linux-kernel - Re: [PATCH] crypto: sha512: remove imaginary and mystifying clearing of variables

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Message-ID: <20210827083842.GF21571@gondor.apana.org.au>
Date:   Fri, 27 Aug 2021 16:38:42 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        linux-crypto@...r.kernel.org,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] crypto: sha512: remove imaginary and mystifying clearing
 of variables

On Sun, Aug 22, 2021 at 12:31:07PM +0200, Lukas Bulwahn wrote:
> The function sha512_transform() assigns all local variables to 0 before
> returning to its caller with the intent to erase sensitive data.
> 
> However, make clang-analyzer warns that all these assignments are dead
> stores, and as commit 7a4295f6c9d5 ("crypto: lib/sha256 - Don't clear
> temporary variables") already points out for sha256_transform():
> 
>   The assignments to clear a through h and t1/t2 are optimized out by the
>   compiler because they are unused after the assignments.
> 
>   Clearing individual scalar variables is unlikely to be useful, as they
>   may have been assigned to registers, and even if stack spilling was
>   required, there may be compiler-generated temporaries that are
>   impossible to clear in any case.
> 
> This applies here again as well. Drop meaningless clearing of local
> variables and avoid this way that the code suggests that data is erased,
> which simply does not happen.
> 
> Signed-off-by: Lukas Bulwahn <lukas.bulwahn@...il.com>
> ---
>  crypto/sha512_generic.c | 3 ---
>  1 file changed, 3 deletions(-)

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt