Re: [PATCH v2] virtio: refresh vring region cache after updating a virtqueue size

[Cornelia Huck]

On Fri, Mar 17 2023, Carlos López <clopez@suse.de> wrote:

> When a virtqueue size is changed by the guest via
> virtio_queue_set_num(), its region cache is not automatically updated.
> If the size was increased, this could lead to accessing the cache out
> of bounds. For example, in vring_get_used_event():
>
>     static inline uint16_t vring_get_used_event(VirtQueue *vq)
>     {
>         return vring_avail_ring(vq, vq->vring.num);
>     }
>
>     static inline uint16_t vring_avail_ring(VirtQueue *vq, int i)
>     {
>         VRingMemoryRegionCaches *caches = vring_get_region_caches(vq);
>         hwaddr pa = offsetof(VRingAvail, ring[i]);
>
>         if (!caches) {
>             return 0;
>         }
>
>         return virtio_lduw_phys_cached(vq->vdev, &caches->avail, pa);
>     }
>
> vq->vring.num will be greater than caches->avail.len, which will
> trigger a failed assertion down the call path of
> virtio_lduw_phys_cached().
>
> Fix this by calling virtio_init_region_cache() after
> virtio_queue_set_num() if we are not already calling
> virtio_queue_set_rings(). In the legacy path this is already done by
> virtio_queue_update_rings().
>
> Signed-off-by: Carlos López <clopez@suse.de>
> ---
> v2: use virtio_init_region_cache() instead of
> virtio_queue_update_rings() in the path for modern devices.
>
>  hw/s390x/virtio-ccw.c      | 1 +
>  hw/virtio/virtio-mmio.c    | 1 +
>  hw/virtio/virtio-pci.c     | 1 +
>  hw/virtio/virtio.c         | 2 +-
>  include/hw/virtio/virtio.h | 1 +
>  5 files changed, 5 insertions(+), 1 deletion(-)
>

Reviewed-by: Cornelia Huck <cohuck@redhat.com>

We can always do any ccw reshuffling on top.