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:
- The child calls qdisc_tree_reduce_backlog(), which, now that the child
- hfsc_dequeue() then calls update_vf(cl, qdisc_pkt_len(skb), cur_time)
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.