crypto: hash - Fix page length clamping in hash walk

commit 13f4bb78 upstream.

The crypto hash walk code is broken when supplied with an offset
greater than or equal to PAGE_SIZE.  This patch fixes it by adjusting
walk->pg and walk->offset when this happens.
Reported-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

crypto/ahash.c

@@ -68,8 +68,9 @@ static int hash_walk_new_entry(struct crypto_hash_walk *walk)
 	struct scatterlist *sg;
 
 	sg = walk->sg;
-	walk->pg = sg_page(sg);
 	walk->offset = sg->offset;
+	walk->pg = sg_page(walk->sg) + (walk->offset >> PAGE_SHIFT);
+	walk->offset = offset_in_page(walk->offset);
 	walk->entrylen = sg->length;
 
 	if (walk->entrylen > walk->total)