Re: [edk2-devel] [RFC PATCH] OvmfPkg/SecurityPkg: Add build option for coexistance of vTPM and RTMR.

["Lendacky, Thomas via groups.io" <thomas.lendacky=amd.com@groups.io>]

On 4/11/24 05:33, Ard Biesheuvel wrote:
> On Thu, 11 Apr 2024 at 12:29, Gerd Hoffmann <kraxel@redhat.com> wrote:
>>
>> On Thu, Apr 11, 2024 at 09:56:48AM +0000, Yao, Jiewen wrote:
>>> Please allow me to clarify what you are proposing:
>>> Do you mean in vTPM case, we extend both, but we only need TCG event log, NOT CC event log?
>>
>> Elsewhere in this thread it was mentioned that writing both vTPM and
>> RTMR events to the event log is problematic because the event log format
>> has no field to specify whenever a given event was measured to vTPM or
>> RTMR.
>>
>> If the firmware can make sure all events are measured to both vTPM and
>> RTMR the need to trace them separately goes away.
>>
>> So, yes, in case a vTPM is present the firmware would:
>>    (a) expose EFI_TCG2_PROTOCOL, measure to both vTPM + RTMR
>>    (b) not expose EFI_CC_MEASUREMENT_PROTOCOL
>>    (c) log measurements to TCG event log
>>
> 
> A TDX attestation would require the PCR to RTMR mapping used by the
> firmware in order to reconstruct the RTMR values from the TCG event
> log, but that seems feasible to me.
> 
> In any case, I think it should be the guest firmware's job to abstract
> away the difference.

Agreed, this approach seems to be the best way forward.

Thanks,
Tom


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