net/sched: sch_hfsc: Don't make class passive twice

Summary

In the Linux kernel, the following vulnerability has been resolved: net/sched: sch_hfsc: Don't make class passive twice update_vf() is called from two places for the same class during a single dequeue when the class's child qdisc (e.g. codel/fq_codel) drops its last packets while dequeuing: 1. The child calls qdisc_tree_reduce_backlog(), which, now that the child is empty, invokes hfsc_qlen_notify() -> update_vf(cl, 0, 0) and turns the class passive (cl_nactive is decremented up the hierarchy). 2. hfsc_dequeue() then calls update_vf(cl, qdisc_pkt_len(skb), cur_time) to charge the dequeued bytes. On the second call the class is already passive, but its child qdisc is still empty, so update_vf() arms go_passive again: if (cl->qdisc->q.qlen == 0 && cl->cl_flags & HFSC_FSC) go_passive = 1; The leaf is then skipped by the cl_nactive == 0 check inside the loop, which does not clear go_passive, so the stale go_passive propagates to the parent and decrements its cl_nactive a second time. A parent that still has other active children is driven to cl_nactive == 0 and removed from the vttree, even though those siblings are still backlogged. They are never dequeued again and the qdisc stalls. Fix this by only arming go_passive when the class is actually active, so an already-passive class no longer triggers a second passive transition. The byte accounting (cl->cl_total += len) still runs for every ancestor, so dequeued bytes continue to be counted exactly once.

Timeline

1. Aug 15, 2026, 5:57 AM UTCCVE published
2. Aug 17, 2026, 5:16 AM UTCRecord last updated

Affected versions

• Linux Linux

  From 5.10.241 · fixed in 5.10.261; From 5.15.190 · fixed in 5.15.212; From 6.1.138 · fixed in 6.1.178; From 6.6.90 · fixed in 6.6.145; From 6.12.28 · fixed in 6.12.97; From 5.4.297 · fixed in 5.5; From 6.14.6 · fixed in 6.15

• Linux Linux

  6.15

References 8