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

[Eric Biggers <ebiggers@kernel.org>]

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)?

- Eric