linux-kernel - Re: [PATCH] crypto: pkcs7 - use constant-time digest comparison

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Message-ID: <20260201044135.GA71244@quark>
Date: Sat, 31 Jan 2026 20:41:35 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: Daniel Hodges <hodgesd@...a.com>
Cc: David Howells <dhowells@...hat.com>, Lukas Wunner <lukas@...ner.de>,
	Ignat Korchagin <ignat@...udflare.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S . Miller" <davem@...emloft.net>, keyrings@...r.kernel.org,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] crypto: pkcs7 - use constant-time digest comparison

On Sat, Jan 31, 2026 at 07:55:03PM -0800, Daniel Hodges wrote:
> This creates a timing side-channel that could allow an
> attacker to forge valid signatures by measuring verification time
> and recovering the expected digest value byte-by-byte.

Good luck with that.  The memcmp just checks that the CMS object
includes the hash of the data as a signed attribute.  It's a consistency
check of two attacker-controlled values, which happens before the real
signature check.  You may be confusing it with a MAC comparison.

- Eric