dm array: fix releasing a faulty array block twice in dm_array_cursor_end

When dm_bm_read_lock() fails due to locking or checksum errors, it
releases the faulty block implicitly while leaving an invalid output
pointer behind. The caller of dm_bm_read_lock() should not operate on
this invalid dm_block pointer, or it will lead to undefined result.
For example, the dm_array_cursor incorrectly caches the invalid pointer
on reading a faulty array block, causing a double release in
dm_array_cursor_end(), then hitting the BUG_ON in dm-bufio cache_put().

Reproduce steps:

1. initialize a cache device

dmsetup create cmeta --table "0 8192 linear /dev/sdc 0"
dmsetup create cdata --table "0 65536 linear /dev/sdc 8192"
dmsetup create corig --table "0 524288 linear /dev/sdc $262144"
dd if=/dev/zero of=/dev/mapper/cmeta bs=4k count=1
dmsetup create cache --table "0 524288 cache /dev/mapper/cmeta \
/dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 writethrough smq 0"

2. wipe the second array block offline

dmsteup remove cache cmeta cdata corig
mapping_root=$(dd if=/dev/sdc bs=1c count=8 skip=192 \
2>/dev/null | hexdump -e '1/8 "%u\n"')
ablock=$(dd if=/dev/sdc bs=1c count=8 skip=$((4096*mapping_root+2056)) \
2>/dev/null | hexdump -e '1/8 "%u\n"')
dd if=/dev/zero of=/dev/sdc bs=4k count=1 seek=$ablock

3. try reopen the cache device

dmsetup create cmeta --table "0 8192 linear /dev/sdc 0"
dmsetup create cdata --table "0 65536 linear /dev/sdc 8192"
dmsetup create corig --table "0 524288 linear /dev/sdc $262144"
dmsetup create cache --table "0 524288 cache /dev/mapper/cmeta \
/dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 writethrough smq 0"

Kernel logs:

(snip)
device-mapper: array: array_block_check failed: blocknr 0 != wanted 10
device-mapper: block manager: array validator check failed for block 10
device-mapper: array: get_ablock failed
device-mapper: cache metadata: dm_array_cursor_next for mapping failed
------------[ cut here ]------------
kernel BUG at drivers/md/dm-bufio.c:638!

Fix by setting the cached block pointer to NULL on errors.

In addition to the reproducer described above, this fix can be
verified using the "array_cursor/damaged" test in dm-unit:
  dm-unit run /pdata/array_cursor/damaged --kernel-dir <KERNEL_DIR>

Signed-off-by: Ming-Hung Tsai <mtsai@redhat.com>
Fixes: fdd1315 ("dm array: introduce cursor api")
Reviewed-by: Joe Thornber <thornber@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Author: mingnus

drivers/md/persistent-data/dm-array.c

@@ -917,23 +917,27 @@ static int load_ablock(struct dm_array_cursor *c)
 	if (c->block)
 		unlock_ablock(c->info, c->block);
 
-	c->block = NULL;
-	c->ab = NULL;
 	c->index = 0;
 
 	r = dm_btree_cursor_get_value(&c->cursor, &key, &value_le);
 	if (r) {
 		DMERR("dm_btree_cursor_get_value failed");
-		dm_btree_cursor_end(&c->cursor);
+		goto out;
 
 	} else {
 		r = get_ablock(c->info, le64_to_cpu(value_le), &c->block, &c->ab);
 		if (r) {
 			DMERR("get_ablock failed");
-			dm_btree_cursor_end(&c->cursor);
+			goto out;
 		}
 	}
 
+	return 0;
+
+out:
+	dm_btree_cursor_end(&c->cursor);
+	c->block = NULL;
+	c->ab = NULL;
 	return r;
 }