SEV: retrying INIT command because of SECURE_DATA_INVALID error. Retrying once to reset PSP SEV state.");
+
+		rc = __sev_do_init_locked(&psp_ret);
 	}
+
 	if (error)
 		*error = psp_ret;
 
diff --git a/include/uapi/linux/psp-sev.h b/include/uapi/linux/psp-sev.h
index 1ad7f0a7e328..a9ed9e846cd2 100644
--- a/include/uapi/linux/psp-sev.h
+++ b/include/uapi/linux/psp-sev.h
@@ -42,7 +42,7 @@ typedef enum {
 	 * This is (u32)-1 since the firmware error code is represented as a
 	 * 32-bit integer.
 	 */
-	SEV_RET_NO_FW_CALL = 0xffffffff,
+	SEV_RET_NO_FW_CALL = -1,
 	SEV_RET_SUCCESS = 0,
 	SEV_RET_INVALID_PLATFORM_STATE,
 	SEV_RET_INVALID_GUEST_STATE,
  

On Sat, Dec 3, 2022 at 4:26 AM Borislav Petkov <bp@alien8.de> wrote:
>
> On Fri, Nov 04, 2022 at 11:00:37PM +0000, Dionna Glaze wrote:
> > From: Peter Gonda <pgonda@google.com>
> >
> > The PSP can return a "firmware error" code of -1 in circumstances where
> > the PSP is not actually called. To make this protocol unambiguous, we
>
> Please use passive voice in your commit message: no "we" or "I", etc,
> and describe your changes in imperative mood.
>
> > add a constant naming the return value.
> >
> > Cc: Thomas Lendacky <Thomas.Lendacky@amd.com>
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Joerg Roedel <jroedel@suse.de>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Andy Lutomirsky <luto@kernel.org>
> > Cc: John Allen <john.allen@amd.com>
> > Cc: Herbert Xu <herbert@gondor.apana.org.au>
> > Cc: "David S. Miller" <davem@davemloft.net>
> >
> > Signed-off-by: Peter Gonda <pgonda@google.com>
> > Signed-off-by: Dionna Glaze <dionnaglaze@google.com>
> > ---
> >  drivers/crypto/ccp/sev-dev.c | 2 +-
> >  include/uapi/linux/psp-sev.h | 7 +++++++
> >  2 files changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
> > index 06fc7156c04f..97eb3544ab36 100644
> > --- a/drivers/crypto/ccp/sev-dev.c
> > +++ b/drivers/crypto/ccp/sev-dev.c
> > @@ -444,7 +444,7 @@ static int __sev_platform_init_locked(int *error)
> >  {
> >       struct psp_device *psp = psp_master;
> >       struct sev_device *sev;
> > -     int rc = 0, psp_ret = -1;
> > +     int rc = 0, psp_ret = SEV_RET_NO_FW_CALL;
> >       int (*init_function)(int *error);
> >
> >       if (!psp || !psp->sev_data)
>
> Ok, lemme chase down this flow here:
>
> __sev_platform_init_locked() calls that automatic variable function
> pointer ->init_function which already looks funky. See the end of this
> mail for a diff removing it and making the code more readable.
>

I'm fine removing it if possible for the sev-dev.c code, but I'll
still need the enum for the next patches in this series. I added it
specifically because of the uninitialized memory problem with `err`
that I witnessed in user space, and to replace the arbitrary 0xff
value in existing code.

> The called function can be one of two and both get the pointer to
> psp_ret as its only argument.
>
> 1. __sev_init_ex_locked() calls __sev_do_cmd_locked() and passes down
> *psp_ret.
>
> or
>
> 2. __sev_init_locked(). Ditto.
>
> Now, __sev_do_cmd_locked() will overwrite psp_ret when
> sev_wait_cmd_ioc() fails. So far so good.

It doesn't always overwrite psp_ret, such as the initial error checking.
The value remains uninitialized for -ENODEV, -EBUSY, -EINVAL.
Thus *error in __sev_platform_init_locked can be set to uninitialized
memory if psp_ret is not first initialized.
That error points to the kernel copy of the user's argument struct,
which the ioctl always copies back.
In the case of those error codes then, without SEV_RET_NO_FW_CALL,
user space will get uninitialized kernel memory.

>
> In the case __sev_do_cmd_locked() succeeds, it'll put there something
> else:
>
>         if (psp_ret)
>                 *psp_ret = reg & PSP_CMDRESP_ERR_MASK;
>
> So no caller will ever see SEV_RET_NO_FW_CALL, as far as I can tell.
>
> And looking further through the rest of the set, nothing tests
> SEV_RET_NO_FW_CALL - it only gets assigned.
>
> So why are we even bothering with this?
>
> You can hand in *psp_ret uninitialized and you'll get a value in all
> cases. Unless I'm missing an angle.
>

I think my above comment points out the wrinkle.

> > diff --git a/include/uapi/linux/psp-sev.h b/include/uapi/linux/psp-sev.h
> > index 91b4c63d5cbf..1ad7f0a7e328 100644
> > --- a/include/uapi/linux/psp-sev.h
> > +++ b/include/uapi/linux/psp-sev.h
> > @@ -36,6 +36,13 @@ enum {
> >   * SEV Firmware status code
> >   */
> >  typedef enum {
> > +     /*
> > +      * This error code is not in the SEV spec but is added to convey that
> > +      * there was an error that prevented the SEV Firmware from being called.
> > +      * This is (u32)-1 since the firmware error code is represented as a
> > +      * 32-bit integer.
> > +      */
> > +     SEV_RET_NO_FW_CALL = 0xffffffff,
>
> What's wrong with having -1 here?
>

C++ brain not trusting what type enum has even in C. I can change it to -1.

> >       SEV_RET_SUCCESS = 0,
> >       SEV_RET_INVALID_PLATFORM_STATE,
> >       SEV_RET_INVALID_GUEST_STATE,
> > --
>
> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
> index 97eb3544ab36..8bc4209b338b 100644
> --- a/drivers/crypto/ccp/sev-dev.c
> +++ b/drivers/crypto/ccp/sev-dev.c
> @@ -440,12 +440,20 @@ static int __sev_init_ex_locked(int *error)
>         return __sev_do_cmd_locked(SEV_CMD_INIT_EX, &data, error);
>  }
>
> +static inline int __sev_do_init_locked(int *psp_ret)
> +{
> +       if (sev_init_ex_buffer)
> +               return __sev_init_ex_locked(psp_ret);
> +       else
> +
> +               return __sev_init_locked(psp_ret);
> +}
> +
>  static int __sev_platform_init_locked(int *error)
>  {
>         struct psp_device *psp = psp_master;
>         struct sev_device *sev;
> -       int rc = 0, psp_ret = SEV_RET_NO_FW_CALL;
> -       int (*init_function)(int *error);
> +       int rc = 0, psp_ret;
>
>         if (!psp || !psp->sev_data)
>                 return -ENODEV;
> @@ -456,15 +464,12 @@ static int __sev_platform_init_locked(int *error)
>                 return 0;
>
>         if (sev_init_ex_buffer) {
> -               init_function = __sev_init_ex_locked;
>                 rc = sev_read_init_ex_file();
>                 if (rc)
>                         return rc;
> -       } else {
> -               init_function = __sev_init_locked;
>         }
>
> -       rc = init_function(&psp_ret);
> +       rc = __sev_do_init_locked(&psp_ret);
>         if (rc && psp_ret == SEV_RET_SECURE_DATA_INVALID) {
>                 /*
>                  * Initialization command returned an integrity check failure
> @@ -473,9 +478,12 @@ static int __sev_platform_init_locked(int *error)
>                  * initialization function should succeed by replacing the state
>                  * with a reset state.
>                  */
> -               dev_err(sev->dev, "SEV: retrying INIT command because of SECURE_DATA_INVALID error. Retrying once to reset PSP SEV state.");
> -               rc = init_function(&psp_ret);
> +               dev_err(sev->dev,
> +"SEV: retrying INIT command because of SECURE_DATA_INVALID error. Retrying once to reset PSP SEV state.");
> +
> +               rc = __sev_do_init_locked(&psp_ret);
>         }
> +
>         if (error)
>                 *error = psp_ret;
>
> diff --git a/include/uapi/linux/psp-sev.h b/include/uapi/linux/psp-sev.h
> index 1ad7f0a7e328..a9ed9e846cd2 100644
> --- a/include/uapi/linux/psp-sev.h
> +++ b/include/uapi/linux/psp-sev.h
> @@ -42,7 +42,7 @@ typedef enum {
>          * This is (u32)-1 since the firmware error code is represented as a
>          * 32-bit integer.
>          */
> -       SEV_RET_NO_FW_CALL = 0xffffffff,
> +       SEV_RET_NO_FW_CALL = -1,
>         SEV_RET_SUCCESS = 0,
>         SEV_RET_INVALID_PLATFORM_STATE,
>         SEV_RET_INVALID_GUEST_STATE,
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette
  

