Re: net: possible deadlock in skb_queue_tail

[Andrey Konovalov]

On Fri, Feb 24, 2017 at 3:56 AM, Florian Westphal <fw@xxxxxxxxx> wrote:
> Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
>
> [ CC Paolo ]
>
>> I've got the following error report while fuzzing the kernel with syzkaller.
>>
>> On commit c470abd4fde40ea6a0846a2beab642a578c0b8cd (4.10).
>>
>> Unfortunately I can't reproduce it.
>
> This needs NETLINK_BROADCAST_ERROR enabled on a netlink socket
> that then subscribes to netfilter conntrack (ctnetlink) events.
> probably syzkaller did this by accident -- impressive.
>
> (one task is the ctnetlink event redelivery worker
>  which won't be scheduled otherwise).
>
>> ======================================================
>> [ INFO: possible circular locking dependency detected ]
>> 4.10.0-rc8+ #201 Not tainted
>> -------------------------------------------------------
>> kworker/0:2/1404 is trying to acquire lock:
>>  (&(&list->lock)->rlock#3){+.-...}, at: [<ffffffff8335b23f>]
>> skb_queue_tail+0xcf/0x2f0 net/core/skbuff.c:2478
>>
>> but task is already holding lock:
>>  (&(&pcpu->lock)->rlock){+.-...}, at: [<ffffffff8366b55f>] spin_lock
>> include/linux/spinlock.h:302 [inline]
>>  (&(&pcpu->lock)->rlock){+.-...}, at: [<ffffffff8366b55f>]
>> ecache_work_evict_list+0xaf/0x590
>> net/netfilter/nf_conntrack_ecache.c:48
>>
>> which lock already depends on the new lock.
>
> Cong is correct, this is a false positive.
>
> However we should fix this splat.
>
> Paolo, this happens since 7c13f97ffde63cc792c49ec1513f3974f2f05229
> ('udp: do fwd memory scheduling on dequeue'), before this
> commit kfree_skb() was invoked outside of the locked section in
> first_packet_length().
>
> cpu 0 call chain:
> - first_packet_length (hold udp sk_receive_queue lock)
>    - kfree_skb
>       - nf_conntrack_destroy
>          - spin_lock(net->ct.pcpu->lock)
>
> cpu 1 call chain:
> - ecache_work_evict_list
>   - spin_lock( net->ct.pcpu->lock)
>   - nf_conntrack_event
>      - aquire netlink socket sk_receive_queue
>
> So this could only ever deadlock if a netlink socket
> calls kfree_skb while holding its sk_receive_queue lock, but afaics
> this is never the case.
>
> There are two ways to avoid this splat (other than lockdep annotation):
>
> 1. re-add the list to first_packet_length() and free the
> skbs outside of locked section.
>
> 2. change ecache_work_evict_list to not call nf_conntrack_event()
> while holding the pcpu lock.
>
> doing #2 might be a good idea anyway to avoid potential deadlock
> when kfree_skb gets invoked while other cpu holds its sk_receive_queue
> lock, I'll have a look if this is feasible.

Hi!

Any updates on this?

I might have missed the patch if there was one.

Thanks!