[CVE-2026-90053][LOW] net/sched: sch_htb: limit htb_classify inner-class filter hops [ Upstream

[CVE-2026-90053][LOW] net/sched: sch_htb: limit htb_classify inner-class filter hops [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90053
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net/sched: sch_htb: limit htb_classify inner-class filter hops [ Upstream
Commit: 83bb11805b620c498d6417e0e409f26887981489
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=83bb11805b620c498d6417e0e409f26887981489
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90053
Analysis date: Thu, 17 Sep 2026 22:38:38 -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-2026-90053 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90053

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-2026-90053	LOW	https://www.kernelcve.org/list/?q=CVE-2026-90053 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;BEST CVSS score: '5.5';DESCR 'A logic flaw in sch_htb allows filters attached to inner HTB classes to select the same class or form a cycle with other inner classes. htb_classify then follows the cyclic filter graph without a hop limit and loops while the qdisc lock is held and BH processing is disabled. A single packet entering the configured path can cause a CPU soft lockup and can panic the kernel when the lockup detector is configured to panic. For the CVSS the PR:L value is used because a local unprivileged user can create a user and network namespace, obtain CAP_NET_ADMIN inside that namespace, configure the cyclic HTB graph, and send the triggering packet. Systems that disable unprivileged user namespaces generally restrict the trigger to an administrator. The issue is not directly network reachable because packet delivery alone is insufficient without prior control of the HTB qdisc and filter configuration. Impact is denial of service only. The analyzed path provides no UAF, out of bounds access, information disclosure, or write primitive that would support confidentiality or integrity impact.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)  SKIP CVE-2026-90053 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	REMOTE DEADLOCK DANGER NETWORK HARDWARE LINUS QDISC PACKET  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	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
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

Positive signals:

* +1 Local unprivileged trigger. The supplied reproducer explicitly establishes that an ordinary user can use `unshare -Urn`, obtain namespace-scoped `CAP_NET_ADMIN`, configure the cyclic HTB graph, and trigger it with one packet.
* +1 Reliable kernel crash or strong DoS. The infinite classification loop runs with the qdisc lock held and BH disabled, causing a CPU soft lockup and a kernel panic when `softlockup_panic=1`.

Negative signals:

* No negative points. The issue is not penalized merely because HTB must be configured or because kernel configuration options are required.

Material non-awarded signals:

* No UAF, OOB access, stale object, write primitive, or other memory corruption is established.
* No controlled reclaim, arbitrary write, confidentiality impact, integrity impact, or privilege escalation is demonstrated.
* Remote reachability is not established. A packet triggers the loop only after the attacker locally creates the cyclic qdisc configuration.
* No additional Availability point is awarded because the soft lockup and panic are the same DoS consequence.
* Common networking-path exposure is not awarded because the vulnerable traversal requires a specifically configured cyclic HTB filter graph and is not an ordinary default packet path.
* No race or timing penalty applies. Once configured, the trigger is deterministic.

3. Reachability analysis

The complete trigger is positively established below host-admin privilege. An ordinary local user can create a user and network namespace with `unshare -Urn`, obtain `CAP_NET_ADMIN` scoped to that namespace, create the HTB qdisc and classes on a namespace-owned interface such as loopback, install the cyclic filters, and send the triggering packet.

The relevant qdisc configuration operation is therefore namespace-scoped for this demonstrated chain. No separate host administrator, prior privileged provisioning, physical device, special hardware, race, or attacker-inaccessible sysctl is required. The lockup detector is needed for the documented watchdog report, and `softlockup_panic=1` converts the lockup into a panic, but the infinite loop and loss of the affected CPU occur without a memory-corruption primitive.

The issue is not directly remotely exploitable because network traffic alone cannot create the required cyclic HTB configuration.

4. Primitive and severity interpretation

The exact primitive is an attacker-configured nonterminating traversal of cyclic inner-class filters. This is a logic and algorithmic-complexity defect producing a soft lockup while a qdisc lock is held and BH processing is disabled.

There is no stale storage, active-object UAF, repeated release, refcount mutation, ownership violation, or semantic reinterpretation. The demonstrated consequence is local denial of service and possible kernel panic. No useful primitive remains after excluding the lockup or crash.

This is therefore CLASS_C. `ImportantCandidate=NO` because the evidence supports only a deterministic DoS with no unresolved corruption or security-boundary primitive. `AutoDowngradeAllowed=YES` because automatic Moderate handling would not discard evidence of memory corruption, privilege escalation, or authorization bypass.

Likely operational severity is Moderate on systems permitting unprivileged user namespaces. Systems disabling that facility generally restrict configuration to a more privileged actor.

5. One-sentence report phrase

A local unprivileged user can configure cyclic HTB inner-class filters inside an attacker-created network namespace and make one packet cause an infinite classification loop, CPU soft lockup, and possible kernel panic without exposing a memory-corruption or privilege-escalation primitive.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch and reproducer establish a deterministic namespace-local configuration path to DoS, while the affected code exposes no UAF, OOB operation, write primitive, disclosure, or authorization bypass requiring exploitation-leverage analysis.

7. Call-site confidence

Call-site confidence: high

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

Patch: net/sched: sch_htb: limit htb_classify inner-class filter hops [ Upstream
Commit: 83bb11805b620c498d6417e0e409f26887981489
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=83bb11805b620c498d6417e0e409f26887981489

Changed files:
  net/sched/sch_htb.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=83bb11805b620c498d6417e0e409f26887981489

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

The original Linux kernel CVE announcement for CVE-2026-90053 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90053

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: 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).