linux-kernel - Re: [PATCH v3 2/5] crypto: lib/sha256

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Message-ID: <20201023221126.GB180517@gmail.com>
Date:   Fri, 23 Oct 2020 15:11:26 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        David Laight <David.Laight@...lab.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/5] crypto: lib/sha256 - Don't clear temporary
 variables

On Fri, Oct 23, 2020 at 03:22:00PM -0400, Arvind Sankar wrote:
> 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.
> 
> So drop the clearing of a through h and t1/t2.
> 
> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> ---
>  lib/crypto/sha256.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/lib/crypto/sha256.c b/lib/crypto/sha256.c
> index d43bc39ab05e..099cd11f83c1 100644
> --- a/lib/crypto/sha256.c
> +++ b/lib/crypto/sha256.c
> @@ -202,7 +202,6 @@ static void sha256_transform(u32 *state, const u8 *input)
>  	state[4] += e; state[5] += f; state[6] += g; state[7] += h;
>  
>  	/* clear any sensitive info... */
> -	a = b = c = d = e = f = g = h = t1 = t2 = 0;
>  	memzero_explicit(W, 64 * sizeof(u32));
>  }

Looks good,

Reviewed-by: Eric Biggers <ebiggers@...gle.com>