On Sat, Dec 03, 2022 at 10:58:39AM -0800, Dionna Amalie Glaze wrote:
> It doesn't always overwrite psp_ret, such as the initial error checking.
> The value remains uninitialized for -ENODEV, -EBUSY, -EINVAL.
> Thus *error in __sev_platform_init_locked can be set to uninitialized
> memory if psp_ret is not first initialized.

Lemme see if I understand it correctly: you wanna signal that all early
return cases in __sev_do_cmd_locked() are such that no firmware was
called?

I.e., everything before the first iowrite into the command buffer?

But then the commit message says:

"The PSP can return a "firmware error" code of -1 in circumstances where
the PSP is not actually called."

which is confusing. How can the PSP return something if it wasn't called?

Or you mean those cases above where it would fail on some of the checks
before issuing a SEV command? I think you do...

So I see Tom has ACKed this but I have to ask: is the SEV spec not going
to use -1 ever?

Also, if this behavior is going to be user-visible, where are we
documenting it? Especially if nothing in the kernel is looking at
that value but only assigning it to a retval which gets looked at by
userspace. Especially then this should be documented.

Dunno, maybe somewhere in Documentation/x86/amd-memory-encryption.rst or
maybe Tom would have a better idea.

> That error points to the kernel copy of the user's argument struct,
> which the ioctl always copies back. In the case of those error codes
> then, without SEV_RET_NO_FW_CALL, user space will get uninitialized
> kernel memory.

Right, but having a return value which means "firmware wasn't called"
sounds weird. Why does userspace care?

I mean, you can just as well return any of the negative values -ENODEV,
-EBUSY, -EINVAL too, depending on where you exit. Having three different
retvals could tell you where exactly it failed, even.

But the question remains: why does userspace needs to know that the
failure happened and firmware wasn't called, as long as it is getting
something negative to signal an error?

Thx.
  

On Sat, Dec 3, 2022 at 11:37 AM Borislav Petkov <bp@alien8.de> wrote:
>
> On Sat, Dec 03, 2022 at 10:58:39AM -0800, Dionna Amalie Glaze wrote:
> > It doesn't always overwrite psp_ret, such as the initial error checking.
> > The value remains uninitialized for -ENODEV, -EBUSY, -EINVAL.
> > Thus *error in __sev_platform_init_locked can be set to uninitialized
> > memory if psp_ret is not first initialized.
>
> Lemme see if I understand it correctly: you wanna signal that all early
> return cases in __sev_do_cmd_locked() are such that no firmware was
> called?
>
> I.e., everything before the first iowrite into the command buffer?
>
> But then the commit message says:
>
> "The PSP can return a "firmware error" code of -1 in circumstances where
> the PSP is not actually called."
>
> which is confusing. How can the PSP return something if it wasn't called?
>
> Or you mean those cases above where it would fail on some of the checks
> before issuing a SEV command? I think you do...
>
> So I see Tom has ACKed this but I have to ask: is the SEV spec not going
> to use -1 ever?
>

I'll confirm with Tom, since he's changing the GHCB spec for the
throttling value.

> Also, if this behavior is going to be user-visible, where are we
> documenting it? Especially if nothing in the kernel is looking at
> that value but only assigning it to a retval which gets looked at by
> userspace. Especially then this should be documented.
>
> Dunno, maybe somewhere in Documentation/x86/amd-memory-encryption.rst or
> maybe Tom would have a better idea.
>

Agreed it should be in both the Linux documentation and the GHCB spec.

> > That error points to the kernel copy of the user's argument struct,
> > which the ioctl always copies back. In the case of those error codes
> > then, without SEV_RET_NO_FW_CALL, user space will get uninitialized
> > kernel memory.
>
> Right, but having a return value which means "firmware wasn't called"
> sounds weird. Why does userspace care?
>

Arguably it shouldn't ever get this value. We're just not very
selective when we copy back the kernel copy of the ioctl argument.
In all cases user space should treat the value as undefined, but still
we don't want to leak uninitialized kernel stack values.

