Re: [Qemu-devel] [PATCH v3 1/2] virtio: check VirtQueue Vring object is set

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 28 Nov 2017 16:57:34 +0530 (IST)
P J P <ppandit@redhat.com> wrote:

> +-- On Tue, 28 Nov 2017, Stefan Hajnoczi wrote --+
> | > This is conflating different things:
> | > - vq does not exist (num == 0)
> | > - vq is not setup by the guest (desc == 0)
> | > - vq has no valid alignment (which is only relevant for legacy)
> | 
> | I agree.
> 
> Either case, vq would be unfit for use, no?

What is "unfit for use"?

I'm not quite sure what you want to achieve with this patch. I assume
you want to fix the issue that a guest may provide invalid values for
align etc. which can cause qemu to crash or behave badly.

If so, you need to do different things for the different points above.
- The guest should not muck around with a non-existing queue (num == 0)
  in any case, so this should be fenced for any manipulation triggered
  by the guest.
- Processing a non-setup queue (desc == 0; also applies to the other
  buffers for virtio-1) should be skipped. However, _setting_ desc etc.
  to 0 from the guest is fine (as long as it follows the other
  constraints of the spec).
- Setting alignment to 0 only applies to legacy + virtio-mmio. I would
  not overengineer fencing this. A simple check in update_rings should
  be enough.