Re: [PATCH v2 2/6] crypto: lib/sha256 - Don't clear temporary variables

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Oct 20, 2020 at 04:39:53PM -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.
> 
> These variables shouldn't be very sensitive: t1/t2 can be calculated
> from a through h, so they don't reveal any additional information.
> Knowing a through h is equivalent to knowing one 64-byte block's SHA256
> hash (with non-standard initial value) which, assuming SHA256 is secure,
> doesn't reveal any information about the input.
> 
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>

I don't entirely buy the second paragraph.  It could be the case that the input
is less than or equal to one SHA-256 block (64 bytes), in which case leaking
'a' through 'h' would reveal the final SHA-256 hash if the input length is
known.  And note that callers might consider either the input, the resulting
hash, or both to be sensitive information -- it depends.

> ---
>  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));
>  }

Your change itself is fine, though.  As you mentioned, these assignments get
optimized out, so they weren't accomplishing anything.

The fact is, there just isn't any way to guarantee in C code that all sensitive
variables get cleared.

So we shouldn't (and generally don't) bother trying to clear individual u32's,
ints, etc. like this, but rather only structs and arrays, as clearing those is
more likely to work as intended.

- Eric