batman-adv: verify tt len does not exceed packet len

batadv_iv_ogm_process() accesses the packet using the tt_num_changes
attribute regardless of the real packet len (assuming the length check
was done before). Therefore a length check is needed to avoid reading
random memory.
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>

net/batman-adv/bat_iv_ogm.c

@@ -1288,7 +1288,8 @@ static int batadv_iv_ogm_receive(struct sk_buff *skb,
 	batadv_ogm_packet = (struct batadv_ogm_packet *)packet_buff;
 
 	/* unpack the aggregated packets and process them one by one */
-	do {
+	while (batadv_iv_ogm_aggr_packet(buff_pos, packet_len,
+					 batadv_ogm_packet->tt_num_changes)) {
 		tt_buff = packet_buff + buff_pos + BATADV_OGM_HLEN;
 
 		batadv_iv_ogm_process(ethhdr, batadv_ogm_packet, tt_buff,
@@ -1299,8 +1300,7 @@ static int batadv_iv_ogm_receive(struct sk_buff *skb,
 
 		packet_pos = packet_buff + buff_pos;
 		batadv_ogm_packet = (struct batadv_ogm_packet *)packet_pos;
-	} while (batadv_iv_ogm_aggr_packet(buff_pos, packet_len,
-					   batadv_ogm_packet->tt_num_changes));
+	}
 
 	kfree_skb(skb);
 	return NET_RX_SUCCESS;