asix: Add rx->ax_skb = NULL after usbnet_skb_return()

In asix_rx_fixup_internal() there is a risk that rx->ax_skb gets
reused after passing the Ethernet frame into the network stack via
usbnet_skb_return().

The risks include:

a) asynchronously freeing rx->ax_skb after passing the netdev buffer
   to the NAPI layer which might corrupt the backlog queue.

b) erroneously reusing rx->ax_skb such as calling skb_put_data() multiple
   times which causes writing off the end of the netdev buffer.

Therefore add a defensive rx->ax_skb = NULL after usbnet_skb_return()
so that it is not possible to free rx->ax_skb or to apply
skb_put_data() too many times.

Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/asix_common.c

@@ -168,8 +168,10 @@ int asix_rx_fixup_internal(struct usbnet *dev, struct sk_buff *skb,
 		if (rx->ax_skb) {
 			skb_put_data(rx->ax_skb, skb->data + offset,
 				     copy_length);
-			if (!rx->remaining)
+			if (!rx->remaining) {
 				usbnet_skb_return(dev, rx->ax_skb);
+				rx->ax_skb = NULL;
+			}
 		}
 
 		offset += (copy_length + 1) & 0xfffe;