dm-integrity: don't modify bio's immutable bio_vec in integrity_metadata()

__bio_for_each_segment assumes that the first struct bio_vec argument
doesn't change - it calls "bio_advance_iter_single((bio), &(iter),
(bvl).bv_len)" to advance the iterator. Unfortunately, the dm-integrity
code changes the bio_vec with "bv.bv_len -= pos". When this code path
is taken, the iterator would be out of sync and dm-integrity would
report errors. This happens if the machine is out of memory and
"kmalloc" fails.

Fix this bug by making a copy of "bv" and changing the copy instead.

Fixes: 7eada90 ("dm: add integrity target")
Cc: stable@vger.kernel.org	# v4.12+
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Author: Mikulas Patocka

drivers/md/dm-integrity.c

@@ -1755,11 +1755,12 @@ static void integrity_metadata(struct work_struct *w)
 		sectors_to_process = dio->range.n_sectors;
 
 		__bio_for_each_segment(bv, bio, iter, dio->bio_details.bi_iter) {
+			struct bio_vec bv_copy = bv;
 			unsigned int pos;
 			char *mem, *checksums_ptr;
 
 again:
-			mem = bvec_kmap_local(&bv);
+			mem = bvec_kmap_local(&bv_copy);
 			pos = 0;
 			checksums_ptr = checksums;
 			do {
@@ -1768,7 +1769,7 @@ static void integrity_metadata(struct work_struct *w)
 				sectors_to_process -= ic->sectors_per_block;
 				pos += ic->sectors_per_block << SECTOR_SHIFT;
 				sector += ic->sectors_per_block;
-			} while (pos < bv.bv_len && sectors_to_process && checksums != checksums_onstack);
+			} while (pos < bv_copy.bv_len && sectors_to_process && checksums != checksums_onstack);
 			kunmap_local(mem);
 
 			r = dm_integrity_rw_tag(ic, checksums, &dio->metadata_block, &dio->metadata_offset,
@@ -1793,9 +1794,9 @@ static void integrity_metadata(struct work_struct *w)
 			if (!sectors_to_process)
 				break;
 
-			if (unlikely(pos < bv.bv_len)) {
-				bv.bv_offset += pos;
-				bv.bv_len -= pos;
+			if (unlikely(pos < bv_copy.bv_len)) {
+				bv_copy.bv_offset += pos;
+				bv_copy.bv_len -= pos;
 				goto again;
 			}
 		}