Re: SVSM vTPM specification

[James Bottomley <jejb@linux.ibm.com>]

On Thu, 2022-10-13 at 10:14 -0500, Tom Lendacky wrote:
> On 10/12/22 13:44, James Bottomley wrote:
> > On Wed, 2022-10-12 at 18:33 +0100, Dr. David Alan Gilbert wrote:
> > > * Tom Lendacky (thomas.lendacky@amd.com) wrote:
> > [...]
> > > It's important that the VMPL level in the attestation report
> > > reflects the side asking for the attestation; in particular one
> > > TPM story goes that the firmware (in VMPL0) would ask for an
> > > attestation and the attestor would return the vTPM stored
> > > state.  It's important that the state could only be returned to
> > > the vTPM not the guest, so the attestor would check that the VMPL
> > > level in the attestation was 0; any guest attestation would have
> > > a VMPL>0 and so the attestor wouldn't hand it the vTPM state.
> > > Hmm or are you saying such a report would be triggered by the
> > > guest rather than the firmware, but it would be protected by
> > > VMPCK0 so the guest wouldn't be able to read it?
> 
> No, the VMPCK0 key is just used for communication with the PSP.
> 
> While the SVSM would request the attestation report from the PSP,
> the guest would need to request it from the SVSM.

I think this is fine.  The SVSM would do the attestation as it starts
the TPM but the guest would be able to retrieve it at any time. 
Essentially, if you use something like keylime, the agent would request
it on start up to prove it should trust the vTPM, but that could occur
way after VM boot.

> 
> > > I think one of the vTPMs keys should be in the SNP attestation
> > > report (the EK???) - I think that would allow you to attest that
> > > the vTPM you're talking to is a vTPM running in an SNP protected
> > > firmware.
> > 
> > Traditionally the TPM identity is the public EK, so that should
> > definitely be in the report.  Ideally, I think the public storage
> > root key (key derived from the owner seed) should be in there two
> > because it makes it easy to create keys that can only be read by
> > the TPM (keys should be in the owner hierarchy which means they
> > have to be encrypted to the storage seed, so we need to know what a
> > public key corresponding to it is).
> > 
> > One wrinkle of the above is that, when provisioned, the TPM will
> > only have the seeds, not the keys (the keys are derived from the
> > seeds via a TPM2_CreatePrimary command).  The current TPM
> > provisioning guidance:
> > 
> > https://trustedcomputinggroup.org/resource/tcg-tpm-v2-0-provisioning-guidance/
> > 
> > Says that the EK should be at permanent handle
> > 
> > 81010001
> > 
> > And there's an update saying that should be the RSA-2048 key and
> > there should be an EC (NIST-P256) one at 81010002.  The
> > corresponding storage keys should be at 81000001 and 81000002
> > respectively.  I think when the SVSM provisions the TPM, it should
> > run TPM2_CreatePrimary for those four keys and put them into the
> > persistent indexes, then insert the EC keys only for EK and SRK
> > into the attestation report.
> 
> We only have 512 bits to work with for the SVSM-provided data, so
> would hashes of the keys be ok?

Well, if you put the hashes in, the consuming entity would then have to
find out via an additional channel what the actual keys were because
you can't reverse the hash (it's possible, just more effort).  If you
use point compression, an EC key (for the NIST p-256 curve) is only 32
bytes anyway, so it's the same size as a sha256 hash, so I'd say place
the actual public keys into the report to give complete and usable
information

James