[PATCH] KVM: SEV: allow KVM_SEV_GET_ATTESTATION_REPORT for SNP guests

[PATCH] KVM: SEV: allow KVM_SEV_GET_ATTESTATION_REPORT for SNP guests

[Paolo Bonzini]

Even though KVM_SEV_GET_ATTESTATION_REPORT is not one of the commands
that were added for SEV-SNP guests, it can be applied to them.  Filtering
it out, for example, makes the QEMU command query-sev-attestation-report
fail.

Cc: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/svm/sev.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 5c125e4c1096..17307257d632 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -2587,7 +2587,9 @@ int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp)
 	 * Once KVM_SEV_INIT2 initializes a KVM instance as an SNP guest, only
 	 * allow the use of SNP-specific commands.
 	 */
-	if (sev_snp_guest(kvm) && sev_cmd.id < KVM_SEV_SNP_LAUNCH_START) {
+	if (sev_snp_guest(kvm) &&
+	    sev_cmd.id < KVM_SEV_SNP_LAUNCH_START &&
+	    sev_cmd.id != KVM_SEV_GET_ATTESTATION_REPORT) {
 		r = -EPERM;
 		goto out;
 	}
-- 
2.45.2

Re: [PATCH] KVM: SEV: allow KVM_SEV_GET_ATTESTATION_REPORT for SNP guests

[Michael Roth]

On Fri, Aug 02, 2024 at 01:53:33AM +0200, Paolo Bonzini wrote:
> Even though KVM_SEV_GET_ATTESTATION_REPORT is not one of the commands
> that were added for SEV-SNP guests, it can be applied to them.  Filtering

Is the command actually succeeding for an SNP-enabled guest? When I
test this, I get a fw_err code of 1 (INVALID_PLATFORM_STATE), and
after speaking with some firmware folks that seems to be the expected
behavior.

There's also some other things that aren't going to work as expected,
e.g. KVM uses sev->handle as the handle for the guest it wants to fetch
the attestation report for, but in the case of SNP, sev->handle will be
uninitialized since that only happens via KVM_SEV_LAUNCH_UPDATE_DATA,
which isn't usable for SNP guests.

As I understand it, the only firmware commands allowed for SNP guests are
those listed in the SNP firmware ABI, section "Command Reference", and
in any instance where a legacy command from the legacy SEV/SEV-ES firmware
ABI is also applicable for SNP, the legacy command will be defined again
in the "Command Reference" section of the SNP spec.  E.g., GET_ID is
specifically documented in both the SEV/SEV-ES firmware ABI, as well as
the SNP firmware ABI spec. But ATTESTATION (and the similar LAUNCH_MEASURE)
are only mentioned in the SEV/SEV-ES Firmware ABI, so I think it makes
sense that KVM also only allows them for SEV/SEV-ES.

-Mike

> it out, for example, makes the QEMU command query-sev-attestation-report
> fail.
> 
> Cc: Michael Roth <michael.roth@amd.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/svm/sev.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 5c125e4c1096..17307257d632 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2587,7 +2587,9 @@ int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp)
>  	 * Once KVM_SEV_INIT2 initializes a KVM instance as an SNP guest, only
>  	 * allow the use of SNP-specific commands.
>  	 */
> -	if (sev_snp_guest(kvm) && sev_cmd.id < KVM_SEV_SNP_LAUNCH_START) {
> +	if (sev_snp_guest(kvm) &&
> +	    sev_cmd.id < KVM_SEV_SNP_LAUNCH_START &&
> +	    sev_cmd.id != KVM_SEV_GET_ATTESTATION_REPORT) {
>  		r = -EPERM;
>  		goto out;
>  	}
> -- 
> 2.45.2
> 

Re: [PATCH] KVM: SEV: allow KVM_SEV_GET_ATTESTATION_REPORT for SNP guests

[Paolo Bonzini]

On Fri, Aug 2, 2024 at 10:41 PM Michael Roth <michael.roth@amd.com> wrote:
> On Fri, Aug 02, 2024 at 01:53:33AM +0200, Paolo Bonzini wrote:
> > Even though KVM_SEV_GET_ATTESTATION_REPORT is not one of the commands
> > that were added for SEV-SNP guests, it can be applied to them.  Filtering
>
> Is the command actually succeeding for an SNP-enabled guest? When I
> test this, I get a fw_err code of 1 (INVALID_PLATFORM_STATE), and
> after speaking with some firmware folks that seems to be the expected
> behavior.

So is there no equivalent of QEMU's query-sev-attestation-report for
SEV-SNP? (And is there any user of query-sev-attestation-report for
non-SNP?)

