Re: [PATCH -rc] workqueue: Reimplement UAF fix to avoid lockdep worning

[Hillf Danton <hdanton@sina.com>]

On Tue, 4 Jun 2024 21:58:04 +0300 Leon Romanovsky <leon@kernel.org>
> On Tue, Jun 04, 2024 at 06:30:49AM -1000, Tejun Heo wrote:
> > On Tue, Jun 04, 2024 at 02:38:34PM +0300, Leon Romanovsky wrote:
> > > Thanks, it is very rare situation where call to flush/drain queue
> > > (in our case kthread_flush_worker) in the middle of the allocation
> > > flow can be correct. I can't remember any such case.
> > >
> > > So even we don't fully understand the root cause, the reimplementation
> > > is still valid and improves existing code.
> > 
> > It's not valid. pwq release is async and while wq free in the error path
> > isn't. The flush is there so that we finish the async part before
> > synchronize error handling. The patch you posted will can lead to double
> > free after a pwq allocation failure. We can make the error path synchronous
> > but the pwq free path should be updated first so that it stays synchronous
> > in the error path. Note that it *needs* to be asynchronous in non-error
> > paths, so it's going to be a bit subtle one way or the other.
> 
> But at that point, we didn't add newly created WQ to any list which will execute
> that asynchronous release. Did I miss something?
> 
Maybe it is more subtle than thought, but not difficult to make the wq
allocation path sync. See if the patch could survive your test.

--- x/include/linux/workqueue.h
+++ y/include/linux/workqueue.h
@@ -402,6 +402,7 @@ enum wq_flags {
 	 */
 	WQ_POWER_EFFICIENT	= 1 << 7,
 
+	__WQ_INITIALIZING 	= 1 << 14, /* internal: workqueue is initializing */
 	__WQ_DESTROYING		= 1 << 15, /* internal: workqueue is destroying */
 	__WQ_DRAINING		= 1 << 16, /* internal: workqueue is draining */
 	__WQ_ORDERED		= 1 << 17, /* internal: workqueue is ordered */
--- x/kernel/workqueue.c
+++ y/kernel/workqueue.c
@@ -5080,6 +5080,8 @@ static void pwq_release_workfn(struct kt
 	 * is gonna access it anymore.  Schedule RCU free.
 	 */
 	if (is_last) {
+		if (wq->flags & __WQ_INITIALIZING)
+			return;
 		wq_unregister_lockdep(wq);
 		call_rcu(&wq->rcu, rcu_free_wq);
 	}
@@ -5714,8 +5716,10 @@ struct workqueue_struct *alloc_workqueue
 			goto err_unreg_lockdep;
 	}
 
+	wq->flags |= __WQ_INITIALIZING;
 	if (alloc_and_link_pwqs(wq) < 0)
 		goto err_free_node_nr_active;
+	wq->flags &= ~__WQ_INITIALIZING;
 
 	if (wq_online && init_rescuer(wq) < 0)
 		goto err_destroy;
--