linux-cve-announce - CVE-2026-23074: net/sched: Enforce that teql can only be used as root qdisc

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Message-ID: <2026020419-CVE-2026-23074-6bb8@gregkh>
Date: Wed,  4 Feb 2026 17:14:33 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2026-23074: net/sched: Enforce that teql can only be used as root qdisc

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

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

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

net/sched: Enforce that teql can only be used as root qdisc

Design intent of teql is that it is only supposed to be used as root qdisc.
We need to check for that constraint.

Although not important, I will describe the scenario that unearthed this
issue for the curious.

GangMin Kim <km.kim1503@...il.com> managed to concot a scenario as follows:

ROOT qdisc 1:0 (QFQ)
  ├── class 1:1 (weight=15, lmax=16384) netem with delay 6.4s
  └── class 1:2 (weight=1, lmax=1514) teql

GangMin sends a packet which is enqueued to 1:1 (netem).
Any invocation of dequeue by QFQ from this class will not return a packet
until after 6.4s. In the meantime, a second packet is sent and it lands on
1:2. teql's enqueue will return success and this will activate class 1:2.
Main issue is that teql only updates the parent visible qlen (sch->q.qlen)
at dequeue. Since QFQ will only call dequeue if peek succeeds (and teql's
peek always returns NULL), dequeue will never be called and thus the qlen
will remain as 0. With that in mind, when GangMin updates 1:2's lmax value,
the qfq_change_class calls qfq_deact_rm_from_agg. Since the child qdisc's
qlen was not incremented, qfq fails to deactivate the class, but still
frees its pointers from the aggregate. So when the first packet is
rescheduled after 6.4 seconds (netem's delay), a dangling pointer is
accessed causing GangMin's causing a UAF.

The Linux kernel CVE team has assigned CVE-2026-23074 to this issue.

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

	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.6.122 with commit 0686bedfed34155520f3f735cbf3210cb9044380
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.12.68 with commit 4c7e8aa71c9232cba84c289b4b56cba80b280841
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.18.8 with commit 16ed73c1282d376b956bff23e5139add061767ba
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.19-rc7 with commit 50da4b9d07a7a463e2cfb738f3ad4cff6b2c9c3b

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-2026-23074
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_teql.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/0686bedfed34155520f3f735cbf3210cb9044380
	https://git.kernel.org/stable/c/4c7e8aa71c9232cba84c289b4b56cba80b280841
	https://git.kernel.org/stable/c/16ed73c1282d376b956bff23e5139add061767ba
	https://git.kernel.org/stable/c/50da4b9d07a7a463e2cfb738f3ad4cff6b2c9c3b