crypto: ahash - Fix early termination in hash walk

BugLink: http://bugs.launchpad.net/bugs/1765007

commit 900a081f upstream.

When we have an unaligned SG list entry where there is no leftover
aligned data, the hash walk code will incorrectly return zero as if
the entire SG list has been processed.

This patch fixes it by moving onto the next page instead.
Reported-by: Eli Cooper <elicooper@gmx.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

crypto/ahash.c

@@ -91,14 +91,15 @@ int crypto_hash_walk_done(struct crypto_hash_walk *walk, int err)
 
 	if (nbytes && walk->offset & alignmask && !err) {
 		walk->offset = ALIGN(walk->offset, alignmask + 1);
-		walk->data += walk->offset;
-
 		nbytes = min(nbytes,
 			     ((unsigned int)(PAGE_SIZE)) - walk->offset);
 		walk->entrylen -= nbytes;
 
+		if (nbytes) {
+			walk->data += walk->offset;
 			return nbytes;
 		}
+	}
 
 	if (walk->flags & CRYPTO_ALG_ASYNC)
 		kunmap(walk->pg);