From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lamorak.hansenpartnership.com (lamorak.hansenpartnership.com [198.37.111.173]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 31C151DCB09; Fri, 1 Aug 2025 18:03:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.37.111.173 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754071432; cv=none; b=DYO45gJPewoPCCOyh2AFUvPJpbvyftNI971t+ZBKpS0eM1tC/xv3wE63HbvoiQkaLPjZM1Zd6tbF92/Ctl5k2rw/UdeKRt27tyYR3VG3L5gfIQDsjmLDa+4knnDeCJ3nodOvF7H1h15/fS/Mi7EYxvGF1mMmyw0AiWASr+9PjoA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754071432; c=relaxed/simple; bh=jpm9UETghuIrnawF1EnF0l2LDTiuVEUnI/bZIPbrfbQ=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; 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linux-kernel@vger.kernel.org, stable@vger.kernel.org Date: Fri, 01 Aug 2025 14:03:47 -0400 In-Reply-To: <20250801171125.GA1274@sol> References: <20250731215255.113897-1-ebiggers@kernel.org> <20250731215255.113897-2-ebiggers@kernel.org> <3ed1ae7e7f52afe53ce2ff00f362ed153b3eec20.camel@HansenPartnership.com> <20250801030210.GA1495@sol> <20250801171125.GA1274@sol> Autocrypt: addr=James.Bottomley@HansenPartnership.com; prefer-encrypt=mutual; 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 8mT2Eh9SkH+YVOmaKsajgZgrBxA7fWmGoxXswEVxJIFj3vW7yNc0C5HaUdYa5iGOMs4kg2ht4s7yy7NRQuh7BifWjo6BQ6k4S1H+6axZucxhSV1L6zN9d+lr3Xo/vy1unzA== Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.50.3 Precedence: bulk X-Mailing-List: linux-integrity@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 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.=C2=A0 Replace the memcmp() with the correct > > > > > function, crypto_memneq(). > > > >=20 > > > > Um, OK, I'm all for more security but how could there possibly > > > > be a timing attack in the hmac final comparison code?=C2=A0 All it'= s > > > > doing is seeing if the HMAC the TPM returns matches the > > > > calculated one.=C2=A0 Beyond this calculation, there's nothing > > > > secret about the HMAC key. > > >=20 > > > I'm not sure I understand your question.=C2=A0 Timing attacks on MAC > > > validation are a well-known issue that can allow a valid MAC to > > > be guessed without knowing the key.=C2=A0 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. > > >=20 > > > 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)? > >=20 > > 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. >=20 > MAC timing attacks forge MACs; they don't leak the key. > It's true that such attacks don't work with one-time keys.=C2=A0 But here > it's not necessarily a one-time key.=C2=A0 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. > =C2=A0 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.=C2=A0 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