Re: [RFC PATCH 0/2] TPM derived keys

Ignat Korchagin
2024-05-14T09:51:03Z

On Mon, May 13, 2024 at 11:33 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> On Mon, 2024-05-13 at 18:09 +0100, Ignat Korchagin wrote:
> [...]
> > TPM derived keys attempt to address the above use cases by allowing
> > applications to deterministically derive unique cryptographic keys
> > for their own purposes directly from the TPM seed in the owner
> > hierarchy. The idea is that when an application requests a new key,
> > instead of generating a random key and wrapping it with the TPM, the
> > implementation generates a key via KDF(hierarchy seed, application
> > specific info). Therefore, the resulting keys will always be
> > cryptographically bound to the application itself and the device they
> > were generated on.
>
> So I think what confuses me is what the expected cryptographic secrecy
> properties of the derived keys are. I get they're a KDF of seed and
> deterministic properties, but if those mixing values are well known (as
> the path or binary checksum cases) then anyone with access to the TPM
> can derive the key from user space because they can easily obtain the
> mixing parameters and there's no protection to the TPM keyed hash
> operation.
>
> Consider the use case where two users are using derived keys on the
> same system (so same TPM). Assuming they use them to protect sensitive
> information, what prevents user1 from simply deriving user2's key and
> getting the information, or am I missing the point of this?

You are correct: it is possible, but in practice it would be limited
only to privileged users/applications. I remember there was a push to
set a 666 mask for the TPM device file, but it is not how it is done
today by default. Also I think the same applies to trusted keys as
well, at least without any additional authorizations or PCR
restrictions on the blob (I remember I could manually unwrap a trusted
key blob in userspace as root).

It would be fixed if we could limit access to some TPM ops only from
the kernel, but I remember from one of your presentations that it is
generally a hard problem and that some solution was in the works (was
it based on limiting access to a resettable PCR?). I'm happy to
consider adopting it here as well somehow.

> James
>