[CVE-2026-80630][MODERATE 7.0] net/sched: sch_fq_codel: Do not call qdisc_tree_reduce_backlog during peek before restoring qlen [ Upstream

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

CVE: CVE-2026-80630
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net/sched: sch_fq_codel: Do not call qdisc_tree_reduce_backlog during peek before restoring qlen [ Upstream
Commit: 3e515188393e62a718ccebee651ee74514104ff6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3e515188393e62a718ccebee651ee74514104ff6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80630
Analysis date: Fri, 28 Aug 2026 05:52:16 -0400
ActionableScore: 6
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A qlen accounting bug in sch_fq_codel peek handling can incorrectly deactivate a parent qdisc class and lead to wild memory access in qfq, causing a local kernel crash with weak memory-corruption concern when qdisc control is available.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80630 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80630

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-80630	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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-682;*CWE-664;*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 'fq_codel could call qdisc_tree_reduce_backlog during peek before restoring qlen after putting the skb back on gso_skb. If qlen temporarily reaches zero, the parent qdisc can run qlen_notify and incorrectly deactivate a class even though fq_codel still has a packet queued. In a qfq parent setup this can corrupt scheduler state and was observed as a KASAN wild memory access in qfq_deactivate_agg followed by a kernel oops. For the CVSS the PR:L is used for the paranoid score because an untrusted local user may be able to control qdisc state through a network namespace, container, or delegated CAP_NET_ADMIN rather than full host root. The issue is not directly network reachable and is triggered through local traffic control operations, but packet enqueue or peek behavior can be part of the trigger once the qdisc hierarchy is configured. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to memory corruption like behavior, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	REMOTE KASAN OOB NETWORK SKB HARDWARE LINUS QDISC KPANIC PACKET 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Weak/indirect corruption candidate: +1. The report shows KASAN wild memory access in `qfq_deactivate_agg`, caused by inconsistent qdisc/class state after an incorrect `qlen_notify` path.
* Real lifetime/list-state corruption concern: +1. The access involves poisoned list-style addresses in qfq scheduler state, suggesting stale list membership or invalid deactivation state.
* Reliable kernel crash / strong DoS: +1. The provided trace shows a general protection fault and kernel Oops.
* Broad/common networking subsystem exposure: +1. Traffic control and qdisc logic are common kernel networking infrastructure, although the exact fq_codel plus qfq hierarchy is configuration dependent.
* Requires CAP_NET_ADMIN in typical deployments: -2. Creating or modifying qdisc hierarchy via `tc` normally requires administrative network privileges.

Conservative total: 3.

Paranoid additional interpretation:

* Local unprivileged trigger: +1. In some deployments, untrusted users may gain effective qdisc control through user namespaces, containers, delegated `CAP_NET_ADMIN`, or service-level network administration.
* Weak LPE concern: +1. The bug is not a simple NULL dereference or resource leak. It produces scheduler state inconsistency and wild memory access, but no controlled overwrite or reclaim primitive is demonstrated.
* Do not award strong LPE: +0. No attacker-controlled reclaim, arbitrary write, callback control, or reliable object replacement is shown.
* Do not award remote reachable: +0. This is not directly network-triggerable from packet traffic alone.
* Do not add separate availability point: +0. Crash impact is already counted through strong DoS.

Paranoid total: 6.

## 3. Reachability analysis

The realistic trigger requires local control over traffic control configuration, specifically a qdisc hierarchy involving fq_codel behavior and a parent such as qfq where incorrect class deactivation can later be exercised. In ordinary host deployments this requires `CAP_NET_ADMIN`, so the conservative privilege model treats it as privileged local attack surface.

Namespaces and containers matter. If untrusted users can create network namespaces with usable qdisc control, or if a container or service account has delegated `CAP_NET_ADMIN`, the practical privilege level can drop from full host-admin to a lower local-privileged context. The path is not remotely reachable by simply sending packets to the machine, although packet enqueue, dequeue, or peek behavior can participate after the qdisc hierarchy is configured.

The affected subsystem is common networking infrastructure, but the exact reproducer depends on scheduler configuration and qdisc interactions. This makes exploitation more specialized than a default remote packet parser bug, but the observed KASAN wild memory access makes it unsuitable for automatic low-priority closure.

## 4. Severity interpretation

This behaves above ordinary Moderate because it is not only a validation failure or harmless accounting bug. The incorrect ordering around `qdisc_tree_reduce_backlog` can cause parent qdisc state corruption and a demonstrated wild memory access in qfq.

Realistically, the demonstrated impact is local denial of service via kernel crash. Theoretical confidentiality or integrity impact is plausible only as a paranoid concern due to memory-corruption-like scheduler state damage, not because the patch demonstrates a controlled write or a direct privilege escalation primitive.

Overall this is an Actionable Moderate and a weak Strong Important candidate for manual review, especially in products where delegated network administration or unprivileged namespaces make qdisc manipulation reachable by less trusted users.

## 5. One-sentence report phrase

A qlen accounting bug in sch_fq_codel peek handling can incorrectly deactivate a parent qdisc class and lead to wild memory access in qfq, causing a local kernel crash with weak memory-corruption concern when qdisc control is available.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: the issue has a demonstrated kernel Oops with KASAN wild memory access in scheduler list/state handling, and namespace or delegated `CAP_NET_ADMIN` models may reduce the practical privilege requirement.

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

Patch: net/sched: sch_fq_codel: Do not call qdisc_tree_reduce_backlog during peek before restoring qlen [ Upstream
Commit: 3e515188393e62a718ccebee651ee74514104ff6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3e515188393e62a718ccebee651ee74514104ff6

Changed files:
  include/linux/list.h
  net/sched/sch_qfq.c
  net/sched/sch_generic.c
  net/sched/sch_api.c
  net/sched/sch_fq_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=3e515188393e62a718ccebee651ee74514104ff6

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

The original Linux kernel CVE announcement for CVE-2026-80630 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80630

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: 6

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: YES
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).