[Qemu-devel] Re: [PATCHv2 10/12] tap: add vhost/vhostfd options

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Feb 28, 2010 at 10:08:26AM -0600, Anthony Liguori wrote:
> On 02/27/2010 01:44 PM, Michael S. Tsirkin wrote:
>>> and it doesn't
>>> support all of the features of userspace virtio.  Since it's in upstream
>>> Linux without supporting all of the virtio-net features, it's something
>>> we're going to have to deal with for a long time.
>>>      
>> Speaking of vlan filtering etc?  It's just a matter of time before it
>> supports all interesting features. Kernel support is there in net-next
>> already, userspace should be easy too. I should be able to code it up
>> once I finish bothering about upstream merge (hint hint :)).
>>    
>
> :-)  As I've said in the past, I'm willing to live with -net tap,vhost  
> but I really think -net vhost would be better in the long run.
>
> The only two real issues I have with the series is the ring address  
> mapping stability

This one I do not yet understand completely to be able solve.  Is the
only case where PCI BAR overlays RAM?  I think this case is best dealt
with by disabling BAR mapping.



> and the duplicated slot management code.

If you look at qemu-kvm, it's even triplicated :) I just would like to
get the code merged, then work at adding more infrastructure to prettify
it.

> Both have  security implications so I think it's important that they
> be addressed.   Otherwise, I'm pretty happy with how things are.

Care suggesting some solutions?

>
>>> Furthermore, vhost reduces a virtual machine's security.  It offers an
>>> impressive performance boost (particularly when dealing with 10gbit+
>>> networking) but for a user that doesn't have such strong networking
>>> performance requirements, I think it's reasonable for them to not want
>>> to make a security trade off.
>>>      
>> It's hard for me to see how it reduces VM security. If it does, it's
>> not by design and will be fixed.
>>    
>
> If you have a bug in vhost-net (would never happen of course) then it's  
> a host-kernel exploit whereas if we have a bug in virtio-net userspace,  
> it's a local user exploit.  We have a pretty robust architecture to deal  
> with local user exploits (qemu can run unprivilieged, SELinux enforces  
> mandatory access control) but a host-kernel can not be protected against.
> 
> I'm not saying that we should never put things in the kernel, but  
> there's definitely a security vs. performance trade off here.
>
> Regards,
>
> Anthony Liguori

Not sure I get the argument completely. Any kernel service with a bug
might be exploited for priveledge escalation. Yes, more kernel code
gives you more attack surface, but given we use rich interfaces such as
ones exposed by kvm, I am not sure by how much.

Also note that vhost net does not take qemu out of the equation for
everything, just for datapath operations.

-- 
MST