Subject: RE: [RFC] Handling device free after a packet is passed to the network stack

From: "Harout Hedeshian" <[email-masked]>
Date: Mon, 30 Mar 2015 08:19:47 -0600

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]
> On Behalf Of Eric Dumazet
> Sent: Friday, March 27, 2015 8:49 PM
> To: [email-masked]
> Cc: [email-masked]
> Subject: Re: [RFC] Handling device free after a packet is passed to the
> network stack
> 
> On Fri, 2015-03-27 at 19:57 +0000, [email-masked] wrote:
> > We have been coming across a couple of scenarios where the device is
> > freed and the corresponding packets which were already queued up the
> > stack encounter crashes when they find that contents of skb->dev are
> > no longer valid.
> >
> > Specifically, we have observed an instance where a cpu hotplug occurs
> > along with the network driver module unloading. When the packets are
> > being queued up the stack using netif_rx_ni from dev_cpu_callback,
> > get_rps_cpus crashes as it encounters invalid data at skb->dev since
> > it would have been freed.
> >
> > We would like to know if the kernel provides some mechanisms to
> > safeguard against such scenarios.
> 
> This is supposed to be handled in flush_backlog() (net/core/dev.c)

It looks like this only takes care of packets in the softnet_data queues (or am I missing something). Wouldn't we run into similar issues if the packet were stuck in, for example, TCP OFO socket queue (not sure if anything tries to access skb->dev after that)? Or perhaps some other queue which may later try to dereference skb->dev. 

It seems to me that the only way to be absolutely sure that there is no reference on this net_device would be to have the drivers dev_hold() before earch netif_rx/netif_rx_ni/netif_receive_skb; and then later have __kfree_skb() do a dev_put().

Of course, this approach has a number of issues:
- 2 extra operations per packet
- Anything holding onto an SKB can prevent a net_device from deregistering/freeing
- requires skbs to have a valid dev before they can be freed (or we simply null check this field in __kfree_skb())

> Maybe you hit some race condition in on_each_cpu().
> 
> Really, I do not think we have tested such stress conditions ever.
> 
> cpu hotplug was very unstable 2 years ago (last time I tried it, this
> was constantly failing outside of networking land)
> 
> I would try following patch, although I am not sure what would prevent
> dev_cpu_callback() being called too late.
> 
> diff --git a/net/core/dev.c b/net/core/dev.c index
> a0408d497dae04e7caa145f05c915b058aa2d356..fa74887adfe10709bce55ccf1e34d6
> 5c6b7a8fba 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -7149,11 +7149,17 @@ static int dev_cpu_callback(struct
> notifier_block *nfb,
> 
>  	/* Process offline CPU's input_pkt_queue */
>  	while ((skb = __skb_dequeue(&oldsd->process_queue))) {
> -		netif_rx_ni(skb);
> +		if (skb->dev->reg_state != NETREG_REGISTERED)
> +			kfree_skb(skb);
> +		else
> +			netif_rx_ni(skb);
>  		input_queue_head_incr(oldsd);
>  	}
>  	while ((skb = skb_dequeue(&oldsd->input_pkt_queue))) {
> -		netif_rx_ni(skb);
> +		if (skb->dev->reg_state != NETREG_REGISTERED)
> +			kfree_skb(skb);
> +		else
> +			netif_rx_ni(skb);
>  		input_queue_head_incr(oldsd);
>  	}
> 
> 
> 
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