[v3,2/5] crypto: lib/sha256 - Don't clear temporary variables
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Message ID 20201023192203.400040-3-nivedita@alum.mit.edu
State New, archived
Headers show
Series
  • crypto: lib/sha256 - cleanup/optimization
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Commit Message

Arvind Sankar Oct. 23, 2020, 7:22 p.m. UTC
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>
---
 lib/crypto/sha256.c | 1 -
 1 file changed, 1 deletion(-)

Comments

Eric Biggers Oct. 23, 2020, 10:11 p.m. UTC | #1
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@alum.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@google.com>

Patch
diff mbox series

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));
 }