virtio: guard against negative vq notifies

The virtio_queue_notify() function checks that the virtqueue number is
less than the maximum number of virtqueues.  A signed comparison is used
but the virtqueue number could be negative if a buggy or malicious guest
is run.  This results in memory accesses outside of the virtqueue array.

It is risky doing input validation in common code instead of at the
guest<->host boundary.  Note that virtio_queue_set_addr(),
virtio_queue_get_addr(), virtio_queue_get_num(), and many other virtio
functions do *not* validate the virtqueue number argument.

Instead of fixing the comparison in virtio_queue_notify(), move the
comparison to the virtio bindings (just like VIRTIO_PCI_QUEUE_SEL) where
we have a uint32_t value and can avoid ever calling into common virtio
code if the virtqueue number is invalid.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/syborg_virtio.c

@@ -146,7 +146,9 @@ static void syborg_virtio_writel(void *opaque, target_phys_addr_t offset,
             vdev->queue_sel = value;
         break;
     case SYBORG_VIRTIO_QUEUE_NOTIFY:
-        virtio_queue_notify(vdev, value);
+        if (value < VIRTIO_PCI_QUEUE_MAX) {
+            virtio_queue_notify(vdev, value);
+        }
         break;
     case SYBORG_VIRTIO_STATUS:
         virtio_set_status(vdev, value & 0xFF);

hw/virtio-pci.c

@@ -348,7 +348,9 @@ static void virtio_ioport_write(void *opaque, uint32_t addr, uint32_t val)
             vdev->queue_sel = val;
         break;
     case VIRTIO_PCI_QUEUE_NOTIFY:
-        virtio_queue_notify(vdev, val);
+        if (val < VIRTIO_PCI_QUEUE_MAX) {
+            virtio_queue_notify(vdev, val);
+        }
         break;
     case VIRTIO_PCI_STATUS:
         if (!(val & VIRTIO_CONFIG_S_DRIVER_OK)) {

hw/virtio.c

@@ -585,9 +585,7 @@ void virtio_queue_notify_vq(VirtQueue *vq)
 
 void virtio_queue_notify(VirtIODevice *vdev, int n)
 {
-    if (n < VIRTIO_PCI_QUEUE_MAX) {
-        virtio_queue_notify_vq(&vdev->vq[n]);
-    }
+    virtio_queue_notify_vq(&vdev->vq[n]);
 }
 
 uint16_t virtio_queue_vector(VirtIODevice *vdev, int n)