CVE-2026-23074

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. teqls 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 teqls peek always returns NULL), dequeue will never be called and thus the qlen will remain as 0. With that in mind, when GangMin updates 1:2s lmax value, the qfq_change_class calls qfq_deact_rm_from_agg. Since the child qdiscs 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 (netems delay), a dangling pointer is accessed causing GangMins causing a UAF.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/0686bedfed34155520f3f735cbf3210cb9044380
• https://git.kernel.org/stable/c/16ed73c1282d376b956bff23e5139add061767ba
• https://git.kernel.org/stable/c/4c7e8aa71c9232cba84c289b4b56cba80b280841
• https://git.kernel.org/stable/c/50da4b9d07a7a463e2cfb738f3ad4cff6b2c9c3b
• https://git.kernel.org/stable/c/73d970ff0eddd874a84c953387c7f4464b705fc6
• https://git.kernel.org/stable/c/ae810e6a8ac4fe25042e6825d2a401207a2e41fb
• https://git.kernel.org/stable/c/dad49a67c2d817bfec98e6e45121b351e3a0202c