netdev - Re: [PATCH v2 net-next 9/9] l2tp: flush workqueue before draining it

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Message-ID: <Zrzw8oe+LBoTModP@pop-os.localdomain>
Date: Wed, 14 Aug 2024 11:01:22 -0700
From: Cong Wang <xiyou.wangcong@...il.com>
To: James Chapman <jchapman@...alix.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, dsahern@...nel.org,
	tparkin@...alix.com, horms@...nel.org,
	syzbot+0e85b10481d2f5478053@...kaller.appspotmail.com
Subject: Re: [PATCH v2 net-next 9/9] l2tp: flush workqueue before draining it

On Mon, Aug 12, 2024 at 10:24:11AM +0100, James Chapman wrote:
> On 11/08/2024 18:54, Cong Wang wrote:
> > On Wed, Aug 07, 2024 at 07:54:52AM +0100, James Chapman wrote:
> > > syzbot exposes a race where a net used by l2tp is removed while an
> > > existing pppol2tp socket is closed. In l2tp_pre_exit_net, l2tp queues
> > > TUNNEL_DELETE work items to close each tunnel in the net. When these
> > > are run, new SESSION_DELETE work items are queued to delete each
> > > session in the tunnel. This all happens in drain_workqueue. However,
> > > drain_workqueue allows only new work items if they are queued by other
> > > work items which are already in the queue. If pppol2tp_release runs
> > > after drain_workqueue has started, it may queue a SESSION_DELETE work
> > > item, which results in the warning below in drain_workqueue.
> > > 
> > > Address this by flushing the workqueue before drain_workqueue such
> > > that all queued TUNNEL_DELETE work items run before drain_workqueue is
> > > started. This will queue SESSION_DELETE work items for each session in
> > > the tunnel, hence pppol2tp_release or other API requests won't queue
> > > SESSION_DELETE requests once drain_workqueue is started.
> > > 
> > 
> > I am not convinced here.
> > 
> > 1) There is a __flush_workqueue() inside drain_workqueue() too. Why
> > calling it again?
> 
> Once drain_workqueue starts, it considers any new work item queued outside
> of its processing as an unexpected condition. By doing __flush_workqueue
> first, we ensure that all TUNNEL_DELETE items are processed, which means
> that SESSION_DELETE work items are queued for all existing sessions before
> drain_workqueue starts. Now if an API request is processed to delete a
> session, l2tp_session_delete will do nothing because session->dead has
> already been set.

What about the following case?

CPU 0			CPU1
test_and_set_bit()
			__flush_workqueue()
  queue_work()
  			drain_workqueue()

> 
> > 2) What prevents new work items be queued right between your
> > __flush_workqueue() and drain_workqueue()?
> 
> Hmm, to do so, a new tunnel would need to be created and then deleted by API
> request once drain_workqueue starts. Is it possible to create new sockets in
> a net once a net's pre_exit starts? I didn't think so, but I'll recheck.

I don't quite understand why you need to create a new one while tearing
down the old one(s). I think what you need here is some way to properly
waiting for the socket tear down to finish.

Thanks.