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References: <20201025143119.1054168-1-nivedita@alum.mit.edu> <20201025143119.1054168-2-nivedita@alum.mit.edu>
In-Reply-To: <20201025143119.1054168-2-nivedita@alum.mit.edu>
From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Mon, 26 Oct 2020 08:59:07 +0100
Message-ID: <CAMj1kXE618YtdrCg7BRqiD2K4S2x1S7L7gZgS+25WUNM+Q8TFw@mail.gmail.com>
Subject: Re: [PATCH v4 1/6] crypto: lib/sha256 - Use memzero_explicit() for
 clearing state
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:
>
> Without the barrier_data() inside memzero_explicit(), the compiler may
> optimize away the state-clearing if it can tell that the state is not
> used afterwards. At least in lib/crypto/sha256.c:__sha256_final(), the
> function can get inlined into sha256(), in which case the memset is
> optimized away.
>
> 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 | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/crypto/sha256.c b/lib/crypto/sha256.c
> index 2321f6cb322f..d43bc39ab05e 100644
> --- a/lib/crypto/sha256.c
> +++ b/lib/crypto/sha256.c
> @@ -265,7 +265,7 @@ static void __sha256_final(struct sha256_state *sctx, u8 *out, int digest_words)
>                 put_unaligned_be32(sctx->state[i], &dst[i]);
>
>         /* Zeroize sensitive information. */
> -       memset(sctx, 0, sizeof(*sctx));
> +       memzero_explicit(sctx, sizeof(*sctx));
>  }
>
>  void sha256_final(struct sha256_state *sctx, u8 *out)
> --
> 2.26.2
>