[PATCH] btrfs: don't propagate EXTENT_FLAG_LOGGING to split extent maps

[Leo Martins <loemra.dev@gmail.com>]

When btrfs_drop_extent_map_range() splits an extent map, the new split
maps inherit the original map's flags through a local 'flags' variable.
Commit f86f7a75e2fb ("btrfs: use the flags of an extent map to identify
the compression type") changed the EXTENT_FLAG_LOGGING clearing to
operate on em->flags instead of that local 'flags' copy, so a split of
an extent map that is currently being logged wrongly inherits
EXTENT_FLAG_LOGGING.

The flag is then never cleared on the split, and when it is freed while
still on the inode's modified_extents list (for example by the extent
map shrinker) it trips the WARN_ON(!list_empty(&em->list)) in
btrfs_free_extent_map() and leads to a use-after-free.

Clear EXTENT_FLAG_LOGGING from the local 'flags' copy used for the
splits and only clear EXTENT_FLAG_PINNED from em->flags, restoring the
behaviour prior to f86f7a75e2fb.

Fixes: f86f7a75e2fb ("btrfs: use the flags of an extent map to identify the compression type")
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Boris Burkov <boris@bur.io>
Signed-off-by: Leo Martins <loemra.dev@gmail.com>
---
 fs/btrfs/extent_map.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
index fce9c5cc0122..6ad7b39ae358 100644
--- a/fs/btrfs/extent_map.c
+++ b/fs/btrfs/extent_map.c
@@ -866,13 +866,13 @@ void btrfs_drop_extent_map_range(struct btrfs_inode *inode, u64 start, u64 end,
 			goto next;
 		}
 
-		flags = em->flags;
 		/*
 		 * In case we split the extent map, we want to preserve the
 		 * EXTENT_FLAG_LOGGING flag on our extent map, but we don't want
 		 * it on the new extent maps.
 		 */
-		em->flags &= ~(EXTENT_FLAG_PINNED | EXTENT_FLAG_LOGGING);
+		flags = em->flags & ~EXTENT_FLAG_LOGGING;
+		em->flags &= ~EXTENT_FLAG_PINNED;
 		modified = !list_empty(&em->list);
 
 		/*
-- 
2.53.0-Meta