Host driver: only on platform init, should just see the negative error
value and not try to interpret the fw_err in the argument.
Still the data is copied back and therefore should not be
uninitialized kernel memory.
Possible name: SEV_RET_UNDEFINED, or a return value -1 anyway with a
comment that the argument is undefined.

Guest driver: The host is issuing a guest request on behalf of the
guest using patch 4/4 of this series.
The guest is responsible for keeping the sequence number in sync with
the PSP, so we want to track if the ghcb_hv_call completed
successfully to know we should continue with the incremented IV.
Otherwise we run the risk of the sequence numbers getting out of sync
and we lock down the VMPCK.

The guest driver actually sets exitinfo2 to  an undocumented 0xff
initial value just in case.
=If the host doesn't write back a documented EXIT_INFO_2 value like
invalid_len or throttled, then the kernel will emit a log with the
initial value 0xff (or -1 after this patch).

I've changed it to -1 to name the same kind of error across host and
guest: the communication with the PSP didn't complete successfully, so
the "error" value is not from the PSP.
This value can also get returned to user space during a -ENOTTY result.
We can call this NO_FW_CALL or UNDEFINED. I have no real preference.

Whatever value we set initially, the VMM can overwrite exitinfo2
during the ghcb_hv_call.
I'd rather that the "undefined" values were the same across both,
because the guest is merely receiving a value from the host's PSP
driver (or should be).
It keeps the enum for return values a bit tidier and not concerned
with whether the value is viewed from the host or guest.

I can see an argument for not using the PSP header for its enum type
and instead defining and documenting and using the separate the 0xff
value elsewhere, but this seemed as good a place as any.


> I mean, you can just as well return any of the negative values -ENODEV,
> -EBUSY, -EINVAL too, depending on where you exit. Having three different
> retvals could tell you where exactly it failed, even.
>

That's true, those values are already being returned to user space as
the result of the ioctl.

> But the question remains: why does userspace needs to know that the
> failure happened and firmware wasn't called, as long as it is getting
> something negative to signal an error?
>

I hope the above discussion is clear that it's purely a defined
"undefined" because being pickier about what to copy_to_user during
exceptional circumstances in order to not overwrite the user's fw_err
value seems an unnecessary amount of code.

> Thx.
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette
  

On 12/5/22 11:05, Dionna Amalie Glaze wrote:
> On Sat, Dec 3, 2022 at 11:37 AM Borislav Petkov <bp@alien8.de> wrote:
>>
>> On Sat, Dec 03, 2022 at 10:58:39AM -0800, Dionna Amalie Glaze wrote:
>>> It doesn't always overwrite psp_ret, such as the initial error checking.
>>> The value remains uninitialized for -ENODEV, -EBUSY, -EINVAL.
>>> Thus *error in __sev_platform_init_locked can be set to uninitialized
>>> memory if psp_ret is not first initialized.
>>
>> Lemme see if I understand it correctly: you wanna signal that all early
>> return cases in __sev_do_cmd_locked() are such that no firmware was
>> called?
>>
>> I.e., everything before the first iowrite into the command buffer?
>>
>> But then the commit message says:
>>
>> "The PSP can return a "firmware error" code of -1 in circumstances where
>> the PSP is not actually called."
>>
>> which is confusing. How can the PSP return something if it wasn't called?
>>
>> Or you mean those cases above where it would fail on some of the checks
>> before issuing a SEV command? I think you do...
>>
>> So I see Tom has ACKed this but I have to ask: is the SEV spec not going
>> to use -1 ever?
>>
> 
> I'll confirm with Tom, since he's changing the GHCB spec for the
> throttling value.

The SEV API error codes are 16-bits in size, so you'll never see a -1.

> 
>> Also, if this behavior is going to be user-visible, where are we
>> documenting it? Especially if nothing in the kernel is looking at
>> that value but only assigning it to a retval which gets looked at by
>> userspace. Especially then this should be documented.
>>
>> Dunno, maybe somewhere in Documentation/x86/amd-memory-encryption.rst or
>> maybe Tom would have a better idea.
>>
> 
> Agreed it should be in both the Linux documentation and the GHCB spec.

Linux documentation, yes, GHCB spec, no.

