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.

https://git.kernel.org/stable/c/15720cd8fa3fc625146128f89a8e11b0449a20a7

https://git.kernel.org/stable/c/3a49bbae676fef1ffe548971e6229ae2adeb9d10

https://git.kernel.org/stable/c/66dbb13eeb2fc339f8f548a9076be4c1a94857b0

https://git.kernel.org/stable/c/90b662ea25f5e83bb3b8ccec5b93ced810b92fb8

https://git.kernel.org/stable/c/9221a594c72a1446137926d4c2aa04e345f798dc

https://git.kernel.org/stable/c/a425c82ff06cda5165e0de3de8c2a445ec1f863e

https://git.kernel.org/stable/c/b2a017bfcf565721918ec7355a373911d2f2a227

https://git.kernel.org/stable/c/fc973ecd1a079b9a87c360478542f3a56dea085b

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