Re: [PATCH 1/2] tpm: Compare HMAC values in constant time

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Aug 01, 2025 at 02:53:09PM -0400, James Bottomley wrote:
> On Fri, 2025-08-01 at 11:40 -0700, Eric Biggers wrote:
> > On Fri, Aug 01, 2025 at 02:03:47PM -0400, James Bottomley wrote:
> > > On Fri, 2025-08-01 at 10:11 -0700, Eric Biggers wrote:
> [...]
> > > > It's true that such attacks don't work with one-time keys.  But
> > > > here it's not necessarily a one-time key.  E.g.,
> > > > tpm2_get_random() sets a key, then authenticates multiple
> > > > messages using that key.
> > > 
> > > The nonces come one from us and one from the TPM.  I think ours
> > > doesn't change if the session is continued although it could,
> > > whereas the TPM one does, so the HMAC key is different for every
> > > communication of a continued session.
> > 
> > Again, tpm2_get_random() sets a HMAC key once and then uses it
> > multiple times.
> 
> No it doesn't.  If you actually read the code, you'd find it does what
> I say above.  Specifically  tpm_buf_fill_hmac_session() which is called
> inside that loop recalculates the hmac key from the nonces.  This
> recalculated key is what is used in tpm_buf_check_hmac_response(), and
> which is where the new tpm nonce is collected for the next iteration.

tpm_buf_fill_hmac_session() computes a HMAC value, but it doesn't modify
the HMAC key.  tpm2_parse_start_auth_session() is the only place where
the HMAC key is changed.  You may be confusing HMAC values with keys.

- Eric