Thanks,
Tom

> 
>>> That error points to the kernel copy of the user's argument struct,
>>> which the ioctl always copies back. In the case of those error codes
>>> then, without SEV_RET_NO_FW_CALL, user space will get uninitialized
>>> kernel memory.
>>
>> Right, but having a return value which means "firmware wasn't called"
>> sounds weird. Why does userspace care?
>>
> 
> Arguably it shouldn't ever get this value. We're just not very
> selective when we copy back the kernel copy of the ioctl argument.
> In all cases user space should treat the value as undefined, but still
> we don't want to leak uninitialized kernel stack values.
> 
> Host driver: only on platform init, should just see the negative error
> value and not try to interpret the fw_err in the argument.
> Still the data is copied back and therefore should not be
> uninitialized kernel memory.
> Possible name: SEV_RET_UNDEFINED, or a return value -1 anyway with a
> comment that the argument is undefined.
> 
> Guest driver: The host is issuing a guest request on behalf of the
> guest using patch 4/4 of this series.
> The guest is responsible for keeping the sequence number in sync with
> the PSP, so we want to track if the ghcb_hv_call completed
> successfully to know we should continue with the incremented IV.
> Otherwise we run the risk of the sequence numbers getting out of sync
> and we lock down the VMPCK.
> 
> The guest driver actually sets exitinfo2 to  an undocumented 0xff
> initial value just in case.
> =If the host doesn't write back a documented EXIT_INFO_2 value like
> invalid_len or throttled, then the kernel will emit a log with the
> initial value 0xff (or -1 after this patch).
> 
> I've changed it to -1 to name the same kind of error across host and
> guest: the communication with the PSP didn't complete successfully, so
> the "error" value is not from the PSP.
> This value can also get returned to user space during a -ENOTTY result.
> We can call this NO_FW_CALL or UNDEFINED. I have no real preference.
> 
> Whatever value we set initially, the VMM can overwrite exitinfo2
> during the ghcb_hv_call.
> I'd rather that the "undefined" values were the same across both,
> because the guest is merely receiving a value from the host's PSP
> driver (or should be).
> It keeps the enum for return values a bit tidier and not concerned
> with whether the value is viewed from the host or guest.
> 
> I can see an argument for not using the PSP header for its enum type
> and instead defining and documenting and using the separate the 0xff
> value elsewhere, but this seemed as good a place as any.
> 
> 
>> I mean, you can just as well return any of the negative values -ENODEV,
>> -EBUSY, -EINVAL too, depending on where you exit. Having three different
>> retvals could tell you where exactly it failed, even.
>>
> 
> That's true, those values are already being returned to user space as
> the result of the ioctl.
> 
>> But the question remains: why does userspace needs to know that the
>> failure happened and firmware wasn't called, as long as it is getting
>> something negative to signal an error?
>>
> 
> I hope the above discussion is clear that it's purely a defined
> "undefined" because being pickier about what to copy_to_user during
> exceptional circumstances in order to not overwrite the user's fw_err
> value seems an unnecessary amount of code.
> 
>> Thx.
>>
>> --
>> Regards/Gruss,
>>      Boris.
>>
>> https://people.kernel.org/tglx/notes-about-netiquette
> 
> 
>
  

On Mon, Dec 05, 2022 at 09:05:19AM -0800, Dionna Amalie Glaze wrote:
> Arguably it shouldn't ever get this value. We're just not very
> selective when we copy back the kernel copy of the ioctl argument.
> In all cases user space should treat the value as undefined, but still
> we don't want to leak uninitialized kernel stack values.

Absolutely.

> I've changed it to -1 to name the same kind of error across host and
> guest: the communication with the PSP didn't complete successfully, so
> the "error" value is not from the PSP.
> This value can also get returned to user space during a -ENOTTY result.
> We can call this NO_FW_CALL or UNDEFINED. I have no real preference.

Me neither as long as this is written down and agreed upon as a possible
value and not leaking kernel stack.

> Whatever value we set initially, the VMM can overwrite exitinfo2
> during the ghcb_hv_call.
> I'd rather that the "undefined" values were the same across both,
> because the guest is merely receiving a value from the host's PSP
> driver (or should be).
> It keeps the enum for return values a bit tidier and not concerned
> with whether the value is viewed from the host or guest.

Ack.

...

> I hope the above discussion is clear that it's purely a defined
> "undefined" because being pickier about what to copy_to_user during
> exceptional circumstances in order to not overwrite the user's fw_err
> value seems an unnecessary amount of code.

Ok, I think we're on the same page. So pls document that NO_FW_CALL or
so value and what it means and that thing should be taken care of.

Thx.