Paolo

> There's also some other things that aren't going to work as expected,
> e.g. KVM uses sev->handle as the handle for the guest it wants to fetch
> the attestation report for, but in the case of SNP, sev->handle will be
> uninitialized since that only happens via KVM_SEV_LAUNCH_UPDATE_DATA,
> which isn't usable for SNP guests.
>
> As I understand it, the only firmware commands allowed for SNP guests are
> those listed in the SNP firmware ABI, section "Command Reference", and
> in any instance where a legacy command from the legacy SEV/SEV-ES firmware
> ABI is also applicable for SNP, the legacy command will be defined again
> in the "Command Reference" section of the SNP spec.  E.g., GET_ID is
> specifically documented in both the SEV/SEV-ES firmware ABI, as well as
> the SNP firmware ABI spec. But ATTESTATION (and the similar LAUNCH_MEASURE)
> are only mentioned in the SEV/SEV-ES Firmware ABI, so I think it makes
> sense that KVM also only allows them for SEV/SEV-ES.

Re: [PATCH] KVM: SEV: allow KVM_SEV_GET_ATTESTATION_REPORT for SNP guests

[Michael Roth]

On Mon, Aug 05, 2024 at 04:32:16PM +0200, Paolo Bonzini wrote:
> On Fri, Aug 2, 2024 at 10:41 PM Michael Roth <michael.roth@amd.com> wrote:
> > On Fri, Aug 02, 2024 at 01:53:33AM +0200, Paolo Bonzini wrote:
> > > Even though KVM_SEV_GET_ATTESTATION_REPORT is not one of the commands
> > > that were added for SEV-SNP guests, it can be applied to them.  Filtering
> >
> > Is the command actually succeeding for an SNP-enabled guest? When I
> > test this, I get a fw_err code of 1 (INVALID_PLATFORM_STATE), and
> > after speaking with some firmware folks that seems to be the expected
> > behavior.
> 
> So is there no equivalent of QEMU's query-sev-attestation-report for
> SEV-SNP?

No, but all the attestation support is via the guest request interface.

It would be possible for KVM to provide the measurement by logging the
digest values

> (And is there any user of query-sev-attestation-report for
> non-SNP?)

No, this would have always returned error, either via KVM, or via
firmware failure.

But maybe QEMU should do the error handling a bit more directly in this
case. I can send a patch for QEMU 9.1 that results in an error when
issued for an SNP guest.

-Mike

> 
> Paolo
> 
> > There's also some other things that aren't going to work as expected,
> > e.g. KVM uses sev->handle as the handle for the guest it wants to fetch
> > the attestation report for, but in the case of SNP, sev->handle will be
> > uninitialized since that only happens via KVM_SEV_LAUNCH_UPDATE_DATA,
> > which isn't usable for SNP guests.
> >
> > As I understand it, the only firmware commands allowed for SNP guests are
> > those listed in the SNP firmware ABI, section "Command Reference", and
> > in any instance where a legacy command from the legacy SEV/SEV-ES firmware
> > ABI is also applicable for SNP, the legacy command will be defined again
> > in the "Command Reference" section of the SNP spec.  E.g., GET_ID is
> > specifically documented in both the SEV/SEV-ES firmware ABI, as well as
> > the SNP firmware ABI spec. But ATTESTATION (and the similar LAUNCH_MEASURE)
> > are only mentioned in the SEV/SEV-ES Firmware ABI, so I think it makes
> > sense that KVM also only allows them for SEV/SEV-ES.
> 

Re: [PATCH] KVM: SEV: allow KVM_SEV_GET_ATTESTATION_REPORT for SNP guests

[Paolo Bonzini]

On Mon, Aug 5, 2024 at 5:39 PM Michael Roth <michael.roth@amd.com> wrote:
> > (And is there any user of query-sev-attestation-report for
> > non-SNP?)
>
> No, this would have always returned error, either via KVM, or via
> firmware failure.

I mean for *non-SNP*. If no one ever used it, we can deprecate the command.

Paolo

Re: [PATCH] KVM: SEV: allow KVM_SEV_GET_ATTESTATION_REPORT for SNP guests

[Michael Roth]

On Mon, Aug 05, 2024 at 06:15:51PM +0200, Paolo Bonzini wrote:
> On Mon, Aug 5, 2024 at 5:39 PM Michael Roth <michael.roth@amd.com> wrote:
> > > (And is there any user of query-sev-attestation-report for
> > > non-SNP?)
> >
> > No, this would have always returned error, either via KVM, or via
> > firmware failure.
> 
> I mean for *non-SNP*. If no one ever used it, we can deprecate the command.

This would have been the main architectured way to fetch an attestation
report from firmware for non-SNP cases, so I think it's likely being used
to some degree. Libvirt itself does not seem to use it directly, however
(opting for the less verbose query-sev-launch-measure), but it seems likely
there would be SEV deployments that do. libvirt might find other bits like
policy/major/minor/etc. to be redundant since it also controls the guest
config, but maybe 3rd party tools prefer to known those values as provided
by firmware so they can ensure they match expected values.

-Mike

> 
> Paolo
>