linux-kernel - Re: [PATCH 1/2] tpm: Compare HMAC values in constant time

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Message-ID: <20250801171125.GA1274@sol>
Date: Fri, 1 Aug 2025 10:11:25 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: James Bottomley <James.Bottomley@...senpartnership.com>
Cc: Peter Huewe <peterhuewe@....de>, Jarkko Sakkinen <jarkko@...nel.org>,
	linux-integrity@...r.kernel.org, Jason Gunthorpe <jgg@...pe.ca>,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/2] tpm: Compare HMAC values in constant time

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.

I guses I'm struggling to understand the point of your comments.  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/.

- Eric