Re: net/packet: use-after-free in packet_rcv_fanout

From: Cong Wang
Date: Fri Feb 10 2017 - 13:09:00 EST

On Thu, Feb 9, 2017 at 7:33 PM, Sowmini Varadhan
<sowmini.varadhan@xxxxxxxxxx> wrote:
> On (02/09/17 19:19), Eric Dumazet wrote:
>> More likely the bug is in fanout_add(), with a buggy sequence in error
>> case, and not correct locking.
>> kfree(po->rollover);
>> po->rollover = NULL;
>> Two cpus entering fanout_add() (using the same af_packet socket,
>> syzkaller courtesy...) might both see po->fanout being NULL.
>> Then they grab the mutex. Too late...
> I'm not sure I follow- aiui the panic was in acceessing the
> sk_receive_queue.lock in a socket that had been closed earlier. I think
> the assumption is that rcu_read_lock_bh in __dev_queue_xmit (and
> rcu_read_lock in dev_queue_xmit_nit?) should make sure that the nit
> packet delivery can be done safely, and the synchronize_net in
> packet_release() makes sure that the Tx paths are quiesced before freeing
> the socket. What is the race-hole here? Does it have to do with the
> _bh and softirq context, somehow?

My understanding about the race here is packet_release() doesn't
wait for flying packets correctly, which leads to a flying packet still
refers to the struct sock which is being released.

This could happen because struct packet_fanout is refcn'ted, it is
still there when this is not the last sock referring it, therefore, the
callback packet_rcv_fanout() is not removed yet. When packet_release()
tries to remove the pointer to struct sock from f->arr[i] in
__fanout_unlink(), a flying packet could race with f->arr[i]:

po = pkt_sk(f->arr[idx]);

Of course, the fix may not be as easy as just adding a synchronize_net(),
perhaps we need the spinlock too in fanout_demux_rollover().

At least I believe this explains the crash Dmitry reported.