[PATCH net] 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() unconditionally
decrements grp->bc_ackers on every inbound group ACK, even when the
sending member has already acknowledged the current broadcast round.

Because bc_ackers is a u16, a single duplicate ACK received after the
legitimate set has drained the counter to zero wraps it to 65535.
Once wrapped, tipc_group_bc_cong() permanently reports congestion,
blocking all subsequent group broadcasts on the affected socket until
the group is recreated.

The member-removal path (tipc_group_delete_member) already handles this
correctly: it only decrements bc_ackers when the counter is non-zero
and the member still owes an ACK for the current broadcast round.

Apply the same forward-progress guard to the GRP_ACK_MSG handler: only
update m->bc_acked and decrement bc_ackers when the inbound ack value
is strictly ahead of what has already been recorded for that member,
and only decrement when bc_ackers is non-zero.

Fixes: 75da2163dbb6 ("tipc: introduce communication groups")
Cc: stable@vger.kernel.org
Signed-off-by: Oleh Konko <security@1seal.org>
---
 net/tipc/group.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/tipc/group.c b/net/tipc/group.c
index e0e6227b433..41fa7bb3091 100644
--- a/net/tipc/group.c
+++ b/net/tipc/group.c
@@ -745,7 +745,7 @@ void tipc_group_proto_rcv(struct tipc_group *grp, bool *usr_wakeup,
 	u32 node = msg_orignode(hdr);
 	u32 port = msg_origport(hdr);
 	struct tipc_member *m, *pm;
-	u16 remitted, in_flight;
+	u16 remitted, in_flight, acked;
 
 	if (!grp)
 		return;
@@ -798,8 +798,13 @@ 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);
-		if (--grp->bc_ackers)
+		acked = msg_grp_bc_acked(hdr);
+		if (less(m->bc_acked, acked)) {
+			m->bc_acked = acked;
+			if (grp->bc_ackers)
+				grp->bc_ackers--;
+		}
+		if (grp->bc_ackers)
 			return;
 		list_del_init(&m->small_win);
 		*m->group->open = true;
-- 
2.50.0