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

From: Sowmini Varadhan <[email-masked]>
Date: Thu, 9 Feb 2017 19:33:06 -0800

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?

--Sowmini