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

From: Florian Westphal <[email-masked]>
Date: Mon, 8 Jul 2024 14:43:28 +0200

Hillf Danton <[email-masked]> wrote:
> On Mon, 8 Jul 2024 13:58:31 +0200 Florian Westphal <[email-masked]>
> > Hillf Danton <[email-masked]> wrote:
> > > On Sun, 7 Jul 2024 10:08:24 +0200 Florian Westphal <[email-masked]>
> > > > Hillf Danton <[email-masked]> 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/[email-masked]/
> > > > 
> > > > 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?
> > 
> You saw the below cpu diagram, no?

It did not explain the problem in a way I understand.

>	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);

This means @work is running on cpu3 and made a snapshot of the list.
I don't even understand how thats relevant, but OK.

>			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);

Means CPU2 has added transaction structures that could
reference @table to list.

It also called schedule_work BEFORE releasing the mutex and
after placing entries on destroy list.

> nft_rcv_nl_event()
> mutex_lock(&nft_net->commit_mutex);
> flush_work(&trans_destroy_work);

Means cpu1 serializes vs. cpu2, @work
was scheduled.

flush_work() must only return if @work is idle, without
any other pending execution.

If it gets scheduled again right after flush_work
returns that is NOT a problem, as I tried to explain several times.

We hold the transaction mutex, only a different netns can queue more
work, and such foreign netns can only see struct nft_table structures
that are private to their namespaces.

> // 3) flush work ends with the refilled destroy list left intact
> tear tables down

Again, I do not understand how its possible.

The postcondition after flush_work returns is:

1. nf_tables_destroy_list must be empty, UNLESS its from unrelated
   net namespaces, they cannot see the tables we're tearing down in 3),
   so they cannot reference them.

2. nf_tables_trans_destroy_work() is NOT running, unless its
   processing entries queued by other netns, after flush work
   returned.

cpu2 does:
   -> add trans->table to @nf_tables_destroy_list
   -> unlock list spinlock
   -> schedule_work
   -> unlock mutex

cpu1 does:
 -> lock mutex
 -> flush work

You say its not enough and that trans->table queued by cpu2 can still
be on @nf_tables_destroy_list.

I say flush_work after taking the mutex guarantees strans->table has been
processed by @work in all cases.