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 B1757153598; Fri, 1 Aug 2025 11:36:04 +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=1754048167; cv=none; b=HEdwSf1BlUNH9//Fu6wgT2vP6rBT09NXl3o30FjAjD2aX5S+RGiOhfP6mFsPN5Dx1t/l1ocDRKWoj1h+Ls21v2YKRityUNlgofM3+tZMoTUGXyb8uzMcLqHoaAVmT7Z+CLbHcCHiwWfVpZyg7nTRfLJSen/vNsN93nJ7wQhq24Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1754048167; c=relaxed/simple; bh=RzOp+Cgdi84+OkyH84IiDbQFTpRjJQue6u/n1F/JBxM=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; 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In-Reply-To: <20250801030210.GA1495@sol> References: <20250731215255.113897-1-ebiggers@kernel.org> <20250731215255.113897-2-ebiggers@kernel.org> <3ed1ae7e7f52afe53ce2ff00f362ed153b3eec20.camel@HansenPartnership.com> <20250801030210.GA1495@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 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 i= s > > 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)? 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. Regards, James