CVE-2026-74284: net/sched: sch_hfsc: Don't make class passive twice

CVE-2026-74284: net/sched: sch_hfsc: Don't make class passive twice

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2026-74284 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.10.241 with commit 72c61ffbeeb8c50f6d4d70c65d3283aa1bac57a7 and fixed in 5.10.261 with commit a425c82ff06cda5165e0de3de8c2a445ec1f863e
	Issue introduced in 5.15.190 with commit a5efc95a33bd4fcb879250852828cc58c7862970 and fixed in 5.15.212 with commit fc973ecd1a079b9a87c360478542f3a56dea085b
	Issue introduced in 6.1.138 with commit 0475c85426b18eccdcb7f9fb58d8f8e9c6c58c87 and fixed in 6.1.178 with commit 15720cd8fa3fc625146128f89a8e11b0449a20a7
	Issue introduced in 6.6.90 with commit 9030a91235ae4845ec71902c3e0cecfc9ed1f2df and fixed in 6.6.145 with commit b2a017bfcf565721918ec7355a373911d2f2a227
	Issue introduced in 6.12.28 with commit d06476714d2819b550e0cc39222347e2c8941c9d and fixed in 6.12.97 with commit 9221a594c72a1446137926d4c2aa04e345f798dc
	Issue introduced in 6.15 with commit 51eb3b65544c9efd6a1026889ee5fb5aa62da3bb and fixed in 6.18.40 with commit 66dbb13eeb2fc339f8f548a9076be4c1a94857b0
	Issue introduced in 6.15 with commit 51eb3b65544c9efd6a1026889ee5fb5aa62da3bb and fixed in 7.1.5 with commit 3a49bbae676fef1ffe548971e6229ae2adeb9d10
	Issue introduced in 6.15 with commit 51eb3b65544c9efd6a1026889ee5fb5aa62da3bb and fixed in 7.2-rc1 with commit 90b662ea25f5e83bb3b8ccec5b93ced810b92fb8
	Issue introduced in 5.4.297 with commit 9a5fd5c2f4d4afdd5e405083ee53e0789ce76956
	Issue introduced in 6.14.6 with commit c1175c4ad01dbc9c979d099861fa90a754f72059

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74284
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/sched/sch_hfsc.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/a425c82ff06cda5165e0de3de8c2a445ec1f863e
	https://git.kernel.org/stable/c/fc973ecd1a079b9a87c360478542f3a56dea085b
	https://git.kernel.org/stable/c/15720cd8fa3fc625146128f89a8e11b0449a20a7
	https://git.kernel.org/stable/c/b2a017bfcf565721918ec7355a373911d2f2a227
	https://git.kernel.org/stable/c/9221a594c72a1446137926d4c2aa04e345f798dc
	https://git.kernel.org/stable/c/66dbb13eeb2fc339f8f548a9076be4c1a94857b0
	https://git.kernel.org/stable/c/3a49bbae676fef1ffe548971e6229ae2adeb9d10
	https://git.kernel.org/stable/c/90b662ea25f5e83bb3b8ccec5b93ced810b92fb8