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
