[CVE-2026-90076][MODERATE REGULAR] net/sched: fq: add overflow bounds to quantum and initial quantum [ Upstream

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

CVE: CVE-2026-90076
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: fq: add overflow bounds to quantum and initial quantum [ Upstream
Commit: d16dac3925be95ad46e986d4b139c9898b6e227f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d16dac3925be95ad46e986d4b139c9898b6e227f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90076
Analysis date: Thu, 17 Sep 2026 23:20:08 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90076 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90076

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 REGULAR
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-2026-90076	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90076 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-190;*CWE-835;**CWE-400;BEST CVSS score: '5.5';DESCR 'An integer overflow in fq_init can make the computed fq quantum and initial quantum wrap to zero when psched_mtu returns an extremely large value from a device MTU and header length. With q quantum equal to zero, fq_dequeue can enter a credit refill loop that never makes progress while holding the qdisc lock, causing a CPU soft lockup and denial of service. For the CVSS the PR:L because reliable triggering requires local code with CAP_NET_ADMIN in a user namespace or equivalent control over qdisc and MTU settings, but it does not require full host root in configurations that allow unprivileged user namespaces. The issue is not directly network reachable and is triggered by local control plane configuration, not by ordinary packet reception. Impact is denial of service only because the patch shows arithmetic wraparound and an infinite loop, with no supported memory corruption, information disclosure, or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	REMOTE DEADLOCK DANGER INIT SIMPLEFIX NETWORK HARDWARE QDISC PACKET  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap changed the score.

2. Signal breakdown

Positive signals:

* +1 Local below-host-admin trigger. The supplied context explicitly says the recreating condition requires CAP_NET_ADMIN in a user namespace, not full host administrator privilege.
* +1 Reliable kernel DoS. q->quantum can become zero and fq_dequeue can loop forever under the qdisc lock, causing a CPU soft lockup.
* +1 Networking qdisc packet-processing path. The bad state is installed through qdisc configuration and manifests in the fq dequeue path during packet scheduling.

Negative signals:

* No host-admin penalty. Host administrator privilege is not positively established and the supplied context points to CAP_NET_ADMIN in a user namespace.
* No race or timing penalty. The issue is a deterministic integer overflow and infinite loop condition, not a narrow race.
* No rare hardware penalty. A dummy device or another device with a sufficiently large MTU is enough according to the supplied context.

Non-awarded signals:

* No UAF or lifetime corruption is established.
* No controlled reclaim is demonstrated.
* No arbitrary or directed kernel write is demonstrated.
* No confidentiality or integrity impact is awarded.
* No extra availability point is awarded because the soft lockup is already counted as the DoS consequence.
* No remote reachability is awarded because the trigger is local control-plane configuration, not ordinary packet traffic from the network.
* No Important-grade primitive is established.

3. Reachability analysis

The trigger requires creating or controlling a network device with an extremely large MTU and attaching or initializing the fq qdisc so that fq_init computes quantum and initial_quantum from psched_mtu. The supplied evidence states that this requires CAP_NET_ADMIN in a user namespace. That is below host-admin-equivalent privilege when unprivileged user namespaces and the needed netdevice or module support are available.

The complete chain appears namespace-scoped from the supplied context: the attacker needs namespace-scoped CAP_NET_ADMIN to configure the device MTU and qdisc state. There is no indication that host CAP_NET_ADMIN over host-owned physical devices is required for the described dummy-device reproducer. There may still be practical dependency on user namespace availability and on the dummy or relevant netdevice support being present, but those are deployment conditions rather than proof of host-admin-only reachability.

The issue is not directly network reachable. Packets may be needed to exercise fq_dequeue after the bad qdisc state is created, but ordinary remote packet delivery alone does not create the malformed qdisc parameters.

4. Primitive and severity interpretation

The exact primitive is an integer overflow in fq_init. psched_mtu can return a value large enough that 2 * mtu or 10 * mtu wraps in 32-bit arithmetic, producing q->quantum == 0. The resulting bad scheduler state causes fq_dequeue to refill credit by adding zero and loop without progress.

This is Class C: demonstrated behavior is an invalid arithmetic state leading to an infinite loop and CPU soft lockup. It is not a memory-corruption primitive. There is no UAF, no OOB access, no refcount mutation, no object lifetime violation, no COW or ownership violation, and no authorization bypass.

The demonstrated consequence is local denial of service. A stronger consequence such as privilege escalation is speculative and not supported by the patch mechanics. ImportantCandidate=NO because the patch establishes DoS-only behavior without an unresolved corruption or boundary-bypass primitive. AutoDowngradeAllowed=YES because no meaningful Important-grade primitive would be lost by treating this as an ordinary or borderline Moderate DoS issue, although manual review is still useful due to namespace reachability and host-kernel availability impact.

5. One-sentence report phrase

An integer overflow in fq_init can set the fq quantum to zero for a huge MTU device, causing fq_dequeue to loop forever under the qdisc lock and trigger a local namespace reachable denial of service via CPU soft lockup.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is not memory corruption and not an Important candidate, but it is reachable with CAP_NET_ADMIN in a user namespace and can affect host kernel availability, so it should not be silently ignored without confirming namespace exposure in the target product.

7. Call-site confidence

Call-site confidence: high

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

Patch: net/sched: fq: add overflow bounds to quantum and initial quantum [ Upstream
Commit: d16dac3925be95ad46e986d4b139c9898b6e227f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d16dac3925be95ad46e986d4b139c9898b6e227f

Changed files:
  net/sched/sch_fq.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=d16dac3925be95ad46e986d4b139c9898b6e227f

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

The original Linux kernel CVE announcement for CVE-2026-90076 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90076

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: Not available
  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: 3

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 REGULAR

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).