netdev - Re: [Patch nf] netfilter: xt_hashlimit: unregister proc file before releasing mutex

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Date:   Wed, 19 Feb 2020 19:32:13 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Pablo Neira Ayuso <pablo@...filter.org>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Florian Westphal <fw@...len.de>,
        NetFilter <netfilter-devel@...r.kernel.org>,
        syzbot <syzbot+d195fd3b9a364ddd6731@...kaller.appspotmail.com>
Subject: Re: [Patch nf] netfilter: xt_hashlimit: unregister proc file before
 releasing mutex

On Tue, Feb 18, 2020 at 2:05 PM Pablo Neira Ayuso <pablo@...filter.org> wrote:
>
> On Tue, Feb 18, 2020 at 01:40:26PM -0800, Cong Wang wrote:
> > On Tue, Feb 18, 2020 at 1:35 PM Pablo Neira Ayuso <pablo@...filter.org> wrote:
> > >
> > > On Wed, Feb 12, 2020 at 10:53:52PM -0800, Cong Wang wrote:
> > > > Before releasing the global mutex, we only unlink the hashtable
> > > > from the hash list, its proc file is still not unregistered at
> > > > this point. So syzbot could trigger a race condition where a
> > > > parallel htable_create() could register the same file immediately
> > > > after the mutex is released.
> > > >
> > > > Move htable_remove_proc_entry() back to mutex protection to
> > > > fix this. And, fold htable_destroy() into htable_put() to make
> > > > the code slightly easier to understand.
> > >
> > > Probably revert previous one?
> >
> > The hung task could appear again if we move the cleanup
> > back under mutex.
>
> How could the hung task appear again by reverting
> c4a3922d2d20c710f827? Please elaborate.

Because the cfg.max could be as large as 8*HASHLIMIT_MAX_SIZE:

 311         if (hinfo->cfg.max == 0)
 312                 hinfo->cfg.max = 8 * hinfo->cfg.size;
 313         else if (hinfo->cfg.max < hinfo->cfg.size)
 314                 hinfo->cfg.max = hinfo->cfg.size;

Not sure whether we can finish cleaning up 8*HASHLIMIT_MAX_SIZE
entries within the time a hung task tolerates. This largely depends on
how much contention the spinlock has, at least I don't want to bet
on it.

Thanks.