[PATCH net v2 3/4] tipc: prevent snt_unacked underflow on CONN_ACK

[Michael Bommarito]

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.

Validate the ACK count at the start of the CONN_ACK block and drop the
message if it acknowledges more messages than are outstanding. 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@xxxxxxxxx>
---
v2:
- Validate msg_conn_ack() at the beginning of the CONN_ACK block and
  drop invalid messages instead of capping the peer-supplied value.

 net/tipc/socket.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index 9329919fb07f0..80ad973cda16e 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c
@@ -1362,9 +1362,14 @@ 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);
+
+		if (conn_ack > tsk->snt_unacked)
+			goto exit;
+
 		was_cong = tsk_conn_cong(tsk);
 		tipc_sk_push_backlog(tsk, msg_nagle_ack(hdr));
-		tsk->snt_unacked -= msg_conn_ack(hdr);
+		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