[PATCH net] sctp: prevent peer transport count overflow

sctp_assoc_add_peer() increments the association's 16-bit transport_count
for every new unique peer. Adding the 65,536th transport wraps the count to
zero.

SCTP sock_diag uses transport_count to reserve the INET_DIAG_PEERS payload,
then copies one sockaddr_storage for every entry in transport_addr_list.
After the wrap, a diagnostic dump reserves an empty payload and writes
8 MiB of peer addresses past the skb tail.

Reject a new unique peer when transport_count has reached U16_MAX. Perform
the check after the existing-peer lookup so a duplicate address continues
to return its existing transport at the limit.

Fixes: 8f840e47f190 ("sctp: add the sctp_diag.c file")
Cc: stable@vger.kernel.org
Assisted-by: avom-custom-harness:gpt-5.5-qwen3.6-mod-mix
Signed-off-by: Asim Viladi Oglu Manizada <redacted>
---
 net/sctp/associola.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/sctp/associola.c b/net/sctp/associola.c
index 62d3cc155809..b6ac0966420a 100644
--- a/net/sctp/associola.c
+++ b/net/sctp/associola.c

@@ -614,6 +614,9 @@ struct sctp_transport *sctp_assoc_add_peer(struct sctp_association *asoc,
 		return peer;
 	}
 
+	if (asoc->peer.transport_count == U16_MAX)
+		return NULL;
+
 	peer = sctp_transport_new(asoc->base.net, addr, gfp);
 	if (!peer)
 		return NULL;

-- 
2.53.0