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

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Message-Id: <20240705104821.3202-1-hdanton@sina.com>
Date: Fri,  5 Jul 2024 18:48:21 +0800
From: Hillf Danton <hdanton@...a.com>
To: Florian Westphal <fw@...len.de>
Cc: 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

On Thu, 4 Jul 2024 12:54:18 +0200 Florian Westphal <fw@...len.de>
> Hillf Danton <hdanton@...a.com> wrote:
> > On Wed, 3 Jul 2024 15:01:07 +0200 Florian Westphal <fw@...len.de>
> > > Hillf Danton <hdanton@...a.com> wrote:
> > > > On Wed, 3 Jul 2024 12:52:15 +0200 Florian Westphal <fw@...len.de>
> > > > > Hillf Danton <hdanton@...a.com> wrote:
> > > > > > Given trans->table goes thru the lifespan of trans, your proposal is a bandaid
> > > > > > if trans outlives table.
> > > > > 
> > > > > trans must never outlive table.
> > > > > 
> > > > What is preventing trans from being freed after closing sock, given
> > > > trans is freed in workqueue?
> > > > 
> > > > 	close sock
> > > > 	queue work
> > > 
> > > The notifier acquires the transaction mutex, locking out all other
> > > transactions, so no further transactions requests referencing
> > > the table can be queued.
> > > 
> > As per the syzbot report, trans->table could be instantiated before
> > notifier acquires the transaction mutex. And in fact the lock helps
> > trans outlive table even with your patch.
> > 
> > 	cpu1			cpu2
> > 	---			---
> > 	transB->table = A
> > 				lock trans mutex
> > 				flush work
> > 				free A
> > 				unlock trans mutex
> > 
> > 	queue work to free transB
> 
> Can you show a crash reproducer or explain how this assign
> and queueing happens unordered wrt. cpu2?
> 
Not so difficult.

> This should look like this:
> 
>  	cpu1			cpu2
>  	---			---
> 	lock trans mutex
>   				lock trans mutex -> blocks
>  	transB->table = A
>   	queue work to free transB
> 	unlock trans mutex
> 				lock trans mutex returns
>   				flush work
>   				free A
>   				unlock trans mutex
> 
If your patch is correct, it should survive a warning.

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git  1c5fc27bc48a

--- x/net/netfilter/nf_tables_api.c
+++ y/net/netfilter/nf_tables_api.c
@@ -11552,9 +11552,10 @@ static int nft_rcv_nl_event(struct notif
 
 	gc_seq = nft_gc_seq_begin(nft_net);
 
-	if (!list_empty(&nf_tables_destroy_list))
-		nf_tables_trans_destroy_flush_work();
+	nf_tables_trans_destroy_flush_work();
 again:
+	WARN_ON(!list_empty(&nft_net->commit_list));
+
 	list_for_each_entry(table, &nft_net->tables, list) {
 		if (nft_table_has_owner(table) &&
 		    n->portid == table->nlpid) {
--