dm crypt: allow unaligned bv_offset

When slub_debug is enabled kmalloc returns unaligned memory. XFS uses
this unaligned memory for its buffers (if an unaligned buffer crosses a
page, XFS frees it and allocates a full page instead - see the function
xfs_buf_allocate_memory).

dm-crypt checks if bv_offset is aligned on page size and these checks
fail with slub_debug and XFS.

Fix this bug by removing the bv_offset checks. Switch to checking if
bv_len is aligned instead of bv_offset (this check should be sufficient
to prevent overruns if a bio with too small bv_len is received).

Fixes: 8f0009a225 ("dm crypt: optionally support larger encryption sector size")
Cc: stable@vger.kernel.org # v4.12+
Reported-by: Bruno Prémont <bonbons@sysophe.eu>
Tested-by: Bruno Prémont <bonbons@sysophe.eu>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-crypt.c

@@ -1075,7 +1075,7 @@ static int crypt_convert_block_aead(struct crypt_config *cc,
 	BUG_ON(cc->integrity_iv_size && cc->integrity_iv_size != cc->iv_size);
 
 	/* Reject unexpected unaligned bio. */
-	if (unlikely(bv_in.bv_offset & (cc->sector_size - 1)))
+	if (unlikely(bv_in.bv_len & (cc->sector_size - 1)))
 		return -EIO;
 
 	dmreq = dmreq_of_req(cc, req);
@@ -1168,7 +1168,7 @@ static int crypt_convert_block_skcipher(struct crypt_config *cc,
 	int r = 0;
 
 	/* Reject unexpected unaligned bio. */
-	if (unlikely(bv_in.bv_offset & (cc->sector_size - 1)))
+	if (unlikely(bv_in.bv_len & (cc->sector_size - 1)))
 		return -EIO;
 
 	dmreq = dmreq_of_req(cc, req);