[CVE-2026-74267][MODERATE 7.0] net/sched: sch_codel: Do not call qdisc_tree_reduce_backlog during peek before restoring qlen [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-74267
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: sch_codel: Do not call qdisc_tree_reduce_backlog during peek before restoring qlen [ Upstream
Commit: bb9cfd874ee884117b33e92a002b9a45b48202f4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bb9cfd874ee884117b33e92a002b9a45b48202f4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74267
Analysis date: Sun, 16 Aug 2026 08:35:28 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-74267 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74267

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE 7.0
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74267	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-664;*CWE-672;*CWE-416;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A state ordering bug in sch_codel can call qdisc_tree_reduce_backlog() during peek before the dequeued skb is requeued and qlen is restored. If qlen temporarily reaches zero, the parent qdisc can run qlen_notify and incorrectly deactivate a class even though Codel still has a packet queued. With qfq as a parent and codel as a child, this can leave scheduler class state inconsistent and later trigger wild memory access in qfq_deactivate_agg during reset or graft operations. For the CVSS the PR:L is used in the paranoid score because delegated CAP_NET_ADMIN inside containers or network namespaces may allow an untrusted local actor to configure the affected qdisc hierarchy, while a normal host setup is closer to PR:H. The issue is not directly network reachable and is triggered through local traffic control configuration and queue operations. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to stale list or UAF style memory corruption, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 4)	YES	REMOTE KASAN OOB NETWORK SKB LOG HARDWARE LINUS QDISC PACKET  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=5
ActionableScoreLower=3

1. **ActionableScore**
* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

2. **Signal breakdown**
* Weak/indirect corruption candidate: +1. The trace shows KASAN wild memory access near poisoned list values in `qfq_deactivate_agg()`, caused by inconsistent qdisc/class state.
* Real lifetime or stale-list corruption concern: +1. The parent class can be deactivated while the child CoDel qdisc still owns an skb, suggesting stale scheduler/list state rather than a pure accounting bug.
* Reliable kernel crash / strong DoS: +1. The commit includes an Oops/general protection fault with KASAN wild-memory-access.
* Broad kernel networking scheduler path: +1. `net/sched` is a common kernel networking subsystem, although this exact qdisc hierarchy is not default.
* Requires CAP_NET_ADMIN or equivalent in typical deployments: -1 conservative. Full host root is not always required because CAP_NET_ADMIN may be delegated to containers or network namespaces, so I do not apply the full -2 in the practical score.
* Local unprivileged or reduced-privilege trigger in paranoid interpretation: +1. If unprivileged user namespaces allow a user to obtain CAP_NET_ADMIN in a network namespace, the trigger may be reachable by a non-host-root local actor.
* Weak LPE concern in paranoid interpretation: +1. The demonstrated wild memory access is not a proven exploit primitive, but stale list or UAF-style scheduler corruption is enough to require manual review.
* Hard/specific conditions: -1. Triggering requires a specific qdisc arrangement such as qfq with codel as child plus the peek/drop/qlen ordering condition.

3. **Reachability analysis**
The direct trigger is local control-plane manipulation of traffic control qdiscs, typically via `tc`, so ordinary remote packet traffic alone is not sufficient. In standard host deployments this usually requires CAP_NET_ADMIN, but namespace or container delegation may reduce the practical privilege barrier. The affected path is not a default qdisc setup and appears to require a specific parent/child scheduler composition and queue state. Call-site confidence: high, because the patch and trace show the relevant codel peek path, `qdisc_tree_reduce_backlog()`, parent `qlen_notify`, and qfq crash path.

4. **Severity interpretation**
This is more than an ordinary Moderate DoS because the failure mode is a wild memory access in scheduler state, not just a clean NULL dereference or resource leak. Realistic demonstrated impact is local kernel crash/DoS. Practical privilege escalation is not demonstrated, and there is no shown attacker-controlled reclaim, overwrite target, or callback control, so it should not be treated as a confirmed Important LPE. However, the stale-list/UAF-style behavior makes it an Actionable Moderate and possibly an Important candidate pending manual review.

5. **One-sentence report phrase**
A state ordering bug in sch_codel can restore qlen too late during peek, allowing a parent qdisc such as qfq to deactivate a class incorrectly and later hit wild memory access, causing local DoS with possible stale-list or UAF-style corruption concerns.

6. **Manual review recommendation**
MANUAL CHECK REQUIRED. The crash is demonstrated and the corruption is in qdisc list/class state with KASAN wild-memory-access, so the issue should not be auto-closed even though a reliable privilege-escalation primitive is not proven.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: net/sched: sch_codel: Do not call qdisc_tree_reduce_backlog during peek before restoring qlen [ Upstream
Commit: bb9cfd874ee884117b33e92a002b9a45b48202f4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bb9cfd874ee884117b33e92a002b9a45b48202f4

Changed files:
  include/linux/list.h
  net/sched/sch_qfq.c
  net/sched/sch_generic.c
  net/sched/sch_api.c
  net/sched/sch_codel.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bb9cfd874ee884117b33e92a002b9a45b48202f4

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-74267 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74267

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 3
  Paranoid ActionableScore: 5

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE 7.0

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).