[CVE-2026-90073][MODERATE REGULAR] net/sched: hhf: clamp quantum before hhf_change() to avoid overflow [ Upstream

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

CVE: CVE-2026-90073
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: hhf: clamp quantum before hhf_change() to avoid overflow [ Upstream
Commit: 2e0f0729f922c8de13801004e2131b57646c7c5f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2e0f0729f922c8de13801004e2131b57646c7c5f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90073
Analysis date: Thu, 17 Sep 2026 17:39:40 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
HHF qdisc can accept an oversized MTU-derived quantum that overflows signed deficit accounting in hhf_dequeue and causes an infinite dequeue spin, requiring namespace scoped CAP_NET_ADMIN and resulting in local denial of service only.

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

The original Linux kernel CVE announcement for CVE-2026-90073 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90073

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.

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

CVE-2026-90073	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90073 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;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'HHF qdisc initialization can accept an oversized quantum derived from a very large device MTU. When this value is later multiplied by the HHF weight, the signed deficit calculation in hhf_dequeue can overflow and the dequeue path may spin forever. This is a local control plane issue rather than a packet driven remote network attack. For the CVSS the PR:L is used for the paranoid score because CAP_NET_ADMIN inside a user namespace may be available to a local unprivileged user on affected systems. Impact is denial of service through high CPU usage or soft lockup. Confidentiality and integrity impact are not assigned because the patch indicates an integer overflow leading to an infinite loop, not a memory corruption primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	REMOTE DANGER SIMPLEFIX NETWORK HARDWARE QDISC  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 was applied.

## 2. Signal breakdown

Positive signals:

* +1 Reliable strong DoS: oversized `q->quantum` derived from huge MTU can make `weight * quantum` overflow signed deficit handling in `hhf_dequeue()`, causing an infinite dequeue spin.
* +1 Below-host-admin namespace-scoped reachability: supplied context states the reproducer requires `CAP_NET_ADMIN` in a user namespace, not host-admin-equivalent privilege.
* +1 Attacker-controlled active qdisc configuration: attacker can configure the relevant HHF qdisc and MTU/quantum state in the affected namespace so the invalid arithmetic state reaches active scheduler processing.

Non-awarded signals:

* RealUAF not awarded: no stale pointer, object lifetime violation, double free, or refcount misuse is shown.
* Memory corruption not awarded: the patch shows integer overflow leading to infinite loop, not OOB write, UAF, controlled overwrite, or arbitrary write.
* Confidentiality and integrity not awarded: no disclosure path or unauthorized modification primitive is established.
* Remote reachability not awarded: this is local control-plane configuration via `tc`/qdisc, not packet-driven remote exploitation.
* Strong LPE or weak LPE not awarded: no controlled reclaim, object replacement, credential impact, callback control, or security-boundary bypass is shown.
* Extra availability point not awarded: the infinite spin/soft lockup is already counted as the DoS consequence.

Repeated release/refcount checks:

* First release can end lifetime: no.
* Exact same reference survives: no.
* Second lifetime/refcount mutation: no.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no.

## 3. Reachability analysis

The trigger is local and requires configuring a network device with an extreme MTU and attaching the HHF qdisc. The supplied context explicitly says this requires `CAP_NET_ADMIN` in a user namespace, which supports below-host-admin reachability on systems where unprivileged user namespaces and namespace-local network setup are available.

The relevant operations are qdisc setup and device MTU manipulation. These are control-plane operations, not packet-processing inputs from a remote host. The complete trigger appears namespace-scoped based on the supplied statement, although real product exposure still depends on whether unprivileged user namespaces are enabled and whether the needed device/qdisc support is available.

No special physical hardware is required. A dummy-like device with permissive `max_mtu` is sufficient according to the supplied context. This is not a race. The failure is deterministic once the invalid MTU-derived quantum state is installed.

## 4. Primitive and severity interpretation

The primitive is an integer overflow in scheduler arithmetic: `q->quantum` can be derived from an extremely large MTU, and multiplying it by HHF weight can overflow the signed deficit logic. The demonstrated consequence is an infinite spin in `hhf_dequeue()`, causing local denial of service through CPU soft lockup or similar host responsiveness loss.

This is Class C because the demonstrated behavior is DoS/invalid-state behavior without an independently useful corruption primitive. There is no UAF, no active stale object, no repeated lifetime mutation, no controlled reclaim, and no arbitrary read/write.

ImportantCandidate is NO because the patch does not establish memory corruption, security-boundary bypass, filesystem ownership violation, or unresolved lifetime/refcount exploitation leverage. AutoDowngradeAllowed is YES because this remains a DoS-only arithmetic/control-plane issue, although product-specific namespace exposure should be checked before closing.

Likely operational severity is Moderate-like, with higher practical relevance on systems that allow unprivileged users to create user/network namespaces and obtain namespace-scoped `CAP_NET_ADMIN`.

## 5. One-sentence report phrase

HHF qdisc can accept an oversized MTU-derived quantum that overflows signed deficit accounting in hhf_dequeue and causes an infinite dequeue spin, requiring namespace scoped CAP_NET_ADMIN and resulting in local denial of service only.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: no memory corruption or LPE primitive is established, but namespace-scoped `CAP_NET_ADMIN` reachability and host-impacting CPU DoS should be checked against the target product policy for unprivileged user namespaces and qdisc availability.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: net/sched: hhf: clamp quantum before hhf_change() to avoid overflow [ Upstream
Commit: 2e0f0729f922c8de13801004e2131b57646c7c5f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2e0f0729f922c8de13801004e2131b57646c7c5f

Changed files:
  net/sched/sch_hhf.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=2e0f0729f922c8de13801004e2131b57646c7c5f

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

The original Linux kernel CVE announcement for CVE-2026-90073 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90073

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:H/UI:N/S:U/C:N/I:N/A:H
  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).