On Fri, 2025-08-01 at 10:11 -0700, Eric Biggers wrote: > On Fri, Aug 01, 2025 at 07:36:02AM -0400, James Bottomley wrote: > > 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. > > MAC timing attacks forge MACs; they don't leak the key.
> 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. > I guses I'm struggling to understand the point of your comments. Your commit message, still quoted above, begins "To prevent timing attacks ..." but I still don't think there are any viable timing attacks against this code. However, that statement gives the idea that it's fixing a crypto vulnerablility and thus is going to excite the AI based CVE producers. > Even if in a follow-up message you're finally able to present a > correct argument for why memcmp() is okay, it's clearly subtle enough > that we should just use crypto_memneq() anyway, just like everywhere > else in the kernel that validates MACs. If you're worried about > performance, you shouldn't be: it's a negligible difference that is > far outweighed by all the optimizations I've been making to > lib/crypto/. So if you change the justification to something like "crypto people would like to update hmac compares to be constant time everywhere to avoid having to check individual places for correctness" I think I'd be happy. Regards, James
