CVE-2026-23074

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@gmail.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.

PUBLISHED Reserved 2026-01-13 | Published 2026-02-04 | Updated 2026-02-09 | Assigner Linux

Product status

References

git.kernel.org/...c/73d970ff0eddd874a84c953387c7f4464b705fc6

git.kernel.org/...c/ae810e6a8ac4fe25042e6825d2a401207a2e41fb

git.kernel.org/...c/dad49a67c2d817bfec98e6e45121b351e3a0202c

git.kernel.org/...c/0686bedfed34155520f3f735cbf3210cb9044380

git.kernel.org/...c/4c7e8aa71c9232cba84c289b4b56cba80b280841

git.kernel.org/...c/16ed73c1282d376b956bff23e5139add061767ba

git.kernel.org/...c/50da4b9d07a7a463e2cfb738f3ad4cff6b2c9c3b

cve.org (CVE-2026-23074)

nvd.nist.gov (CVE-2026-23074)

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