linux-kernel - Re: [PATCH v4 3/6] crypto: lib/sha256

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Date:   Mon, 26 Oct 2020 08:59:37 +0100
From:   Ard Biesheuvel <ardb@...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>,
        Eric Biggers <ebiggers@...nel.org>,
        David Laight <David.Laight@...lab.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH v4 3/6] crypto: lib/sha256 - Don't clear temporary variables

On Sun, 25 Oct 2020 at 15:31, Arvind Sankar <nivedita@...m.mit.edu> 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>
> Reviewed-by: Eric Biggers <ebiggers@...gle.com>

Acked-by: Ard Biesheuvel <ardb@...nel.org>

> ---
>  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));
>  }
>
> --
> 2.26.2
>