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Date: Wed, 13 May 2026 16:31:55 +0200
From: Lukas Wunner <lukas@wunner.de>
To: Anastasia Tishchenko <sv3iry@gmail.com>
Cc: Stefan Berger <stefanb@linux.ibm.com>,
	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,
	stable@vger.kernel.org
Subject: Re: [PATCH v2] crypto: ecc - Fix carry overflow in vli multiplication
Message-ID: <agSLW1XpMab2VYNV@wunner.de>
References: <20260513105741.55534-1-sv3iry@gmail.com>
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In-Reply-To: <20260513105741.55534-1-sv3iry@gmail.com>

On Wed, May 13, 2026 at 01:57:40PM +0300, Anastasia Tishchenko wrote:
> The carry flag calculation fails when r01.m_high is saturated
> (0xFFFFFFFFFFFFFFFF) and addition of lower bits overflows.
> 
> The condition (r01.m_high < product.m_high) doesn't handle the case
> where r01.m_high == product.m_high and an additional carry exists
> from lower-bit overflow.
> 
> When commit 3c4b23901a0c ("crypto: ecdh - Add ECDH software support")
> introduced crypto/ecc.c, it split the muladd() function in the
> micro-ecc library into separate mul_64_64() and add_128_128() helpers.
> It seems the check got lost in translation.
> 
> Add proper handling for this boundary by accounting for the carry
> from the lower addition.
> 
> Fixes: 3c4b23901a0c ("crypto: ecdh - Add ECDH software support")
> Signed-off-by: Anastasia Tishchenko <sv3iry@gmail.com>
> Cc: stable@vger.kernel.org # v4.8+

Reviewed-by: Lukas Wunner <lukas@wunner.de>