Medium [CVE-2026-52997] drain both C-queue and L-queue in dualpi2_change

In the Linux kernel, the following vulnerability has been resolved: net/sched: sch_dualpi2: drain both C-queue and L-queue in dualpi2_change() Fix dualpi2_change() to correctly enforce updated limit and memlimit values after a configuration change of the dualpi2 qdisc.

Before this patch, dualpi2_change() always attempted to dequeue packets via the root qdisc (C-queue) when reducing backlog or memory usage, and unconditionally assumed that a valid skb will be returned.

When traffic classification results in packets being queued in the L-queue while the C-queue is empty, this leads to a NULL skb dereference during limit or memlimit enforcement. This is fixed by first dequeuing from the C-queue path if it is non-empty.

Once the C-queue is empty, packets are dequeued directly from the L-queue. Return values from qdisc_dequeue_internal() are checked for both queues.

When dequeuing from the L-queue, the parent qdisc qlen and backlog counters are updated explicitly to keep overall qdisc statistics consistent. A flaw was found in the Linux kernel's `sch_dualpi2` qdisc (queueing discipline) component.

This can lead to a NULL skb (socket buffer) dereference, potentially causing a system crash or a Denial of Service (DoS). Red Hat severity: Moderate.

Weakness: CWE-476.