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Commit 3ee7e869 authored by Jan Kara's avatar Jan Kara Committed by Jens Axboe
Browse files

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.

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: default avatarsyzbot <syzbot+4a7438e774b21ddd8eca@syzkaller.appspotmail.com>
Acked-by: default avatarTejun Heo <tj@kernel.org>
Signed-off-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 0ae52ddf

include/linux/backing-dev-defs.h

+1 −1
Original line number Diff line number Diff line
@@ -22,7 +22,6 @@ struct dentry; 
 */

enum wb_state {

	WB_registered,		/* bdi_register() was done */

	WB_shutting_down,	/* wb_shutdown() in progress */

	WB_writeback_running,	/* Writeback is in progress */

	WB_has_dirty_io,	/* Dirty inodes on ->b_{dirty|io|more_io} */

	WB_start_all,		/* nr_pages == 0 (all) work pending */
@@ -189,6 +188,7 @@ struct backing_dev_info { 
#ifdef CONFIG_CGROUP_WRITEBACK

	struct radix_tree_root cgwb_tree; /* radix tree of active cgroup wbs */

	struct rb_root cgwb_congested_tree; /* their congested states */

	struct mutex cgwb_release_mutex;  /* protect shutdown of wb structs */

#else

	struct bdi_writeback_congested *wb_congested;

#endif

mm/backing-dev.c

+7 −13
Original line number Diff line number Diff line
@@ -359,15 +359,8 @@ static void wb_shutdown(struct bdi_writeback *wb) 
	spin_lock_bh(&wb->work_lock);

	if (!test_and_clear_bit(WB_registered, &wb->state)) {

		spin_unlock_bh(&wb->work_lock);

		/*

		 * Wait for wb shutdown to finish if someone else is just

		 * running wb_shutdown(). Otherwise we could proceed to wb /

		 * bdi destruction before wb_shutdown() is finished.

		 */

		wait_on_bit(&wb->state, WB_shutting_down, TASK_UNINTERRUPTIBLE);

		return;

	}

	set_bit(WB_shutting_down, &wb->state);

	spin_unlock_bh(&wb->work_lock);



	cgwb_remove_from_bdi_list(wb);
@@ -379,12 +372,6 @@ 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));

	/*

	 * Make sure bit gets cleared after shutdown is finished. Matches with

	 * the barrier provided by test_and_clear_bit() above.

	 */

	smp_wmb();

	clear_and_wake_up_bit(WB_shutting_down, &wb->state);

}



static void wb_exit(struct bdi_writeback *wb)
@@ -508,10 +495,12 @@ static void cgwb_release_workfn(struct work_struct *work) 
	struct bdi_writeback *wb = container_of(work, struct bdi_writeback,

						release_work);



	mutex_lock(&wb->bdi->cgwb_release_mutex);

	wb_shutdown(wb);



	css_put(wb->memcg_css);

	css_put(wb->blkcg_css);

	mutex_unlock(&wb->bdi->cgwb_release_mutex);



	fprop_local_destroy_percpu(&wb->memcg_completions);

	percpu_ref_exit(&wb->refcnt);
@@ -697,6 +686,7 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi) 


	INIT_RADIX_TREE(&bdi->cgwb_tree, GFP_ATOMIC);

	bdi->cgwb_congested_tree = RB_ROOT;

	mutex_init(&bdi->cgwb_release_mutex);



	ret = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL);

	if (!ret) {
@@ -717,7 +707,10 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi) 
	spin_lock_irq(&cgwb_lock);

	radix_tree_for_each_slot(slot, &bdi->cgwb_tree, &iter, 0)

		cgwb_kill(*slot);

	spin_unlock_irq(&cgwb_lock);



	mutex_lock(&bdi->cgwb_release_mutex);

	spin_lock_irq(&cgwb_lock);

	while (!list_empty(&bdi->wb_list)) {

		wb = list_first_entry(&bdi->wb_list, struct bdi_writeback,

				      bdi_node);
@@ -726,6 +719,7 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi) 
		spin_lock_irq(&cgwb_lock);

	}

	spin_unlock_irq(&cgwb_lock);

	mutex_unlock(&bdi->cgwb_release_mutex);

}



/**