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

[Tung Quang Nguyen]

>Subject: [PATCH net v3] 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: 2f487712b893 ("tipc: guarantee that group broadcast doesn't bypass
>group unicast")
>Cc: stable@xxxxxxxxxxxxxxx
>Signed-off-by: Oleh Konko <security@xxxxxxxxx>
>---
>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
Reviewed-by: Tung Nguyen <tung.quang.nguyen@xxxxxxxx>