virtio_net: clear MTU when out of range

virtio attempts to clear the MTU feature bit if the value is out of the
supported range, but this has no real effect since FEATURES_OK has
already been set.

Fix this up by checking the MTU in the new validate callback.

Fixes: 14de9d114a82 ("virtio-net: Add initial MTU advice feature")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/net/virtio_net.c

@@ -2230,14 +2230,8 @@ static bool virtnet_validate_features(struct virtio_device *vdev)
 #define MIN_MTU ETH_MIN_MTU
 #define MAX_MTU ETH_MAX_MTU
 
-static int virtnet_probe(struct virtio_device *vdev)
+static int virtnet_validate(struct virtio_device *vdev)
 {
-	int i, err;
-	struct net_device *dev;
-	struct virtnet_info *vi;
-	u16 max_queue_pairs;
-	int mtu;
-
 	if (!vdev->config->get) {
 		dev_err(&vdev->dev, "%s failure: config access disabled\n",
 			__func__);
@@ -2247,6 +2241,25 @@ static int virtnet_probe(struct virtio_device *vdev)
 	if (!virtnet_validate_features(vdev))
 		return -EINVAL;
 
+	if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {
+		int mtu = virtio_cread16(vdev,
+					 offsetof(struct virtio_net_config,
+						  mtu));
+		if (mtu < MIN_MTU)
+			__virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
+	}
+
+	return 0;
+}
+
+static int virtnet_probe(struct virtio_device *vdev)
+{
+	int i, err;
+	struct net_device *dev;
+	struct virtnet_info *vi;
+	u16 max_queue_pairs;
+	int mtu;
+
 	/* Find if host supports multiqueue virtio_net device */
 	err = virtio_cread_feature(vdev, VIRTIO_NET_F_MQ,
 				   struct virtio_net_config,
@@ -2362,12 +2375,17 @@ static int virtnet_probe(struct virtio_device *vdev)
 				     offsetof(struct virtio_net_config,
 					      mtu));
 		if (mtu < dev->min_mtu) {
-			__virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);
-		} else {
-			dev->mtu = mtu;
-			dev->max_mtu = mtu;
+			/* Should never trigger: MTU was previously validated
+			 * in virtnet_validate.
+			 */
+			dev_err(&vdev->dev, "device MTU appears to have changed "
+				"it is now %d < %d", mtu, dev->min_mtu);
+			goto free_stats;
 		}
 
+		dev->mtu = mtu;
+		dev->max_mtu = mtu;
+
 		/* TODO: size buffers correctly in this case. */
 		if (dev->mtu > ETH_DATA_LEN)
 			vi->big_packets = true;
@@ -2548,6 +2566,7 @@ static struct virtio_driver virtio_net_driver = {
 	.driver.name =	KBUILD_MODNAME,
 	.driver.owner =	THIS_MODULE,
 	.id_table =	id_table,
+	.validate =	virtnet_validate,
 	.probe =	virtnet_probe,
 	.remove =	virtnet_remove,
 	.config_changed = virtnet_config_changed,