CVE-2026-22976 — net/sched: sch_qfq: Fix NULL deref when deactivating inactive aggregate in qfq_reset

Affected: Linux, Siemens RUGGEDCOM RST2428P

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

net/sched: sch_qfq: Fix NULL deref when deactivating inactive aggregate in qfq_reset

`qfq_class->leaf_qdisc->q.qlen > 0` does not imply that the class itself is active.

Two qfq_class objects may point to the same leaf_qdisc. This happens when:

1. one QFQ qdisc is attached to the dev as the root qdisc, and

2. another QFQ qdisc is temporarily referenced (e.g., via qdisc_get() / qdisc_put()) and is pending to be destroyed, as in function tc_new_tfilter.

When packets are enqueued through the root QFQ qdisc, the shared leaf_qdisc->q.qlen increases. At the same time, the second QFQ qdisc triggers qdisc_put and qdisc_destroy: the qdisc enters qfq_reset() with its own q->q.qlen == 0, but its class's leaf qdisc->q.qlen > 0. Therefore, the qfq_reset would wrongly deactivate an inactive aggregate and trigger a null-deref in qfq_deactivate_agg:

[ 0.903172] BUG: kernel NULL pointer dereference, address: 0000000000000000 [ 0.903571] #PF: supervisor write access in kernel mode [ 0.903860] #PF: error_code(0x0002) - not-present page [ 0.904177] PGD 10299b067 P4D 10299b067 PUD 10299c067 PMD 0 [ 0.904502] Oops: Oops: 0002 [#1] SMP NOPTI [ 0.904737] CPU: 0 UID: 0 PID: 135 Comm: exploit Not tainted 6.19.0-rc3+ #2 NONE [ 0.905157] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014 [ 0.905754] RIP: 0010:qfq_deactivate_agg (include/linux…

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CVE-2026-22976 — net/sched: sch_qfq: Fix NULL deref when deactivating inactive aggregate in qfq_reset