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

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Thu, 2025-07-31 at 20:02 -0700, Eric Biggers wrote:
> On Thu, Jul 31, 2025 at 10:28:49PM -0400, James Bottomley wrote:
> > On Thu, 2025-07-31 at 14:52 -0700, Eric Biggers wrote:
> > > To prevent timing attacks, HMAC value comparison needs to be
> > > constant time.  Replace the memcmp() with the correct function,
> > > crypto_memneq().
> > 
> > Um, OK, I'm all for more security but how could there possibly be a
> > timing attack in the hmac final comparison code?  All it's doing is
> > seeing if the HMAC the TPM returns matches the calculated one. 
> > Beyond this calculation, there's nothing secret about the HMAC key.
> 
> I'm not sure I understand your question.  Timing attacks on MAC
> validation are a well-known issue that can allow a valid MAC to be
> guessed without knowing the key.  Whether it's practical in this
> particular case for some architecture+compiler+kconfig combination is
> another question, but there's no reason not to use the constant-time
> comparison function that solves this problem.
> 
> Is your claim that in this case the key is public, so the MAC really
> just serves as a checksum (and thus the wrong primitive is being
> used)?

The keys used for TPM HMAC calculations are all derived from a shared
secret and updating parameters making them one time ones which are
never reused, so there's no benefit to an attacker working out after
the fact what the key was.

Regards,

James