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

commit 2b14e0b upstream.

Fix verity_fec_ctr() to reject too-small FEC devices by correctly
computing the number of parity blocks as 'f->rounds * f->roots'.
Previously it incorrectly used 'div64_u64(f->rounds * f->roots,
v->fec->roots << SECTOR_SHIFT)' which is a much smaller value.

Note that the units of 'rounds' are blocks, not bytes.  This matches the
units of the value returned by dm_bufio_get_device_size(), which are
also blocks.  A later commit will give 'rounds' a clearer name.

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 +688,7 @@ int verity_fec_ctr(struct dm_verity *v)
 {
 	struct dm_verity_fec *f = v->fec;
 	struct dm_target *ti = v->ti;
-	u64 hash_blocks, fec_blocks;
+	u64 hash_blocks;
 	int ret;
 
 	if (!verity_fec_is_enabled(v)) {
@@ -769,8 +769,7 @@ int verity_fec_ctr(struct dm_verity *v)
 
 	dm_bufio_set_sector_offset(f->bufio, f->start << (v->data_dev_block_bits - SECTOR_SHIFT));
 
-	fec_blocks = div64_u64(f->rounds * f->roots, v->fec->roots << SECTOR_SHIFT);
-	if (dm_bufio_get_device_size(f->bufio) < fec_blocks) {
+	if (dm_bufio_get_device_size(f->bufio) < f->rounds * f->roots) {
 		ti->error = "FEC device is too small";
 		return -E2BIG;
 	}