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References: <20201025143119.1054168-1-nivedita@alum.mit.edu> <20201025143119.1054168-4-nivedita@alum.mit.edu>
In-Reply-To: <20201025143119.1054168-4-nivedita@alum.mit.edu>
From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Mon, 26 Oct 2020 08:59:37 +0100
Message-ID: <CAMj1kXGOQxymx7zKCV8dmy2-0ypvVumDb1thGKCqrKXQrSbZ9Q@mail.gmail.com>
Subject: Re: [PATCH v4 3/6] crypto: lib/sha256 - Don't clear temporary variables
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        "linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>,
        Eric Biggers <ebiggers@kernel.org>,
        David Laight <David.Laight@aculab.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Eric Biggers <ebiggers@google.com>
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On Sun, 25 Oct 2020 at 15:31, Arvind Sankar <nivedita@alum.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@alum.mit.edu>
> Reviewed-by: Eric Biggers <ebiggers@google.com>

Acked-by: Ard Biesheuvel <ardb@kernel.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
>