dm-verity-fec: correctly reject too-small hash devices

commit 4355142 upstream.

Fix verity_fec_ctr() to reject too-small hash devices by correctly
taking hash_start into account.

Note that this is necessary because dm-verity doesn't call
dm_bufio_set_sector_offset() on the hash device's bufio client
(v->bufio).  Thus, dm_bufio_get_device_size(v->bufio) returns a size
relative to 0 rather than hash_start.  An alternative fix would be to
call dm_bufio_set_sector_offset() on v->bufio, but then all the code
that reads from the hash device would have to be adjusted accordingly.

Fixes: a739ff3 ("dm verity: add support for forward error correction")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Eric Biggers

drivers/md/dm-verity-fec.c

@@ -688,7 +688,8 @@ int verity_fec_ctr(struct dm_verity *v)
 	 * it to be large enough.
 	 */
 	f->hash_blocks = f->blocks - v->data_blocks;
-	if (dm_bufio_get_device_size(v->bufio) < f->hash_blocks) {
+	if (dm_bufio_get_device_size(v->bufio) <
+	    v->hash_start + f->hash_blocks) {
 		ti->error = "Hash device is too small for "
 			DM_VERITY_OPT_FEC_BLOCKS;
 		return -E2BIG;