Re: [RFC PATCH v2 0/4] tsm: Runtime measurement registers ABI

[Dan Middleton]

On 2/2/24 12:24 AM, James Bottomley wrote:
> On Sun, 2024-01-28 at 22:25 +0100, Samuel Ortiz wrote:
> > All architectures supporting RTMRs expose a similar interface to
> > their TVMs: An extension command/call that takes a measurement value
> > and an RTMR index to extend it with, and a readback command for
> > reading an RTMR value back (taking an RTMR index as an argument as
> > well). This patch series builds an architecture agnostic, configfs-
> > based ABI for userspace to extend and read RTMR values back. It
> > extends the current TSM ops structure and each confidential computing
> > architecture can implement this extension to provide RTMR support.
> What's the actual use case for this?  At the moment the TPM PCRs only
> provide a read interface to userspace (via /sys/class/tpm/tpmX/pcr-
> shaY/Z) and don't have any extension ability becuase nothing in
> userspace currently extends them.
>
> The only current runtime use for TPM PCRs is IMA, which is in-kernel
> (and which this patch doesn't enable).
>
> Without the ability to log, this interface is unusable anyway, but even
> with that it's not clear that you need the ability separately to extend
> PCRs because the extension and log entry should be done atomically to
> prevent the log going out of sync with the PCRs, so it would seem a log
> first interface would be the correct way of doing this rather than a
> PCR first one.
>
> James

While we clearly need to cover PCR-like usages, I think Confidential
Computing affords usages that go beyond TPM.

For example, Attested Containers [1] (and similar explorations in CNCF
Confidential Containers [2]) extends the measurement chain into the guest.
There, a trusted agent measures container images, and extends an RTMR
with those measurements. Particularly in the case of containers, the 
existing
runtime infrastructure is user mode oriented. However the generalization
here is in providing a mechanism to strongly identify an application or
behavior provided by the TVM.

Less concretely, I think this is an area for developer creativity.
Attestation is one of the main APIs that CC gives application developers and
these runtime extendable fields provide a further degree of creativity.

[1] ACON https://github.com/intel/acon
[2] CoCo 
https://github.com/confidential-containers/guest-components/commit/3c75201a8ba0327fb41b68b7e1521ff517e3ca9f

Regards,
Dan