Re: [PATCH/RFC] user_ns: fix missing limiting of user_ns counts

[Vasily Kulikov <segoon-cxoSlKxDwOJWk0Htik3J/w@public.gmane.org>]

On Fri, Dec 28, 2012 at 20:05 -0800, Eric W. Biederman wrote:
> > A related issue which is NOT FIXED HERE is limits for all resources
> > available for containerized pseudo roots.  E.g. I succeeded creating
> > thousands of veth network devices without problems by a non-root user,
> > there seems no limit in number of network devices.  I suspect it is
> > possible to setup routing and net_ns'es the way it will be very
> > time-consuming for kernel to handle IP packets inside of ksoftirq, which
> > is not counted as this user scheduler time.   I suppose the issue is not
> > veth-specific, almost all newly available for unprivileged users code
> > pathes are vulnerable to DoS attacks.
> 
> veth at least should process packets synchronously so I don't see how
> you will get softirq action.

What do you mean -- synchronously?  From my limited understanding of
veth job, it is handled like every network packet in system, via:

    veth_xmit() -> dev_forward_skb() -> netif_rx() -> enqueue_to_backlog()

enqueue_to_backlog() adds the packet to softnet_data->input_pkt_queue. 

Then inside of softirq process_backlog() moves ->input_pkt_queue to
->process_queue and calls __netif_receive_skb(), which does all networking
stack magic.

AFAICS, one could create user_ns, net_ns inside of it, and setup routing
tables and netfilter to infinitely pass few network packets from and to
veth, abusing ksoftirq.

-- 
Vasily Kulikov
http://www.openwall.com - bringing security into open computing environments