Linux kernel TIPC bc_ackers underflow on duplicate GRP_ACK_MSG
CVE-2026-31662 is an availability-impacting flaw in the Linux kernel's TIPC group messaging logic. In the GRP_ACK_MSG handling path inside tipc_group_proto_rcv(), the bc_ackers counter is decremented for every inbound group ACK, even if the same member has already acknowledged the current broadcast round. Because bc_ackers is a u16, receipt of a duplicate ACK after the final legitimate ACK can cause the counter to underflow and wrap to 65535. After this wraparound, tipc_group_bc_cong() continues to report broadcast congestion, causing subsequent group broadcasts on the affected socket to remain blocked until the group is recreated. The fix makes ACK processing idempotent by ignoring duplicate or stale ACKs before updating bc_acked or bc_ackers.
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