linux-kernel - Re: [PATCH nf] netfilter: nf_tables: unconditionally flush pending work before notifier

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Message-ID: <20240708115831.GA1289@breakpoint.cc>
Date: Mon, 8 Jul 2024 13:58:31 +0200
From: Florian Westphal <fw@...len.de>
To: Hillf Danton <hdanton@...a.com>
Cc: Florian Westphal <fw@...len.de>, Tejun Heo <tj@...nel.org>,
	netfilter-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, syzkaller-bugs@...glegroups.com,
	syzbot+4fd66a69358fc15ae2ad@...kaller.appspotmail.com
Subject: Re: [PATCH nf] netfilter: nf_tables: unconditionally flush pending
 work before notifier

Hillf Danton <hdanton@...a.com> wrote:
> On Sun, 7 Jul 2024 10:08:24 +0200 Florian Westphal <fw@...len.de>
> > Hillf Danton <hdanton@...a.com> wrote:
> > > > I think this change might be useful as it also documents
> > > > this requirement.
> > > 
> > > Yes it is boy and the current reproducer triggered another warning [1,2].
> > > 
> > > [1] https://lore.kernel.org/lkml/20240706231332.3261-1-hdanton@sina.com/
> > 
> > The WARN is incorrect.  The destroy list can be non-empty; i already
> > tried to explain why.
> >
> That warning as-is could be false positive but it could be triggered with a
> single netns.

How?

> 	cpu1		cpu2		cpu3
> 	---		---		---
> 					nf_tables_trans_destroy_work()
> 					spin_lock(&nf_tables_destroy_list_lock);
> 
> 					// 1) clear the destroy list
> 					list_splice_init(&nf_tables_destroy_list, &head);
> 					spin_unlock(&nf_tables_destroy_list_lock);
> 
> 			nf_tables_commit_release()
> 			spin_lock(&nf_tables_destroy_list_lock);
> 
> 			// 2) refill the destroy list
> 			list_splice_tail_init(&nft_net->commit_list, &nf_tables_destroy_list);
> 			spin_unlock(&nf_tables_destroy_list_lock);
> 			schedule_work(&trans_destroy_work);
> 			mutex_unlock(&nft_net->commit_mutex);

So you're saying work can be IDLE after schedule_work()?

I'm not following at all.