linux-kernel - Re: [PATCH v2 11/14] x86/sev: Extend the config-fs attestation support for an SVSM

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Date: Tue, 12 Mar 2024 08:29:34 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>,
 linux-kernel@...r.kernel.org, x86@...nel.org
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>,
 Dan Williams <dan.j.williams@...el.com>, Michael Roth
 <michael.roth@....com>, Ashish Kalra <ashish.kalra@....com>
Subject: Re: [PATCH v2 11/14] x86/sev: Extend the config-fs attestation
 support for an SVSM

On 3/12/24 00:57, Kuppuswamy Sathyanarayanan wrote:
> 
> On 3/11/24 9:16 AM, Tom Lendacky wrote:
>> On 3/10/24 00:06, Kuppuswamy, Sathyanarayanan wrote:
>>>
>>> On 3/8/24 10:35 AM, Tom Lendacky 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.
>>>>
>>>>     - '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_manifest_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.
>>>>
>>>>     - 'manifestblob' (output)
>>>>       Used to return the service manifest associated with the attestation
>>>>       report.
>>>>
>>>> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
>>>> ---
>>>>    Documentation/ABI/testing/configfs-tsm  |  59 ++++++++++
>>>>    arch/x86/include/asm/sev.h              |  31 ++++-
>>>>    arch/x86/kernel/sev.c                   |  50 ++++++++
>>>>    drivers/virt/coco/sev-guest/sev-guest.c | 147 ++++++++++++++++++++++++
>>>>    drivers/virt/coco/tsm.c                 |  95 ++++++++++++++-
>>>>    include/linux/tsm.h                     |  11 ++
>>>>    6 files changed, 390 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/Documentation/ABI/testing/configfs-tsm b/Documentation/ABI/testing/configfs-tsm
>>>> index dd24202b5ba5..a4663610bf7c 100644
>>>> --- a/Documentation/ABI/testing/configfs-tsm
>>>> +++ b/Documentation/ABI/testing/configfs-tsm
>>

>>>> +
>>>> +What:        /sys/kernel/config/tsm/report/$name/service_guid
>>>> +Date:        January, 2024
>>>> +KernelVersion:    v6.9
>>>> +Contact:    linux-coco@...ts.linux.dev
>>>> +Description:
>>>> +        (WO) Attribute is visible if a TSM implementation provider
>>>> +        supports the concept of attestation reports for TVMs running
>>>> +        under an SVSM, like SEV-SNP. Specifying a empty or null GUID
>>>> +        (00000000-0000-0000-0000-000000) requests all active services
>>>> +        within the SVSM be part of the attestation report. Specifying
>>>> +        a non-null GUID requests an attestation report of just the
>>>> +        specified service using the manifest form specified by the
>>>> +        service_manifest_version attribute.
>>>> +        Secure VM Service Module for SEV-SNP Guests v1.00 Section 7.
>>>> +        https://www.amd.com/content/dam/amd/en/documents/epyc-technical-docs/specifications/58019.pdf
>>>> +
>>>
>>> I think it will be useful to the user if there is a attribute to list the service GUIDs
>>> supported. It can help prevent user using incorrect or unsupported GUIDs.
>>
>> A list of supported GUIDs can be obtained from the manifest of a all-services attestation request.
> 
> So they have to make a request twice? Once with a NULL GUID to get the
> manifest with all service list, and another to make service-specific request?
> There should be a fixed list of service GUIDs, right? Why not list them by
> default?

It's not a fixed list. It may appear that way today, but as other services 
are added, then it is impossible for a kernel to know what services are 
present in the SVSM without querying it.

> 
>>

>>
>>>
>>>> +    if (guid_len == UUID_STRING_LEN) {
>>>> +        rc = guid_parse(buf, &report->desc.service_guid);
>>>> +        if (rc)
>>>> +            return rc;
>>>> +    } else {
>>>> +        report->desc.service_guid = guid_null;
>>>
>>> I think the default value will be guid_null right, why reset it to NULL for every failed attempt?
>>
>> Default, yes. But what if it is written once, then a second time with an invalid GUID. Should the previously written GUID still be used?
>>
> 
> If the user write fails, why update the state? IMO, we can leave it at the old value. But, lets see what others think.

Sounds good.

Thanks,
Tom

> 
>> Thanks,
>> Tom
>>
>>>