[CVE-2026-98021][LOW] net: reject oversized tx_queue_len at netlink parse time [ Upstream

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

CVE: CVE-2026-98021
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net: reject oversized tx_queue_len at netlink parse time [ Upstream
Commit: 45ca9f59b6c7ea70e0a17054902b8a556913db34
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=45ca9f59b6c7ea70e0a17054902b8a556913db34
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98021
Analysis date: Fri, 25 Sep 2026 14:46:44 -0400
ActionableScore: 2
ActionableScore lower bound: 1
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-2026-98021 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98021

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-2026-98021	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98021 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-789;**CWE-400;CWE-1284;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 insufficient range check for the IFLA_TXQLEN netlink attribute allows a local user to create or modify a network device with an oversized tx_queue_len. The oversized value can bypass the normal S16_MAX limit and later cause excessive queue memory allocations in pfifo_fast tun or tap. For the CVSS the PR:L is used because a local process must create a user and network namespace and issue rtnetlink operations, but root privileges are not required when unprivileged user namespaces are enabled. The issue is not network reachable through packet traffic. Impact is denial of service through memory pressure or resource exhaustion. The base score uses A:L because actual host exhaustion depends on memory limits and configuration. The paranoid score uses A:H because sustained creation of oversized queues may exhaust host memory. NO MANUAL CHECK REQUIRED COULD BE AUTOCLOSED.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	YES	REMOTE OOB DANGER HARDWARE LINUS 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
ActionableScoreLower=1
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::

No floor or cap was applied.

2. Signal breakdown

* +1 Local unprivileged trigger. The supplied reproducer establishes that an unprivileged user can create a fresh user and network namespace and create a veth device with an oversized IFLA_TXQLEN value.
* +1 Paranoid availability impact. Sustained creation or modification of devices with oversized tx_queue_len can inflate pfifo_fast, tun, or tap queue allocations and may cause meaningful memory pressure or resource exhaustion.

Important non-awarded signals:

* No UAF, double free, stale reference, refcount misuse, OOB access, arbitrary write, or controlled kernel corruption is established.
* No confidentiality or integrity impact is established.
* No remote reachability is established. The vulnerable operation is rtnetlink control plane configuration, not packet processing.
* No persistent resource leak is established. The patch prevents oversized allocations but does not fix a missing release or reference leak.
* Reliable host-wide memory exhaustion is not demonstrated. The conservative score therefore does not award an availability point beyond the unprivileged reachability signal.
* No security-boundary bypass is established beyond the ability to use namespace-scoped network administration capabilities normally granted inside an attacker-created user and network namespace.

3. Reachability analysis

The supplied reproducer establishes a below-host-admin attacker model. An ordinary local user can create a fresh user namespace and network namespace when unprivileged user namespaces are enabled, obtain CAP_NET_ADMIN in the new user namespace, create a veth pair, and submit the oversized IFLA_TXQLEN through rtnetlink.

The relevant network-management operations are namespace-scoped for this demonstrated path. The full supplied trigger uses only the newly created user and network namespace and does not require host CAP_NET_ADMIN, host root, special hardware, packet reachability, or a prior host-created device.

Required configuration is CONFIG_NET_SCHED, CONFIG_VETH, CONFIG_USER_NS, and CONFIG_NET_NS. These are common optional kernel features, but the issue remains a local resource-allocation condition rather than a memory-corruption primitive. No race is involved.

4. Primitive and severity interpretation

The exact primitive is insufficient range validation for IFLA_TXQLEN. A user-controlled U32 value could exceed the intended S16_MAX bound during link creation or modification and later cause oversized queue-related allocations in pfifo_fast, tun, or tap.

This is neither active structured-object lifetime corruption nor fixed stale storage. There is no cleanup/destructor UAF, no repeated release/refcount operation, and no first lifetime operation that may free an object followed by a second mutation.

The demonstrated consequence is increased kernel memory consumption and potential denial of service through resource exhaustion. The patch-supported evidence does not establish immediate host-wide failure, allocation-accounting bypass, cross-container memory isolation bypass, kernel crash, controlled memory corruption, or privilege escalation.

PrimitiveClass is NONE because the patch establishes an input-validation and resource-consumption issue without a meaningful corruption, lifetime, ownership, authorization, or boundary-bypass primitive. ImportantCandidate is NO and AutoDowngradeAllowed is YES because the remaining risk is resource exhaustion only and no unresolved corruption leverage is present.

Likely operational severity is Low to ordinary Moderate. Systems allowing unprivileged user namespaces should apply the fix through normal maintenance, but the issue does not require urgent manual exploitation analysis based on the supplied evidence.

5. One-sentence report phrase

An oversized IFLA_TXQLEN value could bypass the intended S16_MAX limit during rtnetlink device creation or update and cause excessive queue memory allocations leading to local resource exhaustion.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a local input-validation and resource-exhaustion issue only, with no demonstrated memory corruption, lifetime violation, privilege escalation, confidentiality impact, integrity impact, or security-boundary bypass.

7. Call-site confidence

Call-site confidence: high

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

Patch: net: reject oversized tx_queue_len at netlink parse time [ Upstream
Commit: 45ca9f59b6c7ea70e0a17054902b8a556913db34
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=45ca9f59b6c7ea70e0a17054902b8a556913db34

Changed files:
  net/core/rtnetlink.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=45ca9f59b6c7ea70e0a17054902b8a556913db34

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

The original Linux kernel CVE announcement for CVE-2026-98021 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98021

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: 1
  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).