batman-adv: Only read OGM tvlv_len after buffer len check

Multiple batadv_ogm_packet can be stored in an skbuff. The functions
batadv_iv_ogm_send_to_if()/batadv_iv_ogm_receive() use
batadv_iv_ogm_aggr_packet() to check if there is another additional
batadv_ogm_packet in the skb or not before they continue processing the
packet.

The length for such an OGM is BATADV_OGM_HLEN +
batadv_ogm_packet->tvlv_len. The check must first check that at least
BATADV_OGM_HLEN bytes are available before it accesses tvlv_len (which is
part of the header. Otherwise it might try read outside of the currently
available skbuff to get the content of tvlv_len.

Fixes: ef261577 ("batman-adv: tvlv - basic infrastructure")
Reported-by: syzbot+355cab184197dbbfa384@syzkaller.appspotmail.com
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/bat_iv_ogm.c

@@ -277,17 +277,23 @@ static u8 batadv_hop_penalty(u8 tq, const struct batadv_priv *bat_priv)
  * batadv_iv_ogm_aggr_packet() - checks if there is another OGM attached
  * @buff_pos: current position in the skb
  * @packet_len: total length of the skb
- * @tvlv_len: tvlv length of the previously considered OGM
+ * @ogm_packet: potential OGM in buffer
  *
  * Return: true if there is enough space for another OGM, false otherwise.
  */
-static bool batadv_iv_ogm_aggr_packet(int buff_pos, int packet_len,
-				      __be16 tvlv_len)
+static bool
+batadv_iv_ogm_aggr_packet(int buff_pos, int packet_len,
+			  const struct batadv_ogm_packet *ogm_packet)
 {
 	int next_buff_pos = 0;
 
-	next_buff_pos += buff_pos + BATADV_OGM_HLEN;
-	next_buff_pos += ntohs(tvlv_len);
+	/* check if there is enough space for the header */
+	next_buff_pos += buff_pos + sizeof(*ogm_packet);
+	if (next_buff_pos > packet_len)
+		return false;
+
+	/* check if there is enough space for the optional TVLV */
+	next_buff_pos += ntohs(ogm_packet->tvlv_len);
 
 	return (next_buff_pos <= packet_len) &&
 	       (next_buff_pos <= BATADV_MAX_AGGREGATION_BYTES);
@@ -315,7 +321,7 @@ static void batadv_iv_ogm_send_to_if(struct batadv_forw_packet *forw_packet,
 
 	/* adjust all flags and log packets */
 	while (batadv_iv_ogm_aggr_packet(buff_pos, forw_packet->packet_len,
-					 batadv_ogm_packet->tvlv_len)) {
+					 batadv_ogm_packet)) {
 		/* we might have aggregated direct link packets with an
 		 * ordinary base packet
 		 */
@@ -1704,7 +1710,7 @@ static int batadv_iv_ogm_receive(struct sk_buff *skb,
 
 	/* unpack the aggregated packets and process them one by one */
 	while (batadv_iv_ogm_aggr_packet(ogm_offset, skb_headlen(skb),
-					 ogm_packet->tvlv_len)) {
+					 ogm_packet)) {
 		batadv_iv_ogm_process(skb, ogm_offset, if_incoming);
 
 		ogm_offset += BATADV_OGM_HLEN;