Re: [PATCH v3 3/3] selftests/tdx: Test GetQuote TDX attestation feature

On Tue, Jun 27, 2023 at 06:36:07PM -0700, Dionna Amalie Glaze wrote:

On Tue, Jun 27, 2023 at 5:13 PM Dan Williams [off-list ref] wrote:

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[..]

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The VMPL-based separation that will house the supervisor module known
as SVSM can have protocols that implement a TPM command interface, or
an RTMR-extension interface, and will also need to have an
SVSM-specific protocol attestation report format to keep the secure
chain of custody apparent. We'd have different formats and protocols
in the kernel, at least, to speak to each technology.

That's where I hope the line can be drawn, i.e. that all of this vendor
differentiation really only matters inside the kernel in the end.

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I'm not sure it's worth the trouble of papering over all the... 3-4
technologies with similar but still weirdly different formats and ways
of doing things with an abstracted attestation ABI, especially since
the output all has to be interpreted in an architecture-specific way
anyway.

This is where I need help. Can you identify where the following
assertion falls over:

"The minimum viable key-server is one that can generically validate a
blob with an ECDSA signature".

I.e. the fact that SEV and TDX send different length blobs is less
important than validating that signature.

If it is always the case that specific fields in the blob need to be
decoded then yes, that weakens the assertion. However, maybe that means
that kernel code parses the blob and conveys that parsed info along with
vendor attestation payload all signed by a Linux key. I.e. still allow
for a unified output format + signed vendor blob and provide a path to
keep all the vendor specific handling internal to the kernel.

All the specific fields of the blob have to be decoded and subjected
to an acceptance policy. That policy will most always be different
across different platforms and VM owners. I wrote all of
github.com/google/go-sev-guest, including the verification and
validation logic, and it's going to get more complicated, and the
sources of the data that provide validators with notions of what
values can be trusted will be varied. The formats are not
standardized. The Confidential Computing Consortium should be working
toward that, but it's a slow process. There's IETF RATS. There's
in-toto.io attestations. There's Azure's JWT thing. There's a signed
serialized protocol buffer that I've decided is what Google is going
to produce while we figure out all the "right" formats to use. There
will be factions and absolute gridlock for multiple years if we
require solidifying an abstraction for the kernel to manage all this
logic before passing a report on to user space.

I agree with most of the above, but all that nightmate^Wcomplexity is
handled on the remote attestation side. If I understand the current
discussion, it's about how to abstract a guest attestation evidence
generation request in a vendor agnostic way. And I think what's proposed
here is simply to pass a binary payload (The evidence request from the
guest userspace) to the kernel key subsystem, hook it into vendor
specific handler and get userspace an attestation evidence (a platform
key signed blob) back to the guest app. The guest app can then give that
to an attestation service, and that's when all the above described
complexity takes place. Am I missing something?

Now, not only are the field contents important, the certificates of
the keys that signed the report are important. Each platform has its
own special x509v3 extensions and key hierarchy to express what parts
of the report should be what value if signed by this key, and in TDX's
case there are extra endpoints that you need to query to determine if
there's an active CVE on the associated TCB version. This is how they
avoid adding every cpu's key to the leaf certificate's CRL.

You really shouldn't be putting attestation validation logic in the
kernel. 

AFAIU, that's not part of the proposal/PoC/mockup. It's all about
funneling an attestation evidence request down to the TSM/PSP/firmware
for it to generate an actually verifiable attestation evidence.

Cheers,
Samuel.