[PATCH] batman-adv: mcast: validate tracker padding before forwarding

[Kyle Zeng <kylebot@openai.com>]

A multicast tracker with an even number of destinations needs two
padding bytes after its destination list. batadv_mcast_forw_packet()
currently only checks that the count field and the addresses fit in
the tracker TVLV.

If an unpadded even tracker is compacted to an odd number of
destinations, batadv_mcast_forw_shrink_update_headers() includes those
missing padding bytes in the amount to remove. The subsequent memmove
can then write two bytes past the old packet tail. With no tailroom,
this corrupts skb_shared_info.

Include the required padding in the existing tracker length check.
This rejects the malformed tracker before either received or locally
generated packets can reach the compactor.

Fixes: 2dfe644a1ce0 ("batman-adv: mcast: shrink tracker packet after scrubbing")
Cc: stable@vger.kernel.org
Signed-off-by: Kyle Zeng <kylebot@openai.com>
---
I have a reproducer for this issue and can share via DM

 net/batman-adv/multicast_forw.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/batman-adv/multicast_forw.c b/net/batman-adv/multicast_forw.c
index ac97cd8b81dc..9ffed1f237b6 100644
--- a/net/batman-adv/multicast_forw.c
+++ b/net/batman-adv/multicast_forw.c
@@ -950,8 +950,11 @@ static int batadv_mcast_forw_packet(struct batadv_priv *bat_priv,
 
 	dest = (u8 *)mcast_tracker + sizeof(*mcast_tracker);
 
-	/* check if full tracker tvlv is within skb length */
+	/* check if full tracker tvlv, including padding, is within skb length */
 	tvlv_len = sizeof(*mcast_tracker) + ETH_ALEN * num_dests;
+	if (!(num_dests % 2))
+		tvlv_len += 2;
+
 	if (tvlv_len > skb_network_header_len(skb))
 		return -EINVAL;
 

base-commit: d89e1d186ec9271bf78e62771779e9fc03bf0fc3
-- 
2.56.0