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: > > 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.
Again, tpm2_get_random() sets a HMAC key once and then uses it multiple times. > > 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. Sure, provided that memcmp() is actually secure here. So far, it hasn't been particularly convincing when each argument you've given for it being secure has been incorrect. But I do see that each call to tpm_buf_check_hmac_response() is paired with a call to tpm_buf_append_hmac_session() which generates a fresh nonce. That nonce is then sent to the other endpoint (the one that claims to be a TPM) and then implicitly becomes part of the response message (but is not explicitly transmitted back in it). That may be the real reason: messages are guaranteed to not be repeated, so a MAC timing attack can't be done. Do you agree that is the actual reason? - Eric
