tipc: fix bc_ackers underflow on duplicate GRP_ACK_MSG

The GRP_ACK_MSG handler in tipc_group_proto_rcv() currently decrements
bc_ackers on every inbound group ACK, even when the same member has
already acknowledged the current broadcast round.

Because bc_ackers is a u16, a duplicate ACK received after the last
legitimate ACK wraps the counter to 65535. Once wrapped,
tipc_group_bc_cong() keeps reporting congestion and later group
broadcasts on the affected socket stay blocked until the group is
recreated.

Fix this by ignoring duplicate or stale ACKs before touching bc_acked or
bc_ackers. This makes repeated GRP_ACK_MSG handling idempotent and
prevents the underflow path.

Fixes: 2f48771 ("tipc: guarantee that group broadcast doesn't bypass group unicast")
Cc: stable@vger.kernel.org
Signed-off-by: Oleh Konko <security@1seal.org>
Reviewed-by: Tung Nguyen <tung.quang.nguyen@est.tech>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/41a4833f368641218e444fdcff822039.security@1seal.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: 1seal

net/tipc/group.c

@@ -746,6 +746,7 @@ void tipc_group_proto_rcv(struct tipc_group *grp, bool *usr_wakeup,
 	u32 port = msg_origport(hdr);
 	struct tipc_member *m, *pm;
 	u16 remitted, in_flight;
+	u16 acked;
 
 	if (!grp)
 		return;
@@ -798,7 +799,10 @@ void tipc_group_proto_rcv(struct tipc_group *grp, bool *usr_wakeup,
 	case GRP_ACK_MSG:
 		if (!m)
 			return;
-		m->bc_acked = msg_grp_bc_acked(hdr);
+		acked = msg_grp_bc_acked(hdr);
+		if (less_eq(acked, m->bc_acked))
+			return;
+		m->bc_acked = acked;
 		if (--grp->bc_ackers)
 			return;
 		list_del_init(&m->small_win);