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Date: Sat, 11 Jul 2026 07:19:37 +0200
From: Greg KH <gregkh@linuxfoundation.org>
To: Eric Biggers <ebiggers@kernel.org>
Cc: "David C.C.M. Gall" <david.ccm.gall@googlemail.com>,
	Lukas Wunner <lukas@wunner.de>, Ignat Korchagin <ignat@linux.win>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	"David S. Miller" <davem@davemloft.net>,
	linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] crypto: rsassa-pkcs1: use constant-time comparison for
 digest and signature verification
Message-ID: <2026071156-masculine-unsold-3567@gregkh>
References: <alEr_e-G0L2nxxv-@fudgebox>
 <20260710213718.GD1911@quark>
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In-Reply-To: <20260710213718.GD1911@quark>

On Fri, Jul 10, 2026 at 05:37:18PM -0400, Eric Biggers wrote:
> On Fri, Jul 10, 2026 at 07:29:33PM +0200, David C.C.M. Gall wrote:
> > Replace memcmp() with crypto_memneq() for cryptographic digest and
> > signature comparisons to prevent timing side-channel attacks.
> > 
> > crypto/rsassa-pkcs1.c: RSA signature digest verification used memcmp
> > which can leak valid prefix length via timing analysis, user data
> > could reach the leaky comparison via the digest argument to verify.
> > 
> > Assisted-by: gregkh_clanker_t1000
> > Signed-off-by: David C.C.M. Gall <david.ccm.gall@googlemail.com>
> 
> While we should use crypto_memneq() on MACs, auth tags, and other secret
> data, I don't think we should let it creep into domains where it is
> clearly not needed, like public key signature verification.

But isn't this user-controlled data and so a user could use it to figure
out the key?

thanks,

greg k-h