Re: [PATCH] bdi: Fix another oops in wb_workfn()

Tejun Heo
2018-06-11T16:01:31Z

Hello,

On Mon, Jun 11, 2018 at 11:12:48AM +0200, Jan Kara wrote:
> However this is wrong and so is the patch. 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. The writeback structures we are
> accessing at this point can be already freed in principle like:
>
> 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
>
> I'm not 100% sure how to fix this. wb structures can be at various phases of
> shutdown (or there may be other external references still existing) when we
> enter cgwb_bdi_unregister() so I think adding a way for cgwb_bdi_unregister()
> to wait for standard wb shutdown path to finish is the most robust way.
> What do you think about attached patch Tejun? So far only compile tested...
>
> Possible problem with it is that now cgwb_bdi_unregister() will wait for
> all wb references to be dropped so it adds some implicit dependencies to
> bdi shutdown path.

Would something like the following work or am I missing the point
entirely?

Thanks.

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 347cc83..359cacd 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -715,14 +715,19 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
WARN_ON(test_bit(WB_registered, &bdi->wb.state));

spin_lock_irq(&cgwb_lock);
- radix_tree_for_each_slot(slot, &bdi->cgwb_tree, &iter, 0)
- cgwb_kill(*slot);
+ radix_tree_for_each_slot(slot, &bdi->cgwb_tree, &iter, 0) {
+ struct bdi_writeback *wb = *slot;
+
+ wb_get(wb);
+ cgwb_kill(wb);
+ }

while (!list_empty(&bdi->wb_list)) {
wb = list_first_entry(&bdi->wb_list, struct bdi_writeback,
bdi_node);
spin_unlock_irq(&cgwb_lock);
wb_shutdown(wb);
+ wb_put(wb);
spin_lock_irq(&cgwb_lock);
}
spin_unlock_irq(&cgwb_lock);

--
tejun