Re: [PATCH 10/11] x86/sev: Extend the config-fs attestation support for an SVSM

[Tom Lendacky <thomas.lendacky@amd.com>]

On 1/29/24 14:04, Dionna Amalie Glaze wrote:
> On Mon, Jan 29, 2024 at 7:02 AM Tom Lendacky <thomas.lendacky@amd.com> wrote:
>>
>> On 1/26/24 19:27, Dionna Amalie Glaze wrote:
>>> On Fri, Jan 26, 2024 at 2:19 PM Tom Lendacky <thomas.lendacky@amd.com> wrote:
>>>>
>>>> When an SVSM is present, the guest can also request attestation reports
>>>> from the SVSM. These SVSM attestation reports can be used to attest the
>>>> SVSM and any services running within the SVSM.
>>>>
>>>> Extend the config-fs attestation support to allow for an SVSM attestation
>>>> report. This involves creating four (4) new config-fs attributes:
>>>>
>>>>     - 'svsm' (input)
>>>>       This attribute is used to determine whether the attestation request
>>>>       should be sent to the SVSM or to the SEV firmware.
>>>
>>> This is where I'm torn. If there's an SVSM, it's there to provide
>>> paravirtualization for unenlightened guests /or/ it's there to protect
>>
>> An SVSM is for enlightened guests. A para-visor would be for unenlightened
>> guests.
>>
>>> runtime measurement registers. I don't see there being any particular
>>> value in bifurcating the attestation report space by adding this
>>> option. If there's an SVSM present, the configfs-tsm report should
>>> return the full SVSM attestation only.
>>
>> I don't necessarily agree with that. The guest should still be able to
>> request a traditional attestation report.
>>
>> Maybe I can remove the SVSM attribute and direct the call based on
>> requested VMPL level. If VMPL0 is requested, it goes through the SVSM.
>> If VMPL1+ is requested, it goes to the ASP.
>>
>> That would mean that the privlevel_floor would need to stay at zero.
>>
>>>
>>>>
>>>>     - 'service_guid' (input)
>>>>       Used for requesting the attestation of a single service within the
>>>>       SVSM. A null GUID implies that the SVSM_ATTEST_SERVICES call should
>>>>       be used to request the attestation report. A non-null GUID implies
>>>>       that the SVSM_ATTEST_SINGLE_SERVICE call should be used.
>>>>
>>>>     - 'service_version' (input)
>>>>       Used with the SVSM_ATTEST_SINGLE_SERVICE call, the service version
>>>>       represents a specific service manifest version be used for the
>>>>       attestation report.
>>>
>>> I know that this is specified for the SVSM, but I still don't know
>>> what the intended use case is such that we wouldn't simply always
>>> return the full service manifest.
>>> I can see an argument for an evidence requester not being ready for a
>>> new manifest version and wanting the older version until they can
>>> bridge the gap. I don't see that as needing configuration from the
>>> user space. We can either require new service GUIDs for new versions,
>>> require manifest ABIs to be internally versioned to be additive-only
>>> to not break verifiers that understand up to manifest byte X, or we
>>> allow breaking version changes through control plane configuration
>>> that's passed directly to the SVSM.
>>>
>>> New versions get new GUIDs allows for gradual deprecation at the
>>> expense of size. I think that is a reasonable trade-off to keep from
>>> making tsm/report vendor-specific.
>>
>> This was requested and discussed during the SVSM spec review and there
>> were no objections raised. See the this thread where this was discussed:
>>
>> https://lore.kernel.org/linux-coco/09819cb3-1938-fe86-b948-28aaffbe584e@amd.com/
>>
> 
> We also hadn't had the configfs-tsm unification point, so I think it's
> worth folding in that discussion.
> In terms of querying specific services, would you help me with a
> concrete example of where the evidence collector ought to query a
> specific version instead of the service enumeration?

Here is where the request was initially brought up:

https://lore.kernel.org/linux-coco/fbc84da05c5343c5228c5adb697d4b66f1ea6308.camel@HansenPartnership.com/

> 
>> The changes you're requesting would require a new version of the spec
>> and updates to the protocol.
>>
> 
> I think the changes I'm requesting are to just limit the exposure of
> the protocol to linux. What specifically about what I wrote requires a
> change to the spec? Is it the lack of plural handling of 'its GUID
> value' in "Each service will document its GUID value and the format of
> its manifest content."?

The spec is currently written so that a service has a single GUID. If I
understand correctly, you are asking that each version of the manifest
for a service gets a unique GUID. That would require a change to the
specification to document such a behavior and possibly a protocol
modification to somehow indicate to ignore the version field when
requesting a single service attestation or a new protocol that does not
take/use a version.

Thanks,
Tom

> 
>>>
>>>>
>>>>     - 'manifestblob' (output)
>>>>       Used to return the service manifest associated with the attestation
>>>>       report.
>>>
>>> Given the above, I think we can just append the manifest to the report
>>> since the report size is known a priori.
>>
>> We could have theoretically done the same thing with the auxblob (certs
>> data), but that is separate. I prefer the idea of having an individual
>> entry per piece of data being returned.
> 
> Fair enough, another RO blob seems okay enough.
> 
>>
>> Thanks,
>> Tom
>>
>>>