Re: [PATCH 1/1] Fix qcow2 corruption on discard

[Kevin Wolf <kwolf@redhat.com>]

Am 23.11.2020 um 16:49 hat Maxim Levitsky geschrieben:
> 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.

Do you have more details on this last paragraph? I'm trying to come up
with a reproducer, but I don't see how qcow2_free_any_clusters() could
flush the L2 table cache. (It's easy to get it to flush the refcount
block cache, but that's useless for a reproducer.)

The only way I see to flush any cache with it is in update_refcount()
the qcow2_cache_set_dependency() call. This will always flush the cache
that the L2 cache depends on - which will never be the L2 cache itself,
but always either the refcount cache or nothing.

There are more options in alloc_refcount_block() if we're allocating a
new refcount block, but in the context of freeing clusters we'll never
need to do that.

Whatever I tried, at the end of zero_in_l2_slice(), I have a dirty L2
table and a dirty refcount block in the cache, with a dependency that
makes sure that the L2 table will be written out first.

If you don't have the information yet, can you try to debug your manual
reproducer a bit more to find out how this happens?

Kevin

> 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
>