linux-kernel - Re: [PATCH 1/3] KEYS: trusted_tpm1: Compare HMAC values in constant time

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Message-ID: <20250805173227.GD1286@sol>
Date: Tue, 5 Aug 2025 10:32:27 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: James Bottomley <James.Bottomley@...senpartnership.com>,
	Mimi Zohar <zohar@...ux.ibm.com>, keyrings@...r.kernel.org,
	David Howells <dhowells@...hat.com>,
	linux-integrity@...r.kernel.org, linux-crypto@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/3] KEYS: trusted_tpm1: Compare HMAC values in constant
 time

On Tue, Aug 05, 2025 at 04:44:27PM +0300, Jarkko Sakkinen wrote:
> On Thu, Jul 31, 2025 at 02:23:52PM -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().
> > 
> > Fixes: d00a1c72f7f4 ("keys: add new trusted key-type")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Eric Biggers <ebiggers@...nel.org>
> 
> Was crypto_memneq() available at the time?

No.  The Fixes commit is still correct, though, as it's the commit that
introduced the memcmp().  Technically it was still a bug at that time,
even if there wasn't a helper function available yet.

- Eric