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>

1 files changed, 1 insertions, 3 deletions

diff --git a/hw/virtio.c b/hw/virtio.c
index 6e8814c..a651860 100644
--- a/hw/virtio.c
+++ b/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)