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

From: ebiederm-aS9lmoZGLiVWk0Htik3J/[email-masked] (Eric W. Biederman)
Date: Fri, 16 Nov 2012 18:46:11 -0800

Ben Hutchings <bhutchings-s/n/[email-masked]> writes:

> On Fri, 2012-11-16 at 06:32 -0800, Eric W. Biederman wrote:
>> Glauber Costa <[email-masked]> 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_SEEPROM => brick the NIC
> ETHTOOL_FLASHDEV => brick the NIC; own the system if it's not using an IOMMU

These are prevented by not having access to real hardware by default. A
physical network interface must be moved into a network namespace for
you to have access to it.

There are a handful of software network devices that are generally safe
macvlan, veth, tun, ipip tunnels, etc.  Using those network devices is
very interesting and about as performant as you can get while still
being safe.

A buffer overflow in an ethtool command looks as likely to me as being
able to own the system by reflashing the NIC.

Access to a real physical NIC is an act of trust.  Given the general
linux policy that drivers are merged when they mostly work I don't
currently know of any trust models between "I trust you with full access
to this device" and "I don't trust you with direct access to this
device" that I would feel confident giving to an untrusted user.

Which is a convoluted way of saying "ip link set eth0 netns bob" is the
moral equivalent of "chown bob.bob /dev/eth0; chmod u+rwx /dev/eth0"

> ETHTOOL_SMSGLVL => fill up the system log

That one might be worth doing something about, as there is non-local
effect.  Still I don't believe for any of the software based "safe"
networking devices ETHTOOL_SMSGLVL has any effect, and being able to
tweak the debug level could be important for debugging if you do have
direct access to the NIC.

Eric