Commit 205fa50750 ("qcow2: Add subcluster support to zero_in_l2_slice()")
introduced a subtle change to code in zero_in_l2_slice:
It swapped the order of
1. qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
2. set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO);
3. qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST);
To
1. qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
2. qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST);
3. set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO);
It seems harmless, however the call to qcow2_free_any_clusters
can trigger a cache flush which can mark the L2 table as clean,
and assuming that this was the last write to it,
a stale version of it will remain on the disk.
Now we have a valid L2 entry pointing to a freed cluster. Oops.
Fixes: 205fa50750 ("qcow2: Add subcluster support to zero_in_l2_slice()")
Signed-off-by: Maxim Levitsky <[email protected]>
---
block/qcow2-cluster.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 485b4cb92e..267b46a4ca 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -2010,11 +2010,11 @@ static int zero_in_l2_slice(BlockDriverState *bs,
uint64_t offset,
continue;
}
- qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
if (unmap) {
qcow2_free_any_cluster(bs, old_l2_entry, QCOW2_DISCARD_REQUEST);
}
set_l2_entry(s, l2_slice, l2_index + i, new_l2_entry);
+ qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
if (has_subclusters(s)) {
set_l2_bitmap(s, l2_slice, l2_index + i, new_l2_bitmap);
}
--
2.26.2
