[PATCH] bdi: Fix another oops in wb_workfn()

Tetsuo Handa
2018-05-27T02:21:25Z

>From 8a8222698163d1fe180258566e9a3ff43f54fcd9 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Sun, 27 May 2018 11:08:20 +0900
Subject: [PATCH] bdi: Fix another oops in wb_workfn()

syzbot is still hitting NULL pointer dereference at wb_workfn() [1].
This might be because we overlooked that delayed_work_timer_fn() does not
check WB_registered before calling __queue_work() while mod_delayed_work()
does not wait for already started delayed_work_timer_fn() because it uses
del_timer() rather than del_timer_sync().

Make wb_shutdown() be careful about wb_wakeup_delayed() path.

[1] https://syzkaller.appspot.com/bug?id=e0818ccb7e46190b3f1038b0c794299208ed4206

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+4a7438e774b21ddd8eca@syzkaller.appspotmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
---
mm/backing-dev.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 7441bd9..31e1d7e 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -372,11 +372,24 @@ static void wb_shutdown(struct bdi_writeback *wb)

cgwb_remove_from_bdi_list(wb);
/*
+ * mod_delayed_work() is not appropriate here, for
+ * delayed_work_timer_fn() from wb_wakeup_delayed() does not check
+ * WB_registered before calling __queue_work().
+ */
+ del_timer_sync(&wb->dwork.timer);
+ /*
+ * Clear WORK_STRUCT_PENDING_BIT in order to make sure that next
+ * queue_delayed_work() actually enqueues this work to the tail, for
+ * wb_wakeup_delayed() already set WORK_STRUCT_PENDING_BIT before
+ * scheduling delayed_work_timer_fn().
+ */
+ cancel_delayed_work_sync(&wb->dwork);
+ /*
* Drain work list and shutdown the delayed_work. !WB_registered
* tells wb_workfn() that @wb is dying and its work_list needs to
* be drained no matter what.
*/
- mod_delayed_work(bdi_wq, &wb->dwork, 0);
+ queue_delayed_work(bdi_wq, &wb->dwork, 0);
flush_delayed_work(&wb->dwork);
WARN_ON(!list_empty(&wb->work_list));
/*
--
1.8.3.1