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

Arvind Sankar
2020-10-23T03:18:04Z

On Wed, Oct 21, 2020 at 09:58:50PM -0700, Eric Biggers wrote:
> 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.

The "non-standard initial value" was just parenthetical -- my thinking
was that revealing the hash, whether the real SHA hash or an
intermediate one starting at some other initial value, shouldn't reveal
the input; not that you get any additional security from being an
intermediate block. But if the hash itself could be sensitive, yeah then
a-h are sensitive anyway.

>
> > ---
> > 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

Ok, I'll just drop the second paragraph from the commit message then.