[PATCH 1/1] Fix qcow2 corruption on discard

[Maxim Levitsky <mlevitsk@redhat.com>]

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 <mlevitsk@redhat.com>
---
 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