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: syzbot <syzbot+4a7438e774b21ddd8eca@syzkaller.appspotmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 0bd432a4d7bd..24251762c20c 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -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