[Qemu-devel] Re: [PATCH 5/5] virtio-serial: Error out if guest sends unexpected vq elements

[Amit Shah <amit.shah@redhat.com>]

On (Fri) Dec 10 2010 [15:17:58], Paul Brook wrote:
> > On (Fri) Dec 10 2010 [13:59:50], Paul Brook wrote:
> > > > Check if the guest really sent any items in the out_vq before using
> > > > them.  Similarly, check if there is a buffer to send data in before
> > > > writing.
> > > 
> > > Can this actually happen? If so why/how?
> > > Why does it need a special case in this device?
> > 
> > A malicious guest (ie, a guest with the virtio_console module suitably
> > modified) could send in buffers with the 'input' bit set instead of
> > output as expected or vice-versa.
> 
> So what? Who cares if they get it wrong?

Just let the error_report() be there and continue as if nothing
happened?

> It's entirely unclear whether this is actually an error. If a request has zero 
> size then we just transfer zero bytes, exactly as requested.
> 
> Even if you accept this should be a diagnosable error, I suspect your patch is 
> still insufficient. I don't see any code to check that input queue requests 
> have zero output segments, nor do I see anything to handle zero-length 
> segments.

virtio actually supports sending both, input as well as output types of
buffers in one go.

> > > If this is guest triggerable then calling abort() is wrong.
> > 
> > It's either a guest bug or a malicious guest.  What action is
> > recommended?
> 
> Killing the whole VM in response to a malformed request to a device is clearly 
> a bug in that device.  You should report an error to the guest in the normal 
> manner.  IIRC virtio lacks any consistent error reporting mechanisms, and the 
> usual response when asked to do something impossible is to reset the device.

OK, agreed.

		Amit