virtio-net: validate backend queue numbers against bus limitation

We don't validate the backend queue numbers against bus limitation,
this will easily crash qemu if it exceeds the limitation which will
hit the abort() in virtio_del_queue(). An example is trying to
starting a virtio-net device with 256 queues. E.g:

./qemu-system-x86_64 -netdev tap,id=hn0,queues=256 -device
virtio-net-pci,netdev=hn0

Fixing this by doing the validation and fail early.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: qemu-stable <qemu-stable@nongnu.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/net/virtio-net.c

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 27adcc5..59f76bc 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -1588,6 +1588,13 @@
     virtio_init(vdev, "virtio-net", VIRTIO_ID_NET, n->config_size);
 
     n->max_queues = MAX(n->nic_conf.peers.queues, 1);
+    if (n->max_queues * 2 + 1 > VIRTIO_PCI_QUEUE_MAX) {
+        error_setg(errp, "Invalid number of queues (= %" PRIu32 "), "
+                   "must be a postive integer less than %d.",
+                   n->max_queues, (VIRTIO_PCI_QUEUE_MAX - 1) / 2);
+        virtio_cleanup(vdev);
+        return;
+    }
     n->vqs = g_malloc0(sizeof(VirtIONetQueue) * n->max_queues);
     n->vqs[0].rx_vq = virtio_add_queue(vdev, 256, virtio_net_handle_rx);
     n->curr_queues = 1;