Re: [PATCH 2/9] tpm: Allow PCR 23 to be restricted to kernel-only use

[James Bottomley]

On Sat, 2021-02-20 at 01:32 +0000, Matthew Garrett wrote:
> Under certain circumstances it might be desirable to enable the
> creation of TPM-backed secrets that are only accessible to the
> kernel. In an ideal world this could be achieved by using TPM
> localities, but these don't appear to be available on consumer
> systems.

I don't understand this ... the localities seem to work fine on all the
systems I have ... is this some embedded thing?

>  An alternative is to simply block userland from modifying one of the
> resettable PCRs, leaving it available to the kernel. If the kernel
> ensures that no userland can access the TPM while it is carrying out
> work, it can reset PCR 23, extend it to an arbitrary value, create or
> load a secret, and then reset the PCR again. Even if userland somehow
> obtains the sealed material, it will be unable to unseal it since PCR
> 23 will never be in the appropriate state.

This seems a bit arbitrary: You're removing this PCR from user space
accessibility, but PCR 23 is defined as "Application Support" how can
we be sure no application will actually want to use it (and then fail)?

Since PCRs are very scarce, why not use a NV index instead.  They're
still a bounded resource, but most TPMs have far more of them than they
do PCRs, and the address space is much bigger so picking a nice
arbitrary 24 bit value reduces the chance of collisions.

James