[PATCH 4.19 078/139] btrfs: relocation: set trans to be NULL after ending transaction

[Greg Kroah-Hartman]

4.19-stable 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")
CC: stable@xxxxxxxxxxxxxxx # 4.4+
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/btrfs/relocation.c |    1 +
 1 file changed, 1 insertion(+)

--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -3963,6 +3963,7 @@ static noinline_for_stack int relocate_b
 restart:
 		if (update_backref_cache(trans, &rc->backref_cache)) {
 			btrfs_end_transaction(trans);
+			trans = NULL;
 			continue;
 		}