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