[PATCH net 8/9] batman-adv: mcast: avoid OOB read of num_dests header

[Simon Wunderlich <sw@simonwunderlich.de>]

From: Sven Eckelmann <sven@narfation.org>

Before the access to struct batadv_tvlv_mcast_tracker's num_dests, it is
attempted to check whether enough space is actually in the network header.
But instead of using offsetofend() to check for the whole size (2) which
must be accessible, offsetof() of is called. The latter is always returning
0. The comparison with the network header length will always return that
enough data is available - even when only 1 or 0 bytes are accessible.

Instead of using offsetofend(), use the more common check for the whole
header.

Cc: stable@vger.kernel.org
Fixes: 07afe1ba288c ("batman-adv: mcast: implement multicast packet reception and forwarding")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
---
 net/batman-adv/multicast_forw.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/net/batman-adv/multicast_forw.c b/net/batman-adv/multicast_forw.c
index b8668a80b94a1..1404a3b7adfb1 100644
--- a/net/batman-adv/multicast_forw.c
+++ b/net/batman-adv/multicast_forw.c
@@ -927,11 +927,11 @@ static int batadv_mcast_forw_packet(struct batadv_priv *bat_priv,
 {
 	struct batadv_tvlv_mcast_tracker *mcast_tracker;
 	struct batadv_neigh_node *neigh_node;
-	unsigned long offset, num_dests_off;
 	struct sk_buff *nexthop_skb;
 	unsigned char *skb_net_hdr;
 	bool local_recv = false;
 	unsigned int tvlv_len;
+	unsigned long offset;
 	bool xmitted = false;
 	u8 *dest, *next_dest;
 	u16 num_dests;
@@ -940,9 +940,8 @@ static int batadv_mcast_forw_packet(struct batadv_priv *bat_priv,
 	/* (at least) TVLV part needs to be linearized */
 	SKB_LINEAR_ASSERT(skb);
 
-	/* check if num_dests is within skb length */
-	num_dests_off = offsetof(struct batadv_tvlv_mcast_tracker, num_dests);
-	if (num_dests_off > skb_network_header_len(skb))
+	/* check if batadv_tvlv_mcast_tracker header is within skb length */
+	if (sizeof(*mcast_tracker) > skb_network_header_len(skb))
 		return -EINVAL;
 
 	skb_net_hdr = skb_network_header(skb);
-- 
2.47.3