On 6/22/18 2:52 AM, Jan Kara wrote: > On Mon 18-06-18 10:40:14, Tejun Heo wrote: >> On Mon, Jun 18, 2018 at 03:46:58PM +0200, Jan Kara wrote: >>> 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. >>> >>> The problem is in cgwb_bdi_unregister() which does cgwb_kill() and thus >>> drops bdi's reference to wb structures before going through the list of >>> wbs again and calling wb_shutdown() on each of them. This way the loop >>> iterating through all wbs can easily miss a wb if that wb has already >>> passed through cgwb_remove_from_bdi_list() called from wb_shutdown() >>> from cgwb_release_workfn() and as a result fully shutdown bdi although >>> wb_workfn() for this wb structure is still running. In fact there are >>> also other ways cgwb_bdi_unregister() can race with >>> cgwb_release_workfn() leading e.g. to use-after-free issues: >>> >>> CPU1 CPU2 >>> cgwb_bdi_unregister() >>> cgwb_kill(*slot); >>> >>> cgwb_release() >>> queue_work(cgwb_release_wq, &wb->release_work); >>> cgwb_release_workfn() >>> wb = list_first_entry(&bdi->wb_list, ...) >>> spin_unlock_irq(&cgwb_lock); >>> wb_shutdown(wb); >>> ... >>> kfree_rcu(wb, rcu); >>> wb_shutdown(wb); -> oops use-after-free >>> >>> We solve these issues by synchronizing writeback structure shutdown from >>> cgwb_bdi_unregister() with cgwb_release_workfn() using a new mutex. That >>> way we also no longer need synchronization using WB_shutting_down as the >>> mutex provides it for CONFIG_CGROUP_WRITEBACK case and without >>> CONFIG_CGROUP_WRITEBACK wb_shutdown() can be called only once from >>> bdi_unregister(). >>> >>> Reported-by: syzbot <syzbot+4a7438e774b21ddd8eca@xxxxxxxxxxxxxxxxxxxxxxxxx> >>> Signed-off-by: Jan Kara <jack@xxxxxxx> >> >> Acked-by: Tejun Heo <tj@xxxxxxxxxx> > > OK, Jens, can you please pick up the fix? Thanks! Applied, thanks! -- Jens Axboe
