Re: [PATCH v4 4/6] crypto: lib/sha256 - Clear W[] in sha256_update() instead of sha256_transform()

[Ard Biesheuvel <ardb@kernel.org>]

On Sun, 25 Oct 2020 at 15:31, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> The temporary W[] array is currently zeroed out once every call to
> sha256_transform(), i.e. once every 64 bytes of input data. Moving it to
> sha256_update() instead so that it is cleared only once per update can
> save about 2-3% of the total time taken to compute the digest, with a
> reasonable memset() implementation, and considerably more (~20%) with a
> bad one (eg the x86 purgatory currently uses a memset() coded in C).
>
> 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 | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/lib/crypto/sha256.c b/lib/crypto/sha256.c
> index 099cd11f83c1..c6bfeacc5b81 100644
> --- a/lib/crypto/sha256.c
> +++ b/lib/crypto/sha256.c
> @@ -43,10 +43,9 @@ static inline void BLEND_OP(int I, u32 *W)
>         W[I] = s1(W[I-2]) + W[I-7] + s0(W[I-15]) + W[I-16];
>  }
>
> -static void sha256_transform(u32 *state, const u8 *input)
> +static void sha256_transform(u32 *state, const u8 *input, u32 *W)
>  {
>         u32 a, b, c, d, e, f, g, h, t1, t2;
> -       u32 W[64];
>         int i;
>
>         /* load the input */
> @@ -200,15 +199,13 @@ static void sha256_transform(u32 *state, const u8 *input)
>
>         state[0] += a; state[1] += b; state[2] += c; state[3] += d;
>         state[4] += e; state[5] += f; state[6] += g; state[7] += h;
> -
> -       /* clear any sensitive info... */
> -       memzero_explicit(W, 64 * sizeof(u32));
>  }
>
>  void sha256_update(struct sha256_state *sctx, const u8 *data, unsigned int len)
>  {
>         unsigned int partial, done;
>         const u8 *src;
> +       u32 W[64];
>
>         partial = sctx->count & 0x3f;
>         sctx->count += len;
> @@ -223,11 +220,13 @@ void sha256_update(struct sha256_state *sctx, const u8 *data, unsigned int len)
>                 }
>
>                 do {
> -                       sha256_transform(sctx->state, src);
> +                       sha256_transform(sctx->state, src, W);
>                         done += 64;
>                         src = data + done;
>                 } while (done + 63 < len);
>
> +               memzero_explicit(W, sizeof(W));
> +
>                 partial = 0;
>         }
>         memcpy(sctx->buf + partial, src, len - done);
> --
> 2.26.2
>