[PATCH net 3/4] tipc: prevent snt_unacked underflow on CONN_ACK

[Michael Bommarito <michael.bommarito@gmail.com>]

tipc_sk_conn_proto_rcv() subtracts the peer-supplied connection ack
count from the unsigned 16-bit send counter snt_unacked without
checking that it does not exceed the number of messages actually
outstanding:

	tsk->snt_unacked -= msg_conn_ack(hdr);

msg_conn_ack() is read straight from a received CONN_MANAGER/CONN_ACK
message. If the ack count is larger than snt_unacked the subtraction
wraps to a near-maximum value, leaving tsk_conn_cong() permanently true
and starving the connection of further transmits.

Cap the ack to the outstanding count before subtracting. A peer (or,
for a local connection, the connected peer socket) can otherwise wedge
a TIPC connection's send side by sending an oversized connection ack.

Fixes: 10724cc7bb78 ("tipc: redesign connection-level flow control")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 net/tipc/socket.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index 9329919fb07f0..9c739a3cea126 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c
@@ -1362,9 +1362,16 @@ static void tipc_sk_conn_proto_rcv(struct tipc_sock *tsk, struct sk_buff *skb,
 			__skb_queue_tail(xmitq, skb);
 		return;
 	} else if (mtyp == CONN_ACK) {
+		u16 conn_ack = msg_conn_ack(hdr);
+
 		was_cong = tsk_conn_cong(tsk);
 		tipc_sk_push_backlog(tsk, msg_nagle_ack(hdr));
-		tsk->snt_unacked -= msg_conn_ack(hdr);
+		/* Cap a peer-supplied ack so a forged value cannot underflow
+		 * the unsigned counter and wedge connection flow control.
+		 */
+		if (conn_ack > tsk->snt_unacked)
+			conn_ack = tsk->snt_unacked;
+		tsk->snt_unacked -= conn_ack;
 		if (tsk->peer_caps & TIPC_BLOCK_FLOWCTL)
 			tsk->snd_win = msg_adv_win(hdr);
 		if (was_cong && !tsk_conn_cong(tsk))
-- 
2.53.0