[CVE-2026-98009][LOW] net/sched: ets: clamp quantum in parse and fallback paths [ Upstream

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

CVE: CVE-2026-98009
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net/sched: ets: clamp quantum in parse and fallback paths [ Upstream
Commit: 99ae477b892e203a37985ebc56f0063e072f7f4b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=99ae477b892e203a37985ebc56f0063e072f7f4b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98009
Analysis date: Fri, 25 Sep 2026 09:11:52 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98009 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98009

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: LOW
Manual review required: NO

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

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

CVE-2026-98009	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98009 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-835;**CWE-400;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'An ETS qdisc can enter an unbounded deficit refill loop while holding the qdisc lock when a zero or excessively small quantum is combined with a crafted packet size table and a headerless device whose psched_mtu is zero. A local actor can create such a qdisc on a dummy device and cause a CPU soft lockup, leaving the affected qdisc locked and consuming CPU time. For the CVSS the PR:L is selected because reliable triggering requires local control of network namespace configuration and CAP_NET_ADMIN. On systems that permit unprivileged user namespaces this capability can be obtained in an attacker controlled user and network namespace. The issue is not directly network reachable. Impact is denial of service. It is most directly confined to the affected namespace and CPU, but can reduce host availability on constrained systems.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	REMOTE DANGER INIT SIMPLEFIX NETWORK HARDWARE QDISC  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Local unprivileged trigger. The supplied reproduction states that `unshare -Urn` is sufficient, allowing an ordinary user on systems permitting unprivileged user namespaces to obtain CAP_NET_ADMIN in an attacker controlled user and network namespace.
* +1 Reliable denial of service. A crafted ETS qdisc configuration can make the deficit refill loop spin while holding the qdisc lock, causing CPU soft lockup and preventing normal progress for that qdisc.

No additional points:

* No UAF, stale object access, OOB access, refcount corruption, ownership violation, or authorization bypass is established.
* No controlled reclaim, arbitrary write, callback control, disclosure, integrity impact, or privilege escalation primitive is demonstrated.
* No remote reachability is established. The vulnerable path is reached through local qdisc configuration, not packet traffic.
* No local unprivileged high-control kernel data plane API point is awarded. The attacker configures a qdisc, but does not obtain control over a memory layout, object lifetime, allocator state, or kernel payload relevant to exploitation.
* No second Availability point is awarded. The CPU soft lockup and qdisc lock stall are the same DoS consequence.
* No paranoid-only point is awarded. The higher CVSS Availability interpretation reflects possible operational impact on constrained systems, but does not establish an additional exploitation primitive for ActionableScore purposes.

3. Reachability analysis

The attacker is a local ordinary user on systems where unprivileged user namespaces are enabled. The supplied reproduction explicitly states that `unshare -Urn` suffices.

The relevant qdisc and rtnetlink operations are namespace scoped. The attacker can create an attacker controlled user namespace and network namespace, receive CAP_NET_ADMIN in that namespace, create or use a dummy network device there, attach the ETS qdisc, and configure the low quantum values that trigger the loop. Therefore the complete demonstrated trigger is namespace scoped and does not require host administrator privilege.

The affected kernel must be built with `CONFIG_NET_SCH_ETS=y` or otherwise provide the ETS qdisc. This is an affected-kernel feature requirement, not evidence that a separate host administrator must prepare the attack path at runtime. No special hardware, firmware, race, privileged host device, or prior host-created topology is required by the supplied reproduction.

4. Primitive and severity interpretation

The exact primitive is an invalid scheduler configuration leading to an unbounded deficit refill loop. A zero or too-small quantum can combine with a very large packet length and a zero `psched_mtu()` fallback value, causing ETS dequeue processing to spin while holding the qdisc lock.

This is not a memory corruption issue. There is no active structured stale object, no use-after-free, no repeated release or refcount mutation, no arbitrary or directed write, and no attacker controlled post-free state. The demonstrated consequence is CPU consumption, qdisc lock retention, and a CPU soft lockup. The issue is therefore `CLASS_C`, representing an invalid-state DoS condition.

The impact can be meaningful in a constrained environment, especially on systems with few CPUs or where the affected qdisc is important for local network processing. However, the demonstrated impact remains denial of service. There is no patch-supported basis for confidentiality, integrity, privilege escalation, host escape, or Important-class exploitation leverage.

ImportantCandidate is NO because the patch establishes only a local scheduler-loop DoS primitive. AutoDowngradeAllowed is YES because no unresolved kernel corruption, lifetime, ownership, authorization, or boundary-bypass primitive would be discarded by an ordinary Moderate classification.

5. One-sentence report phrase

A local user can configure an ETS qdisc with an invalidly small quantum in an attacker controlled network namespace, causing an unbounded deficit refill loop under the qdisc lock and resulting in CPU soft lockup and denial of service.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The issue is a namespace-reachable local DoS caused by an infinite scheduler loop, with no demonstrated memory corruption, privilege escalation, security-boundary bypass, confidentiality impact, or integrity impact.

7. Call-site confidence

Call-site confidence: high

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

Patch: net/sched: ets: clamp quantum in parse and fallback paths [ Upstream
Commit: 99ae477b892e203a37985ebc56f0063e072f7f4b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=99ae477b892e203a37985ebc56f0063e072f7f4b

Changed files:
  net/sched/sch_ets.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=99ae477b892e203a37985ebc56f0063e072f7f4b

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

The original Linux kernel CVE announcement for CVE-2026-98009 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98009

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:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: unknown
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).