[CVE-2026-89520][MODERATE REGULAR] sched/core: Make core-sched flips wait for in-flight selections [ Upstream

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

CVE: CVE-2026-89520
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: sched/core: Make core-sched flips wait for in-flight selections [ Upstream
Commit: 9d69422ddaa13526c2223444e237d948d6e44728
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9d69422ddaa13526c2223444e237d948d6e44728
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89520
Analysis date: Sat, 12 Sep 2026 02:09:53 -0400
ActionableScore: 3
ActionableScore lower bound: 2
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-2026-89520 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89520

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-2026-89520	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89520 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;CWE-667;CWE-662;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race in sched core can occur when a core wide pick_next_task() selection temporarily drops the shared rq lock and a concurrent core scheduling flip completes before the selection finishes. The selection can then continue with rq_lockp() rebound underneath it, touching sibling runqueue state without the intended lock protection and later releasing a lock that was not taken while leaking another lock. For the CVSS the PR:L is used because reliable triggering requires local code execution or a local component able to manipulate core scheduling cookie lifetime events, but does not normally require full administrator privileges. The issue is not network reachable and is tied to local scheduler control plane behavior on systems with core scheduling enabled. Impact is at least local denial of service via scheduler lock imbalance, soft lockup, or kernel crash. In a conservative review, limited confidentiality or integrity impact cannot be fully ruled out because scheduler state may be accessed without the intended lock, but no direct arbitrary write or information leak primitive is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	LEAK HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	YES	NO	checked

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

ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2. No floor or cap applied.
* Raw paranoid score: 3
* Paranoid score: 3. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative signals:

* +1 Local below-host-admin reachability. The supplied context supports PR:L: a local process or local component can manipulate core scheduling cookie lifetime events without normally requiring full host administrator privilege.
* +1 Reliable kernel crash / strong DoS. The race can produce scheduler lock imbalance, access to sibling rq state without the intended lock, wrong lock release, and leaked lock state.
* +1 Weak / indirect active-state corruption candidate. The patch establishes incorrect scheduler lock protection and active runqueue state access under a rebinding rq_lockp condition, but not a concrete memory corruption primitive.
* -1 Hard timing / race condition. The vulnerable window requires a lock-dropping pick_task() during core-wide selection and a concurrent core-scheduling flip completing mid-selection.

Paranoid difference:

* The paranoid score withholds the -1 hard timing penalty because local repeated cookie-lifetime operations may make attempts repeatable enough that the race should not be automatically minimized.
* No additional C/I or LPE point is awarded.

Important non-awarded signals:

* Real UAF not established.
* Concrete repeated release/refcount mutation not established.
* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Callback or function-pointer control not demonstrated.
* Remote reachability not established.
* Confidentiality and integrity are not awarded because no disclosure path, arbitrary write, credential modification, or security-boundary bypass is shown.
* Extra Availability is not awarded because the scheduler crash or lockup is already counted once.
* ImportantCandidate is not set because the evidence supports a serious synchronization DoS, not a strong unresolved exploitation primitive.

3. Reachability analysis

The evaluated attacker is a local user or local process able to cause core-scheduling cookie lifetime changes while scheduler core-wide selection is occurring. The issue is not network reachable and is not triggered by packet processing.

Host-admin-equivalent privilege is not positively established. The supplied prior analysis supports PR:L rather than PR:H, so HostAdminRequired=NO. Namespace-scoped capability is not the relevant model here, so NamespaceScopedPrivilege=NO.

The vulnerable configuration requires CONFIG_SCHED_CORE and active use of core scheduling. This is a configuration prerequisite, but the supplied evidence does not justify treating it as rare or physical-only. The race is timing-sensitive because the flip must occur while pick_next_task() is in an in-flight core-wide selection and a pick_task() has temporarily dropped the shared rq lock. Repeated attempts appear plausible from local control-plane activity, but reliability is unresolved.

4. Primitive and severity interpretation

The exact primitive is a scheduler synchronization and lock-ownership race. A core-wide pick_next_task() selection may temporarily lose the shared rq lock, allowing __sched_core_flip(false) to rebind rq_lockp() before the selection finishes. The resumed selection can then operate on sibling runqueue state without the intended protection and later release a lock that was not taken while leaking the lock that was.

This is active scheduler state misuse, but not a demonstrated UAF, double free, repeated refcount mutation, OOB access, controlled overwrite, or authorization bypass. The demonstrated consequence is local DoS through lock imbalance, soft lockup, or kernel crash. The speculative consequence is broader scheduler state corruption, but the patch does not establish an independently useful exploitation primitive beyond crash or lockup.

PrimitiveClass=CLASS_C because the established behavior is primarily invalid locking and DoS without a concrete memory-corruption or lifetime primitive. ImportantCandidate=NO because meaningful exploitation leverage beyond DoS is not established. AutoDowngradeAllowed=YES because no Class A primitive, real UAF, repeated lifetime mutation, boundary bypass, or controlled corruption remains unresolved.

Likely operational severity is borderline Moderate: not safe to ignore blindly because it is in the core scheduler, but not enough evidence for Important-class treatment.

5. One-sentence report phrase

A local race in sched core can let a core scheduling flip rebind rq_lockp() while pick_next_task() is still selecting sibling runqueues, causing scheduler lock imbalance and a kernel DoS without a demonstrated UAF or privilege escalation primitive.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the bug affects scheduler locking and active runqueue state, but the available evidence supports DoS-oriented invalid-state behavior rather than a concrete Important-grade exploitation primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: sched/core: Make core-sched flips wait for in-flight selections [ Upstream
Commit: 9d69422ddaa13526c2223444e237d948d6e44728
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9d69422ddaa13526c2223444e237d948d6e44728

Changed files:
  kernel/sched/core.c
  kernel/sched/sched.h

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=9d69422ddaa13526c2223444e237d948d6e44728

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

The original Linux kernel CVE announcement for CVE-2026-89520 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89520

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 2
  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).