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

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Date:	Wed, 18 Dec 2013 11:21:32 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Ding Tianhong <dingtianhong@...wei.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>, 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, Dec 18, 2013 at 06:02:33PM +0800, Ding Tianhong wrote:
> On 2013/12/18 17:28, Hannes Frederic Sowa wrote:
> > On Wed, Dec 18, 2013 at 04:57:01PM +0800, Ding Tianhong wrote:
> >> On 2013/12/18 16:41, Hannes Frederic Sowa wrote:
> >>> On Wed, Dec 18, 2013 at 04:19:43PM +0800, Ding Tianhong wrote:
> >>>> 0xffffffff812f8e29 <neigh_timer_handler+265>:   mov    0xe8(%rbx),%rax
> >>>> 0xffffffff812f8e30 <neigh_timer_handler+272>:   mov    %rbp,%rsi
> >>>> 0xffffffff812f8e33 <neigh_timer_handler+275>:   mov    %rbx,%rdi
> >>>> 0xffffffff812f8e36 <neigh_timer_handler+278>:   callq  *0x8(%rax)						<-----crash
> >>>> /usr/src/linux/net/core/neighbour.c: 877
> >>>> 0xffffffff812f8e39 <neigh_timer_handler+281>:   lea    0x3c(%rbx),%rax
> >>>
> >>> For me it looks like this:
> >>>
> >>> %rax is neigh->ops and the function pointer solicit is NULL and causes the the
> >>> page fault.
> >>>
> >>>
> >> yes, it is. So I was trying to find the situation that may free the neighbour when
> >> the timer is running, but I could not yet.
> > 
> > Hm. Ok. It is actually ops which is NULL, not the function pointer, may bad.
> > 
> > Could you try to follow param or table links and check if this is an arp or
> > ndisc one? Maybe some interactions with arp.c or ndisc.c causes this bug?
> > 
> > 
> 
> David and Eric has said that someone may called neigh_release in a wrong place, I agree with that,
> and review the code which calling the function in the kernel, I could not find any obvious problem,
> and doubt with the situation:

Maybe it could also be a reference count overflow and we wrap around to zero
again? Otherwise I agree, it really looks like this is the case.

> CPU0					  CPU1					  CPU2
>       --------		      			--------                		---------
> neigh_timer_handler				
> write_lock(n->lock);		
> 	...
> write_unlock(n->lock);
> n->ref_cnt = 2 or 3(if mode_time)				



> 	...					neigh_flush_dev
> 						write_lock(n->lock);
> 						n->ref_cnt = 2;
> 						n->nud_state = NUD_NONE;
> 						write_unlock(n->lock);
> 						neigh_release()
> 						n->ref_cnt = 1;
> 						...					neigh_periodic_work
> 											write_lock(n->lock);
> 											write_unlock(n->lock);
> 											neigh_release();
> 											kfree(n)
> n->ops->solicit()									...

On CPU0 the neigh_release happens after solicit. So the timer_handler should
still be guarded to not touch already freed memory. The table lock should make
sure that we either see a reference from the hash table or we don't (with
appropriate reference count). It looks consistent for me for now.

I guess you cannot reproduce this?

Greetings,

  Hannes

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