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

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

On Thu, Mar 09, 2023 at 11:43:46AM +0100, Cornelia Huck wrote:
> On Thu, Mar 02 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_queue_update_rings() after
> > virtio_queue_set_num() if we are not already calling
> > virtio_queue_set_rings().
> 
> Don't we instead need to call virtio_init_region_cache() to update the
> caches? virtio_queue_set_rings() will calculate avail and used from
> desc, which looks wrong for modern devices.


Carlos?

> >
> > Signed-off-by: Carlos López <clopez@suse.de>
> > ---
> >  hw/s390x/virtio-ccw.c   | 1 +
> >  hw/virtio/virtio-mmio.c | 5 ++---
> >  hw/virtio/virtio-pci.c  | 1 +
> >  3 files changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
> > index e33e5207ab..89891ac58a 100644
> > --- a/hw/s390x/virtio-ccw.c
> > +++ b/hw/s390x/virtio-ccw.c
> > @@ -237,6 +237,7 @@ static int virtio_ccw_set_vqs(SubchDev *sch, VqInfoBlock *info,
> >                  return -EINVAL;
> >              }
> >              virtio_queue_set_num(vdev, index, num);
> > +            virtio_queue_update_rings(vdev, index);
> 
> Note that this is the non-legacy path.
> 
> >          } else if (virtio_queue_get_num(vdev, index) > num) {
> >              /* Fail if we don't have a big enough queue. */
> >              return -EINVAL;
> > diff --git a/hw/virtio/virtio-mmio.c b/hw/virtio/virtio-mmio.c
> > index 23ba625eb6..c74822308f 100644
> > --- a/hw/virtio/virtio-mmio.c
> > +++ b/hw/virtio/virtio-mmio.c
> > @@ -350,10 +350,9 @@ static void virtio_mmio_write(void *opaque, hwaddr offset, uint64_t value,
> >      case VIRTIO_MMIO_QUEUE_NUM:
> >          trace_virtio_mmio_queue_write(value, VIRTQUEUE_MAX_SIZE);
> >          virtio_queue_set_num(vdev, vdev->queue_sel, value);
> > +        virtio_queue_update_rings(vdev, vdev->queue_sel);
> >  
> > -        if (proxy->legacy) {
> > -            virtio_queue_update_rings(vdev, vdev->queue_sel);
> > -        } else {
> > +        if (!proxy->legacy) {
> >              proxy->vqs[vdev->queue_sel].num = value;
> >          }
> >          break;
> > diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> > index 247325c193..a0a2f2c965 100644
> > --- a/hw/virtio/virtio-pci.c
> > +++ b/hw/virtio/virtio-pci.c
> > @@ -1554,6 +1554,7 @@ static void virtio_pci_common_write(void *opaque, hwaddr addr,
> >          proxy->vqs[vdev->queue_sel].num = val;
> >          virtio_queue_set_num(vdev, vdev->queue_sel,
> >                               proxy->vqs[vdev->queue_sel].num);
> > +        virtio_queue_update_rings(vdev, vdev->queue_sel);
> >          break;
> >      case VIRTIO_PCI_COMMON_Q_MSIX:
> >          vector = virtio_queue_vector(vdev, vdev->queue_sel);