Re: [Qemu-devel] Re: [PATCH qemu-kvm] Add raw(af_packet) network backend to qemu

Anthony Liguori
2010-01-27T14:14:29Z

On 01/27/2010 12:52 AM, Arnd Bergmann wrote:
> On Wednesday 27 January 2010, Anthony Liguori wrote:
>
>>> The raw backend can be attached to a physical device
>>>
>> This is equivalent to bridging with tun/tap except that it has the
>> unexpected behaviour of unreliable host/guest networking (which is not
>> universally consistent across platforms either). This is not a mode we
>> want to encourage users to use.
>>
> It's not the most common scenario, but I've seen systems (I remember
> one on s/390 with z/VM) where you really want to isolate the guest
> network as much as possible from the host network. Besides PCI
> passthrough, giving the host device to a guest using a raw socket
> is the next best approximation of that.
>

But if you care about isolation, it's the worst possible thing to do.
If a guest breaks into qemu, it's one bind() away from accessing any
other guests network.

Using a bridge with a single interface on it is much better from an
isolation perspective.

>> In general, what I would like to see for
>> this is something more user friendly that dealt specifically with this
>> use-case. Although honestly, given the recent security concerns around
>> raw sockets, I'm very concerned about supporting raw sockets in qemu at all.
>>
>> Essentially, you get worse security doing vhost-net + raw + VF then with
>> PCI passthrough + VF because at least in the later case you can run qemu
>> without privileges. CAP_NET_RAW is a very big privilege.
>>
> It can be contained to a large degree with network namespaces. When you
> run qemu in its own namespace and add the VF to that, CAP_NET_RAW
> should ideally have no effect on other parts of the system (except
> bugs in the namespace implementation).
>

That's a pretty big hammer to hit this problem with. QEMU should not
require CAP_NET_RAW and so far has been able to avoid it quite
successfully. So far, I haven't heard a compelling reason that to use
raw other than bridging can be complicated to setup.

If we had the equivalent of a raw socket that could be bound to a socket
and then "locked" such that it could be safely handed to a
non-privileged process, then it would be a different story.

Regards,

Anthony Liguori