ieee802154: 6lowpan: check on valid 802.15.4 frame

This patch adds frame control checks to check if the received frame is
something which could contain a 6LoWPAN packet.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

include/net/mac802154.h

@@ -249,6 +249,21 @@ struct ieee802154_ops {
 						const bool on);
 };
 
+/**
+ * ieee802154_get_fc_from_skb - get the frame control field from an skb
+ * @skb: skb where the frame control field will be get from
+ */
+static inline __le16 ieee802154_get_fc_from_skb(const struct sk_buff *skb)
+{
+	/* return some invalid fc on failure */
+	if (unlikely(skb->mac_len < 2)) {
+		WARN_ON(1);
+		return cpu_to_le16(0);
+	}
+
+	return (__force __le16)__get_unaligned_memmove16(skb_mac_header(skb));
+}
+
 /**
  * ieee802154_be64_to_le64 - copies and convert be64 to le64
  * @le64_dst: le64 destination pointer