netvsc: fix use-after-free in netvsc_change_mtu()

'nvdev' is freed in rndis_filter_device_remove -> netvsc_device_remove ->
free_netvsc_device, so we mustn't access it, before it's re-created in
rndis_filter_device_add -> netvsc_device_add.
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Reviewed-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hyperv/netvsc_drv.c

@@ -859,15 +859,22 @@ static int netvsc_change_mtu(struct net_device *ndev, int mtu)
 	if (ret)
 		goto out;
 
+	memset(&device_info, 0, sizeof(device_info));
+	device_info.ring_size = ring_size;
+	device_info.num_chn = nvdev->num_chn;
+	device_info.max_num_vrss_chns = nvdev->num_chn;
+
 	ndevctx->start_remove = true;
 	rndis_filter_device_remove(hdev, nvdev);
 
+	/* 'nvdev' has been freed in rndis_filter_device_remove() ->
+	 * netvsc_device_remove () -> free_netvsc_device().
+	 * We mustn't access it before it's re-created in
+	 * rndis_filter_device_add() -> netvsc_device_add().
+	 */
+
 	ndev->mtu = mtu;
 
-	memset(&device_info, 0, sizeof(device_info));
-	device_info.ring_size = ring_size;
-	device_info.num_chn = nvdev->num_chn;
-	device_info.max_num_vrss_chns = nvdev->num_chn;
 	rndis_filter_device_add(hdev, &device_info);
 
 out: