Re: [edk2-devel] separate OVMF binary for TDX? [was: OvmfPkg: Reserve the Secrets and Cpuid page for the SEV-SNP guest]

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

On 04/21/21 02:38, Yao, Jiewen wrote:
> Hello 
> Do we have some conclusion on this topic?
> 
> Do we agree the one-binary solution in OVMF or we need more discussion?

Well it's not technically impossible to do, just very ugly and brittle.
And I'm doubtful that this is a unique problem ("just fix the reset
vector") the likes of which will supposedly never return during the
integration of SEV and TDX. Once we make this promise ("one firmware
binary at all costs"), the hacks we accept for its sake will only
accumulate over time, and we'll have more and more precedent to justify
the next hack. Technical debt is not exactly what we don't have enough
of, in edk2.

I won't make a secret out of the fact that I'm slightly annoyed that
this approach is being dictated by Google (as far as I understand, at
this point, anyway). I don't see or recall a lot of Google contributions
in the edk2 history or the bug tracker. I'm not enthusiastic about
complexity without explicit commitment / investment on the beneficiary's
side.

I won't nack the approach personally, but I'm quite unhappy about it.
Can Google at least propose a designated reviewer ("R") for the
"OvmfPkg: Confidential Computing" section of "Maintainers.txt", in a patch?

Thanks
Laszlo

> 
> 
> Thank you
> Yao Jiewen
> 
>> -----Original Message-----
>> From: Erdem Aktas <erdemaktas@google.com>
>> Sent: Friday, April 16, 2021 3:43 AM
>> To: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: devel@edk2.groups.io; jejb@linux.ibm.com; Yao, Jiewen
>> <jiewen.yao@intel.com>; dgilbert@redhat.com; Laszlo Ersek
>> <lersek@redhat.com>; Xu, Min M <min.m.xu@intel.com>;
>> thomas.lendacky@amd.com; Brijesh Singh <brijesh.singh@amd.com>; Justen,
>> Jordan L <jordan.l.justen@intel.com>; Ard Biesheuvel
>> <ardb+tianocore@kernel.org>; Nathaniel McCallum
>> <npmccallum@redhat.com>; Ning Yang <ningyang@google.com>
>> Subject: Re: [edk2-devel] separate OVMF binary for TDX? [was: OvmfPkg:
>> Reserve the Secrets and Cpuid page for the SEV-SNP guest]
>>
>> Thanks Paolo.
>>
>> On Thu, Apr 15, 2021 at 12:59 AM Paolo Bonzini <pbonzini@redhat.com>
>> wrote:
>>>
>>> On 15/04/21 01:34, Erdem Aktas wrote:
>>>> We do not want to generate different binaries for AMD, Intel, Intel
>>>> with TDX, AMD with SEV/SNP etc
>>>
>>> My question is why the user would want a single binary for VMs with and
>>> without TDX/SNP.  I know there is attestation, but why would you even
>>> want the _possibility_ that your guest starts running without TDX or SNP
>>> protection, and only find out later via attestation?
>>
>> There might be multiple reasons why customers want it but we need this
>> requirement for a couple of other reasons too.
>>
>> We do not only have hardware based confidential VMs. We might have
>> some other solutions which measure the initial image before boot.
>> Ultimately we might want to use a common attestation interface where
>> customers might be running different kinds of VMs. Using a single
>> binary will make it easier to manage/verify measurements for both of
>> us and the customers. I am not a PM so I cannot give more context on
>> customer use cases.
>>
>> Another reason is how we deploy and manage guest firmware. We have a
>> lot of optimization and customization to speed up firmware loading
>> time and also reduce the time to deploy new builds on the whole fleet
>> uniformly.  Adding a new firmware binary is a big challenge for us to
>> enable these features. On the top of integration challenges, it will
>> create maintainability issues in the long run for us when we provide
>> tools to verify/reproduce the hashes in the attestation report.
>>
>>> want the _possibility_ that your guest starts running without TDX or SNP
>>> protection, and only find out later via attestation?
>>
>> I am missing the point here. Customers should rely on only the
>> attestation report to establish the trust.
>> -If firmware does not support TDX and TDX is enabled, that firmware
>> will crash at some point.
>> -If firmware is generic firmware that supports TDX and SNP and others,
>> and TDX is enabled or not,  still the customer needs to verify the TDX
>> enablement through attestation.
>> -If firmware is a customized binary compiled to support TDX,
>> irrelevant of TDX being enabled or not,  still the customer needs to
>> verify the TDX enablement through attestation.
>>
>>
>>> For a similar reason, OVMF already supports shipping a binary that fails
>>> to boot if SMM is not available to the firmware, because then secure
>>> boot would be trivially circumvented.
>>>
>>> I can understand having a single binary for both TDX or SNP.  That's not
>>> a problem since you can set up the SEV startup VMSA to 32-bit protected
>>> mode just like TDX wants.
>>
>> I agree that this is doable but I am not sure if we need to also
>> modify the reset vector for AMD SNP in that case. Also it will not
>> solve our problem. If we start to generate a new firmware for every
>> feature , it will not end well for us, I think. Both TDX and SNP are
>> still new features in the same architecture, and it seems to me that
>> they are sharing a lot of common/similar code. AMD has already made
>> some of their patches in (SEV and SEV-ES) which works very nicely for
>> our use case and integration. Looks like Intel just has an issue on
>> how to fix their reset vector problem. Once they solve it and upstream
>> accepts the changes, I do not see any other big blocker. OVMF was
>> doing a great job on abstracting differences and providing a common
>> interface without creating multiple binaries. I do not see why it
>> should not do the same thing here.
>>
>>>> therefore we were expecting the TDX
>>>> changes to be part of the upstream code.
>>>
>>> Having 1 or more binaries should be unrelated to the changes being
>>> upstream (or more likely, I am misunderstanding you).
>>
>> You are right, it is my bad for not clarifying it. What I mean is we
>> want it to be part of the upstream so it can be easier for us to pull
>> the changes and they are compatible with the changes that SNP is doing
>> but we also do not want to use different configuration files to
>> generate different binaries for each use case.
>>
>>
>>> Thanks,
>>>
>>> Paolo
>>>