>From 014c4149f2e24cd26b278b32d5dfda056eecf093 Mon Sep 17 00:00:00 2001 From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> Date: Sat, 9 Jun 2018 22:47:52 +0900 Subject: [PATCH] bdi: Fix another oops in wb_workfn() 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
