[PATCH 3.16 258/366] bdi: Fix oops in wb_workfn()

Ben Hutchings
2018-10-14T23:12:04Z

From: Jan Kara <jack@suse.cz>

commit b8b784958eccbf8f51ebeee65282ca3fd59ea391 upstream.

Syzbot has reported that it can hit a NULL pointer dereference in
wb_workfn() due to wb->bdi->dev being NULL. This indicates that
wb_workfn() was called for an already unregistered bdi which should not
happen as wb_shutdown() called from bdi_unregister() should make sure
all pending writeback works are completed before bdi is unregistered.
Except that wb_workfn() itself can requeue the work with:

mod_delayed_work(bdi_wq, &wb->dwork, 0);

and if this happens while wb_shutdown() is waiting in:

flush_delayed_work(&wb->dwork);

the dwork can get executed after wb_shutdown() has finished and
bdi_unregister() has cleared wb->bdi->dev.

Make wb_workfn() use wakeup_wb() for requeueing the work which takes all
the necessary precautions against racing with bdi unregistration.

CC: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
CC: Tejun Heo <tj@kernel.org>
Fixes: 839a8e8660b6777e7fe4e80af1a048aebe2b5977
Reported-by: syzbot <syzbot+9873874c735f2892e7e9@syzkaller.appspotmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
[bwh: Backported to 3.16:
- Use bdi_wakeup_thread() instead of wb_wakeup()
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/fs-writeback.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -1071,7 +1071,7 @@ void bdi_writeback_workfn(struct work_st
}

if (!list_empty(&bdi->work_list))
- mod_delayed_work(bdi_wq, &wb->dwork, 0);
+ bdi_wakeup_thread(bdi);
else if (wb_has_dirty_io(wb) && dirty_writeback_interval)
bdi_wakeup_thread_delayed(bdi);