netdev - Re: [PATCH net-next 09/17] net: Allow userns root control of the core of the network stack.

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Date:	Sat, 17 Nov 2012 00:28:36 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Glauber Costa <glommer@...allels.com>,
	David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
	Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: [PATCH net-next 09/17] net: Allow userns root control of the
 core of the network stack.

On Fri, 2012-11-16 at 06:32 -0800, Eric W. Biederman wrote:
> Glauber Costa <glommer@...allels.com> writes:
> 
> > On 11/16/2012 05:03 PM, Eric W. Biederman wrote:
> >> +	if (!capable(CAP_NET_ADMIN))
> >> +		return -EPERM;
> >> +
> >>  	return netdev_store(dev, attr, buf, len, change_tx_queue_len);
> >
> > You mean ns_capable here?
> 
> No.  There I meant capable.
> 
> I deliberately call capable here because I don't understand what
> the tx_queue_len well enough to be certain it is safe to relax
> that check to be just ns_capable.
> 
> My get feel is that allowing an unprivileged user to be able to
> arbitrarily change the tx_queue_len on a networking device would be a
> nice way to allow queuing as many network packets as you would like with
> kernel memory and DOSing the machine.
> 
> So since with a quick read of the code I could not convince myself it
> was safe to allow unprivilged users to change tx_queue_len I left it
> protected by capable.  While at the same time I relaxed the check in
> netdev_store to be ns_capable.

Tor the same reason you had better be very selective about which ethtool
commands are allowed based on per-user_ns CAP_NET_ADMIN.  Consider for a
start:

ETHTOOL_SMSGLVL => fill up the system log
ETHTOOL_SEEPROM => brick the NIC
ETHTOOL_FLASHDEV => brick the NIC; own the system if it's not using an IOMMU

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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