wireguard: device: do not generate ICMP for non-IP packets

If skb->protocol doesn't match the actual skb->data header, it's
probably not a good idea to pass it off to icmp{,v6}_ndo_send, which is
expecting to reply to a valid IP packet. So this commit has that early
mismatch case jump to a later error label.

Fixes: e7096c13 ("net: WireGuard secure network tunnel")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/wireguard/device.c

@@ -138,7 +138,7 @@ static netdev_tx_t wg_xmit(struct sk_buff *skb, struct net_device *dev)
 		else if (skb->protocol == htons(ETH_P_IPV6))
 			net_dbg_ratelimited("%s: No peer has allowed IPs matching %pI6\n",
 					    dev->name, &ipv6_hdr(skb)->daddr);
-		goto err;
+		goto err_icmp;
 	}
 
 	family = READ_ONCE(peer->endpoint.addr.sa_family);
@@ -201,12 +201,13 @@ static netdev_tx_t wg_xmit(struct sk_buff *skb, struct net_device *dev)
 
 err_peer:
 	wg_peer_put(peer);
-err:
-	++dev->stats.tx_errors;
+err_icmp:
 	if (skb->protocol == htons(ETH_P_IP))
 		icmp_ndo_send(skb, ICMP_DEST_UNREACH, ICMP_HOST_UNREACH, 0);
 	else if (skb->protocol == htons(ETH_P_IPV6))
 		icmpv6_ndo_send(skb, ICMPV6_DEST_UNREACH, ICMPV6_ADDR_UNREACH, 0);
+err:
+	++dev->stats.tx_errors;
 	kfree_skb(skb);
 	return ret;
 }