netdev - Re: [PATCH net] net: neighbour: add neighbour dead check for neigh_timer

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Date:	Tue, 03 Dec 2013 22:27:37 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ding Tianhong <dingtianhong@...wei.com>
Cc:	David Miller <davem@...emloft.net>, gaofeng@...fujitsu.com,
	yoshfuji@...ux-ipv6.org, joe@...ches.com, vfalico@...hat.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH net] net: neighbour: add neighbour dead check for
 neigh_timer_handler()

On Wed, 2013-12-04 at 14:19 +0800, Ding Tianhong wrote:
> On 2013/12/4 12:21, David Miller wrote:
> > From: Ding Tianhong <dingtianhong@...wei.com>
> > Date: Wed, 4 Dec 2013 12:04:31 +0800
> > 
> >> The destroying neigh could be trigger by userspace, just like set the ip address which
> >> in arp table to the local device ip, some I could not control it, it maybe anytime,
> >> but the timer handler is execute by logic, this is normal, so I think the logic
> >> is no problem, and the process of destroying neigh may conflict with the timer handler,
> >> it is a synchronous problem to make sure the timer should be finished before the
> >> reference neigh is freed.
> > 
> > The more I think about this, the more none of the explanations for this bug
> > make any sense.
> > 
> > neigh_destroy() _ONLY_ runs when:
> > 
> > 	if (atomic_dec_and_test(&neigh->refcnt))
> > 
> > triggers in neigh_release().
> > 
> > This means it triggers if, and only if, neigh_refcnt goes to zero.
> > 
> > If the refcnt goes to zero, NO TIMER can be running.  If the timer is
> > running, then there refcnt must be at least '1'.
> 
> Hi David:
> 
> Yes, you are right, but when the timer is running and prior to get the neigh->lock, the refcnt
> could be dec to 0, you could not stop it by existing mechanism.
> 
> the refcnt of neighbour could only be inc by these actions:
> 
> 1.create neighbour, the refcnt will be set to 1.
> 2.add timer, the refcnt++.
> 3.neigh_lookup, if found the neigh, refcnt++.
> 
> I can show the whole process of my analysis:
> 
> 		CPU 0				CPU 1
> 		-----				-----
> 	create_neigh() => refcnt = 1;		
> 	add timer =>	refcnt++;
> 						<SOFTIRQ>
> 						base->running_timer = neigh->timer;
> 						neigh_timer_handler() => at this time, refcnt is 2;
> 
> user->	neigh_changeaddr()
> 	neigh_flush_dev();
> 	neigh_del_imer, refcnt dec to 1;

Nope : del_timer() would return 0 here, so we do not decrement refcnt.

I can tell you, if this was not the case, a lot of things would be
terribly broken, like TCP stack.

> 	release_neigh(), refcnt is 0,
> 	destroy_neigh()
> 	kfree(neighbour);
> 						write(neigh->lock)
> 
> So in my opinion, the point of the problem is that I should not kfree the neighbour until
> the timer is not running on CPUs and not pending.
> 
> If I miss someghing, pls point out.

As David explained, if a timer is running, refcnt can not reach 0,
untill the timer handler finished.

So _something_ is calling neigh_release(n) without prior neigh_hold()




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