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

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Message-Id: <20240704103514.3035-1-hdanton@sina.com>
Date: Thu,  4 Jul 2024 18:35:14 +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 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

> The work queue is flushed before potentially ripping the table
> out.  After this, no transactions referencing the table can exist
> anymore; the only transactions than can still be queued are those
> coming from a different netns, and tables are scoped per netns.
> 
> Table is torn down.  Transaction mutex is released.
> 
> Next transaction from userspace can't find the table anymore (its gone),
> so no more transactions can be queued for this table.
> 
> As I wrote in the commit message, the flush is dumb, this should first
> walk to see if there is a matching table to be torn down, and then flush
> work queue once before tearing the table down.
> 
> But its better to clearly split bug fix and such a change.