[PATCH 3.16 224/305] btrfs: relocation: set trans to be NULL after ending transaction

[Ben Hutchings]

3.16.63-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Pan Bian <bianpan2016@xxxxxxx>

commit 42a657f57628402c73237547f0134e083e2f6764 upstream.

The function relocate_block_group calls btrfs_end_transaction to release
trans when update_backref_cache returns 1, and then continues the loop
body. If btrfs_block_rsv_refill fails this time, it will jump out the
loop and the freed trans will be accessed. This may result in a
use-after-free bug. The patch assigns NULL to trans after trans is
released so that it will not be accessed.

Fixes: 0647bf564f1 ("Btrfs: improve forever loop when doing balance relocation")
Reviewed-by: Qu Wenruo <wqu@xxxxxxxx>
Signed-off-by: Pan Bian <bianpan2016@xxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/btrfs/relocation.c | 1 +
 1 file changed, 1 insertion(+)

--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -3955,6 +3955,7 @@ static noinline_for_stack int relocate_b
 restart:
 		if (update_backref_cache(trans, &rc->backref_cache)) {
 			btrfs_end_transaction(trans, rc->extent_root);
+			trans = NULL;
 			continue;
 		}