[PATCH] bdi: Fix use after free bug in debugfs_remove()

[Tetsuo Handa]

syzbot is reporting use after free bug in debugfs_remove() [1].

This is because fault injection made memory allocation for
debugfs_create_file() from bdi_debug_register() from bdi_register_va()
fail and continued with setting WB_registered. But when debugfs_remove()
is called from debugfs_remove(bdi->debug_dir) from bdi_debug_unregister()
 from bdi_unregister() from release_bdi() because WB_registered was set
by bdi_register_va(), IS_ERR_OR_NULL(bdi->debug_dir) == false despite
debugfs_remove(bdi->debug_dir) was already called from bdi_register_va().

Fix this by making IS_ERR_OR_NULL(bdi->debug_dir) == true.

[1] https://syzkaller.appspot.com/bug?id=5ab4efd91a96dcea9b68104f159adf4af2a6dfc1

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Reported-by: syzbot <syzbot+049cb4ae097049dac137@xxxxxxxxxxxxxxxxxxxxxxxxx>
Fixes: 97f07697932e6faf ("bdi: convert bdi_debug_register to int")
Cc: weiping zhang <zhangweiping@xxxxxxxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
---
 mm/backing-dev.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 023190c..7fe73fa 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -115,6 +115,7 @@ static int bdi_debug_register(struct backing_dev_info *bdi, const char *name)
 					       bdi, &bdi_debug_stats_fops);
 	if (!bdi->debug_stats) {
 		debugfs_remove(bdi->debug_dir);
+		bdi->debug_dir = NULL;
 		return -ENOMEM;
 	}
 
-- 
1.8.3.1