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

[Hillf Danton <hdanton@sina.com>]

On Tue, 4 Jun 2024 11:09:58 +0300 Leon Romanovsky <leon@kernel.org>
> On Mon, Jun 03, 2024 at 10:10:36AM -1000, Tejun Heo wrote:
> > 
> > And KASAN is reporting use-after-free on a completely unrelated VFS object.
> > I can't tell for sure from the logs alone but lockdep_register_key()
> > iterates entries in the hashtable trying to find whether the key is a
> > duplicate and it could be that that walk is triggering the use-after-free
> > warning. If so, it doesn't really have much to do with workqueue. The
> > corruption happened elsewhere and workqueue just happens to traverse the
> > hashtable afterwards.
> 
> The problem is that revert of commit 643445531829
> ("workqueue: Fix UAF report by KASAN in pwq_release_workfn()")
> fixed these use-after-free reports.
> 
Given revert makes sense,

	if (alloc_and_link_pwqs(wq) < 0)
		goto err_unreg_lockdep;

err_unreg_lockdep:
	wq_unregister_lockdep(wq);
	wq_free_lockdep(wq);
err_free_wq:
	free_workqueue_attrs(wq->unbound_attrs);
	kfree(wq);	<-- freed
	return NULL;

the difference 643445531829 makes is double free.

	alloc_and_link_pwqs(struct workqueue_struct *wq)
	if (ret)
		kthread_flush_worker(pwq_release_worker);
		  pwq_release_workfn()
		  if (is_last) {
			wq_unregister_lockdep(wq);
			call_rcu(&wq->rcu, rcu_free_wq); <-- freed again
		  }