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

[Tetsuo Handa]

syzbot is reporting NULL pointer dereference at wb_workfn() [1] due to
wb->bdi->dev being NULL. And Dmitry confirmed that wb->state was
WB_shutting_down after wb->bdi->dev became NULL. This indicates that
unregister_bdi() failed to call wb_shutdown() on one of wb objects.

Since cgwb_bdi_unregister() from bdi_unregister() cannot call wb_shutdown()
on wb objects which have already passed list_del_rcu() in wb_shutdown(),
cgwb_bdi_unregister() from bdi_unregister() can return and set wb->bdi->dev
to NULL before such wb objects enter final round of wb_workfn() via
mod_delayed_work()/flush_delayed_work().

Since WB_registered is already cleared by wb_shutdown(), only wb_shutdown()
can schedule for final round of wb_workfn(). Since concurrent calls to
wb_shutdown() on the same wb object is safe because of WB_shutting_down
state, I think that wb_shutdown() can safely keep a wb object in the
bdi->wb_list until that wb object leaves final round of wb_workfn().
Thus, make wb_shutdown() call list_del_rcu() after flush_delayed_work().

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

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Reported-by: syzbot <syzbot+4a7438e774b21ddd8eca@xxxxxxxxxxxxxxxxxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Jens Axboe <axboe@xxxxxx>
---
 mm/backing-dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 347cc83..bef4b25 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -370,7 +370,6 @@ static void wb_shutdown(struct bdi_writeback *wb)
 	set_bit(WB_shutting_down, &wb->state);
 	spin_unlock_bh(&wb->work_lock);
 
-	cgwb_remove_from_bdi_list(wb);
 	/*
 	 * Drain work list and shutdown the delayed_work.  !WB_registered
 	 * tells wb_workfn() that @wb is dying and its work_list needs to
@@ -379,6 +378,7 @@ static void wb_shutdown(struct bdi_writeback *wb)
 	mod_delayed_work(bdi_wq, &wb->dwork, 0);
 	flush_delayed_work(&wb->dwork);
 	WARN_ON(!list_empty(&wb->work_list));
+	cgwb_remove_from_bdi_list(wb);
 	/*
 	 * Make sure bit gets cleared after shutdown is finished. Matches with
 	 * the barrier provided by test_and_clear_bit() above.
-- 
1.8.3.1