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

["Erdem Aktas" <erdemaktas@google.com>]

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
>