ieee802154: 6lowpan: fix memory leak

Looking at current situation of memory management in 6lowpan receive
function I detected some invalid handling. After calling
lowpan_invoke_rx_handlers we will do a kfree_skb and then NET_RX_DROP on
error handling. We don't do this before, also on
skb_share_check/skb_unshare which might manipulate the reference
counters.

After running some 'grep -r "dev_add_pack" net/' to look how others
packet-layer receive callbacks works I detected that every subsystem do
a kfree_skb, then NET_RX_DROP without calling skb functions which
might manipulate the skb reference counters. This is the reason why we
should do the same here like all others subsystems. I didn't find any
documentation how the packet-layer receive callbacks handle NET_RX_DROP
return values either.

This patch will add a kfree_skb, then NET_RX_DROP handling for the
"trivial checks", in case of skb_share_check/skb_unshare the kfree_skb
call will be done inside these functions.
Signed-off-by: NAlexander Aring <alex.aring@gmail.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/ieee802154/6lowpan/rx.c

@@ -284,16 +284,16 @@ static int lowpan_rcv(struct sk_buff *skb, struct net_device *wdev,
 	if (wdev->type != ARPHRD_IEEE802154 ||
 	    skb->pkt_type == PACKET_OTHERHOST ||
 	    !lowpan_rx_h_check(skb))
-		return NET_RX_DROP;
+		goto drop;
 
 	ldev = wdev->ieee802154_ptr->lowpan_dev;
 	if (!ldev || !netif_running(ldev))
-		return NET_RX_DROP;
+		goto drop;
 
 	/* Replacing skb->dev and followed rx handlers will manipulate skb. */
 	skb = skb_share_check(skb, GFP_ATOMIC);
 	if (!skb)
-		return NET_RX_DROP;
+		goto out;
 	skb->dev = ldev;
 
 	/* When receive frag1 it's likely that we manipulate the buffer.
@@ -304,10 +304,15 @@ static int lowpan_rcv(struct sk_buff *skb, struct net_device *wdev,
 	    lowpan_is_iphc(*skb_network_header(skb))) {
 		skb = skb_unshare(skb, GFP_ATOMIC);
 		if (!skb)
-			return NET_RX_DROP;
+			goto out;
 	}
 
 	return lowpan_invoke_rx_handlers(skb);
+
+drop:
+	kfree_skb(skb);
+out:
+	return NET_RX_DROP;
 }
 
 static struct packet_type lowpan_packet_type = {