CVE-2026-52997: net/sched: sch_dualpi2: drain both C-queue and L-queue in dualpi2_change()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

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.

The Linux kernel CVE team has assigned CVE-2026-52997 to this issue.


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

	Issue introduced in 6.17 with commit 320d031ad6e4d67e8e1ab08ac71efda02bc85683 and fixed in 6.18.33 with commit 86cf2eba2056bcf9c41fba260e599bd95bf9943b
	Issue introduced in 6.17 with commit 320d031ad6e4d67e8e1ab08ac71efda02bc85683 and fixed in 7.0.10 with commit 3042add80c2c50bd127d570b83319af612efde65
	Issue introduced in 6.17 with commit 320d031ad6e4d67e8e1ab08ac71efda02bc85683 and fixed in 7.1 with commit 478ed6b7d2577439c610f91fa8759a4c878a4264

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-2026-52997
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_dualpi2.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/86cf2eba2056bcf9c41fba260e599bd95bf9943b
	https://git.kernel.org/stable/c/3042add80c2c50bd127d570b83319af612efde65
	https://git.kernel.org/stable/c/478ed6b7d2577439c610f91fa8759a4c878a4264