[PATCH net] tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow

Commit 48a5fe38772b ("tipc: fix bc_ackers underflow on duplicate
GRP_ACK_MSG") rejected duplicate/stale ACKs in tipc_group_proto_rcv()
by returning early when less_eq(acked, m->bc_acked).

However, that check remains incomplete in two ways:

1. When grp->bc_ackers is zero (e.g. on a quiet group, when replicast
   ACKs were not requested, or after all expected members have already
   acknowledged), an unexpected GRP_ACK_MSG with acked > m->bc_acked
   passes less_eq() and unconditionally decrements grp->bc_ackers.
   Because bc_ackers is a u16, this wraps to 65535, causing
   tipc_group_bc_cong() to permanently report congestion and blocking
   all future group broadcasts on the socket.

2. During an active broadcast round (grp->bc_ackers > 0), the sender
   transmits packet S and advances grp->bc_snd_nxt to S + 1. Receivers
   increment their expected counter to S + 1 upon consuming packet S,
   so the only valid ACK value for the current round is strictly
   acked == grp->bc_snd_nxt.

   However, tipc_group_update_bc_members() initializes each member's
   m->bc_acked to prev = grp->bc_snd_nxt - 1 (S - 1 before increment).
   This leaves a 2-sequence gap (S - 1 to S + 1) in sequence space.
   An incoming ACK is therefore neither rejected as duplicate nor
   prevented from decrementing grp->bc_ackers if an unexpected or stale
   value (such as S) is received. A member sending acked = S followed
   by acked = S + 1 could decrement grp->bc_ackers twice in the same
   round, prematurely clearing bc_ackers or underflowing it.

Fix this by:
- Dropping GRP_ACK_MSG immediately if grp->bc_ackers is zero.
- Requiring acked == grp->bc_snd_nxt and rejecting duplicates where
  m->bc_acked == acked. Because replicast broadcast rounds are strictly
  sequential, only grp->bc_snd_nxt can be acknowledged, and each member
  can acknowledge at most once per round.

Note that a related pre-existing issue in tipc_group_delete_member()
(where grp->bc_ackers decrementing to zero upon member departure does
not restore *grp->open or trigger a socket wakeup) will be addressed
in a separate patch.

Fixes: 48a5fe38772b ("tipc: fix bc_ackers underflow on duplicate GRP_ACK_MSG")
Fixes: 2f487712b893 ("tipc: guarantee that group broadcast doesn't bypass group unicast")
Reported-by: James Burton <redacted>
Cc: stable@vger.kernel.org
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
Cc: Oleh Konko <redacted>
Cc: Tung Nguyen <tung.quang.nguyen@est.tech>
Cc: Jon Maloy <jmaloy@redhat.com>
---
 net/tipc/group.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/tipc/group.c b/net/tipc/group.c
index 14e6732624e28edc20ff80163787bca3018cb2dd..74f6d3dac0784d5a54db3e2202ead3878d8df9e2 100644
--- a/net/tipc/group.c
+++ b/net/tipc/group.c

@@ -797,10 +797,10 @@ void tipc_group_proto_rcv(struct tipc_group *grp, bool *usr_wakeup,
 		tipc_group_open(m, usr_wakeup);
 		return;
 	case GRP_ACK_MSG:
-		if (!m)
+		if (!m || !grp->bc_ackers)
 			return;
 		acked = msg_grp_bc_acked(hdr);
-		if (less_eq(acked, m->bc_acked))
+		if (acked != grp->bc_snd_nxt || m->bc_acked == acked)
 			return;
 		m->bc_acked = acked;
 		if (--grp->bc_ackers)

-- 
2.55.0.1007.g17ff1f9808-goog