Re: [PATCH 0/4] tsm: Unified Measurement Register ABI for TVMs

[Dionna Amalie Glaze]

On Tue, Feb 18, 2025 at 4:41 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 2/18/25 15:57, Dionna Amalie Glaze wrote:
> >> If there are actual end users who care about this, it would be great to
> >> see their acks on it as well.
> >>
> > We would like to have this for Google Confidential Space and Kubernetes Engine.
> >
> > Acked-by: Dionna Glaze <dionnaglaze@xxxxxxxxxx>
>
> Great! Thanks for chiming in. Can you talk for a second, though, about
> why this is useful and how you plan to use it? Is it for debugging?

Confidential space on SEV depends on the hypervisor-provided vTPM to
provide remotely attestable quotes of its PCRs, and the corresponding
event logs.
https://github.com/google/go-tpm-tools/blob/main/launcher/agent/agent.go#L97

On TDX and ARM CCA (maybe RISC-V CoVE someday), we don't want to have
to depend on the vTPM.
There are runtime measurement registers and the CCEL.
When we have a sysfs interface to extend these registers, it makes the
user space evidence manager's life easier.
When Dan Williams forced the issue about configfs-tsm, we were told
that it is bad for the kernel to have many platform-specific
interfaces for attestation operations.
This patch series is a way to unify behind the tsm.

As for the ability to read the registers through sysfs instead of just
extend-on-write, that's not something Confidential Space depends on
specifically.
Our attestation policies are evaluated in a verification service
rather than on-node.

For on-node policy evaluation, for instance in kubectl, there is a
benefit to being able to generically read measurement registers that
have been extended generically to execute policy that a certain action
if a measurement register isn't an exact expected value.
This is not far-fetched, since it is used for confidential containers,
and is being discussed for kubernetes engine as a way to poison an
instance when an ssh session is terminated for a human operator.

To have that same capability without a generic read interface, we need
to stuff kubectl with quote parsers of every attestation technology.

Hope that helps.
-- 
-Dionna Glaze, PhD, CISSP, CCSP (she/her)