Re: [edk2-devel] [PATCH 3/3] OvmfPkg/PlatformPei: Mark TPM MMIO range as unencrypted for SEV

["Laszlo Ersek" <lersek@redhat.com>]

On 04/23/21 19:41, Tom Lendacky wrote:
> On 4/23/21 8:04 AM, Laszlo Ersek wrote:

>> I've had a further idea on this.
>>
>> You could add an entirely new PEIM just for this. The entry point
>> function of the PEIM would check for SEV, decrypt the TPM range if SEV
>> were active, and then install gOvmfTpmMmioAccessiblePpiGuid
>> (unconditionally). The exit status of the PEIM would always be
>> EFI_ABORTED, because there would be no need to keep the PEIM resident.
>>
>> The new PEIM would have a DEPEX on gEfiPeiMemoryDiscoveredPpiGuid, to
>> make sure that potential page table splitting for the potential MMIO
>> range decryption could be satisfied from permanent PEI RAM.
>>
>> The new PEIM would be included in the DSC and FDF files of the usual
>> three OVMF platforms, and in the Bhyve platform -- dependent on the
>> TPM_ENABLE build flag.
>>
>> There are several advantages to such a separate PEIM:
>>
>> - For Bhyve, the update is minimal. Just include one line in each of the
>> FDF and the DSC files. No need to customize an existent
>> platform-specific PEIM, no code duplication between two PlatformPei modules.
>>
>> - The new PEIM would depend on the TPM_ENABLE build flag, so it would
>> only be included in the firmware binaries if and only if Tcg2ConfigPei
>> were. No useless PPI installation would occur in the absence of TPM_ENABLE.
>>
>> - No need to check PcdTpmBaseAddress for nullity in the new PEIM, before
>> the decryption, as TPM_ENABLE guarantees (on IA32/X64) that the PCD
>> already has the right value.
>>
>> - The new logic would be properly ordered between PlatformPei and
>> Tcg2ConfigPei, namely due to the use of two such PPI GUIDs in DEPEXes
>> that actually make sense. PlatformPei -> TPM MMIO decryptor PEIM ordered
>> via "memory discovered" (needed for potential page table splitting), TPM
>> MMIO decryptor PEIM -> Tcg2ConfigPei ordered via "TPM MMIO decrypted".
>>
>> You could place the new PEIM at:
>>
>>   OvmfPkg/Tcg/TpmMmioSevDecryptPei
>>
>> If you haven't lost your patience with me yet, I'd really appreciate if
>> you could investigate this!
>>
> 
> So far, this appears to be working nicely. I'm new at the whole PEIM
> thing, so hopefully I haven't missed anything. I should be submitting the
> patches soon for review.

Thanks!

> 
> One thing I found is that the Bhyve package makes reference to the
> OvmfPkg/Bhyve/Tcg directory, but that directory does not exist. So I don't
> think that TPM enablement has been tested. I didn't update the Bhyve
> support for that reason.

That's good to know; thanks for reporting this. I've turned it now into
a BZ ticket (assigned to Rebecca):

https://bugzilla.tianocore.org/show_bug.cgi?id=3354

Thanks!
Laszlo