[PATCH net v3] tipc: fix bc_ackers underflow on duplicate GRP_ACK_MSG

[Oleh Konko <security@1seal.org>]

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: 2f487712b893 ("tipc: guarantee that group broadcast doesn't bypass group unicast")
Cc: stable@vger.kernel.org
Signed-off-by: Oleh Konko <security@1seal.org>
---
v3:
- correct the Fixes tag to the commit that introduced GRP_ACK_MSG and bc_ackers

v2:
- make duplicate or stale GRP_ACK_MSG a full no-op via early return
- place acked in reverse xmas tree style

 net/tipc/group.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/tipc/group.c b/net/tipc/group.c
index e0e6227b433..14e6732624e 100644
--- a/net/tipc/group.c
+++ b/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);
-- 
2.50.0