tipc: reject invalid and unexpected GRP_ACK_MSG to prevent bc_ackers underflow

Summary

In the Linux kernel, the following vulnerability has been resolved: 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.

Timeline

1. Oct 18, 2026, 3:18 PM UTCCVE published
2. Oct 24, 2026, 2:09 PM UTCRecord last updated

Affected versions

• Linux Linux

  From 2f487712b89376fce267223bbb0db93d393d4b09 · fixed in edc6593c442278fceff0120e8cc58bc592168c35; From 2f487712b89376fce267223bbb0db93d393d4b09 · fixed in 9b94a90651e1fa573c6ac6a5a6b269d5874df2c0; From 2f487712b89376fce267223bbb0db93d393d4b09 · fixed in 507a27435b2bb1f22e3a3674f6a6553eb815021b; From 2f487712b89376fce267223bbb0db93d393d4b09 · fixed in 80a9ccfc24d3776ef6753a57315e726475fac7d7; From 2f487712b89376fce267223bbb0db93d393d4b09 · fixed in 478fb08d0c2a53002a721d22b8d6681ec971d90f; From 2f487712b89376fce267223bbb0db93d393d4b09 · fixed in 12cc3a1eac5a2a06b67a7b85f0f48ff241a94d1b; From 2f487712b89376fce267223bbb0db93d393d4b09 · fixed in b0e517961f09b9f60d338d1ea4e17aacdbf206f0; From 2f487712b89376fce267223bbb0db93d393d4b09 · fixed in 99cc2a62e07a44a22254d7beca9ef1f8ad886d0d

• Linux Linux

  4.15

• KernelLinux

  From 4.15.0 · fixed in 5.10.271; From 5.11.0 · fixed in 5.15.222; From 5.16.0 · fixed in 6.1.189; From 6.2.0 · fixed in 6.6.158; From 6.7.0 · fixed in 6.12.112; From 6.13.0 · fixed in 6.18.55; From 6.19.0 · fixed in 7.2.9

References 8