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

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Message-ID: <3ed1ae7e7f52afe53ce2ff00f362ed153b3eec20.camel@HansenPartnership.com>
Date: Thu, 31 Jul 2025 22:28:49 -0400
From: James Bottomley <James.Bottomley@...senPartnership.com>
To: Eric Biggers <ebiggers@...nel.org>, Peter Huewe <peterhuewe@....de>, 
	Jarkko Sakkinen <jarkko@...nel.org>, linux-integrity@...r.kernel.org
Cc: 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 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.

Regards,

James