sctp: prevent peer transport count overflow

commit bd0e9289e2 upstream.

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: 8f840e47f1 ("sctp: add the sctp_diag.c file")
Cc: stable@vger.kernel.org
Signed-off-by: Asim Viladi Oglu Manizada <manizada@pm.me>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20260725032053.521705-1-manizada@pm.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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;