[PATCH v2 1/3] dm array: fix releasing a faulty array block twice in dm_array_cursor_end

[Ming-Hung Tsai]

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.

Signed-off-by: Ming-Hung Tsai <mt...@redhat.com>
Fixes: fdd1315aa5f0 ("dm array: introduce cursor api")
---
 drivers/md/persistent-data/dm-array.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/md/persistent-data/dm-array.c 
b/drivers/md/persistent-data/dm-array.c
index 157c9bd2fed7..4866ff56125f 100644
--- a/drivers/md/persistent-data/dm-array.c
+++ b/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;
 }
 
-- 
2.47.0