[CVE-2026-98017][MODERATE REGULAR] net/sched: defer qdisc freeing after failed creation [ Upstream

[CVE-2026-98017][MODERATE REGULAR] net/sched: defer qdisc freeing after failed creation [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98017
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: defer qdisc freeing after failed creation [ Upstream
Commit: 20bf6fa34b345333971bd4464a322cce87b83f4e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=20bf6fa34b345333971bd4464a322cce87b83f4e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98017
Analysis date: Fri, 25 Sep 2026 08:17:45 -0400
ActionableScore: 4
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-2026-98017 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98017

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-2026-98017	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98017 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use after free in the traffic control qdisc creation error path occurs when a failed RTM_NEWQDISC request publishes a clsact embedded mini Qdisc to lockless tc_run readers and then frees the containing Qdisc synchronously. A malformed qdisc configuration with an invalid TCA_RATE can remove the published pointer while a concurrent tc_run path still holds and dereferences it. This can at least cause a local kernel crash and may provide limited confidentiality or integrity impact because the freed object is accessed in a lockless networking path. For the CVSS the PR:L metric is used for the paranoid score because triggering requires traffic control administration such as CAP_NET_ADMIN or delegated network administration capabilities, which can be available to a container or service account and does not necessarily require full host root access. The issue is not directly network reachable because packet traffic alone does not issue RTM_NEWQDISC requests. Impact is at least local denial of service and the UAF should be reviewed manually for possible memory reuse and privilege escalation conditions.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	REMOTE INIT BPF SIMPLEFIX LINUS QDISC 	NO	NO	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Real UAF and RCU lifetime violation. The failed qdisc creation path can publish an embedded mini_Qdisc to lockless tc_run readers and then synchronously free the containing Qdisc while such a reader may still retain and dereference it.
* +1 Reliable kernel crash or strong DoS. Dereferencing a freed Qdisc container from tc_run can realistically result in a kernel fault, Oops, or crash.
* +1 Attacker influenced lifetime transition. A caller that can issue RTM_NEWQDISC requests can repeatedly drive the relevant create, publish, invalid TCA_RATE failure, and free sequence. This is counted only as lifetime transition control, not as controlled post free reuse.

Non awarded material signals:

* Controlled reclaim or controlled replacement of the freed Qdisc storage is not demonstrated.
* Arbitrary kernel write, directed write, callback control, or refcount takeover is not demonstrated.
* Structured stale object leverage is not separately awarded. The patch establishes that tc_run can hold a stale reference, but does not show attacker controlled replacement contents or a meaningful post reuse field consumption primitive.
* Weak LPE concern is not awarded merely because the bug is a UAF in networking code.
* Remote reachability is not established. RTM_NEWQDISC is a local rtnetlink control plane operation and packet traffic alone does not invoke it.
* No concrete repeated release or refcount mutation exists. The bug is a premature free while a reader remains active, not a second put or free through the same dangling reference.
* No additional Availability point is awarded because the kernel crash consequence is already counted once.
* Confidentiality and integrity impact are not awarded. The patch establishes post free dereference but does not establish disclosure, attacker controlled reuse, or unauthorized modification.

3. Reachability analysis

The immediate trigger is a local RTM_NEWQDISC configuration request that creates a clsact qdisc, binds a populated shared ingress block, and subsequently fails estimator setup due to an invalid TCA_RATE value. The stale use requires concurrent tc_run processing after the mini_Qdisc was published and before an RCU grace period would normally protect readers.

Traffic control qdisc changes normally require CAP_NET_ADMIN in the relevant network namespace. The supplied patch does not establish whether all required stages, including creation of the shared ingress block, clsact setup, traffic generation, and the affected tc_run path, are available to an unprivileged attacker inside an attacker created user and network namespace on the evaluated product.

Therefore HostAdminRequired is UNKNOWN rather than YES. NamespaceScopedPrivilege is also UNKNOWN because rtnetlink capability checks may be namespace scoped, but the complete trigger chain was not shown. No special hardware, boot parameter, or firmware prerequisite is indicated. The race is repeatable in principle through repeated qdisc configuration and traffic activity, but the supplied context does not prove a reliably controlled timing window.

4. Primitive and severity interpretation

The established primitive is a real active object use after free. A Qdisc containing an embedded mini_Qdisc is published to lockless packet processing readers, then freed synchronously on an error unwind despite possible active tc_run readers. The normal destruction path already retires Qdiscs through call_rcu, and the patch applies that same retirement model to failed creation.

This is an active structured object rather than fixed stale DMA storage. However, the available evidence shows only that tc_run may dereference the freed object. It does not establish that an attacker can reclaim the exact memory with attacker shaped state, cause type confusion, influence callbacks, obtain disclosure, or perform a useful write after reuse.

The issue is therefore CLASS_B, not CLASS_A. The demonstrated consequence is local kernel DoS. Privilege escalation or confidentiality and integrity impact remain speculative and are not scored. ImportantCandidate is NO because meaningful exploitation leverage beyond the crash has not been established from the patch. AutoDowngradeAllowed is YES because the currently supported behavior is a constrained stale active object dereference without demonstrated reuse, write, callback, boundary bypass, or repeated lifetime mutation.

Likely operational severity is Borderline Moderate. It should receive focused review because the bug is an RCU lifetime violation in a packet processing path, but the patch alone does not justify Important classification.

5. One-sentence report phrase

A failed RTM_NEWQDISC request can publish a clsact mini_Qdisc to lockless tc_run readers and then free its containing Qdisc before an RCU grace period, resulting in a local use after free and potential kernel crash.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The patch establishes a genuine RCU lifetime UAF in an active traffic control data path, but controlled reclaim, attacker shaped post free state, and below host admin reachability are not established.

7. Call-site confidence

Call-site confidence: medium

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

Patch: net/sched: defer qdisc freeing after failed creation [ Upstream
Commit: 20bf6fa34b345333971bd4464a322cce87b83f4e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=20bf6fa34b345333971bd4464a322cce87b83f4e

Changed files:
  include/net/sch_generic.h
  net/sched/sch_api.c
  net/sched/sch_generic.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=20bf6fa34b345333971bd4464a322cce87b83f4e

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

The original Linux kernel CVE announcement for CVE-2026-98017 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98017

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:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 4

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).