From e1f8b721426e0ec30fed2d5ba4f83d25ebe9e55c Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Thu, 3 May 2018 18:26:26 +0200 Subject: [PATCH] bdi: Fix oops in wb_workfn() 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 CC: Tejun Heo Fixes: 839a8e8660b6777e7fe4e80af1a048aebe2b5977 Reported-by: syzbot Reviewed-by: Dave Chinner Signed-off-by: Jan Kara Signed-off-by: Jens Axboe [bwh: Backported to 3.16: - Use bdi_wakeup_thread() instead of wb_wakeup() - Adjust context] Signed-off-by: Ben Hutchings --- fs/fs-writeback.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 8d6005c248c8..bec5e4b5d3e0 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -1072,7 +1072,7 @@ void bdi_writeback_workfn(struct work_struct *work) } 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);