[CVE-2026-98087][LOW] sched/rt,dl: Skip migrate-disabled tasks when picking a push candidate [ Upstream

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

CVE: CVE-2026-98087
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: sched/rt,dl: Skip migrate-disabled tasks when picking a push candidate [ Upstream
Commit: 755476d7ba92a1f3fe2a376c515d710bc6baebc3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=755476d7ba92a1f3fe2a376c515d710bc6baebc3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98087
Analysis date: Fri, 25 Sep 2026 09:15:13 -0400
ActionableScore: 1
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-2026-98087 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98087

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-2026-98087	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98087 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:N/I:N/A:H';**CWE-400;CWE-770;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A scheduler logic error can keep migrate disabled RT or deadline tasks on the pushable task lists even though such tasks cannot be migrated. RT and deadline load balancing can repeatedly send push IPIs and wake stopper threads for migrations that always fail, consuming CPU time and degrading real time scheduling behavior on the affected CPU. The condition requires a specific timing and workload combination involving migrate disabled tasks and RT or deadline balancing. For the CVSS the PR:L value is used for the conservative score because some deployments may delegate RT scheduling capabilities to service accounts or container workloads. The issue is not network reachable. Impact is availability degradation or local denial of service. No memory corruption, information disclosure, integrity impact, or privilege escalation primitive is indicated by the scheduler code change.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	HARDWARE LINUS 	NO	NO	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Availability impact realistic. The documented retry loop repeatedly sends push IPIs and wakes stopper threads for migrations that cannot succeed. The supplied trace shows substantial CPU loss on the affected real time CPU, with about 37 ms of 89 ms consumed by the stopper thread.
* No reliable kernel crash or panic point. The demonstrated effect is CPU starvation and real time scheduling degradation, not a kernel crash.
* No memory corruption signal. The patch changes scheduler candidate selection and does not establish UAF, stale pointer use, refcount misuse, OOB access, invalid free, or invalid object lifetime.
* No remote reachability point. The condition is driven by local scheduler state, RT or deadline tasks, and migrate_disable state.
* No local unprivileged trigger point. The supplied evidence does not establish that an ordinary unprivileged user can create the complete SCHED_FIFO or SCHED_DEADLINE plus migrate_disable workload.
* No privilege penalty. Host-admin-equivalent privilege is not positively established, and the supplied evidence also does not prove below-host-admin reachability.
* No hard race penalty. The issue is a deterministic scheduler-state retry loop once the relevant workload exists. It is not demonstrated as a narrow or uncontrollable timing race.
* No strong DoS point beyond the availability point. The evidence shows degradation of an affected CPU and missed real time work, not host-wide unavailability, persistent exhaustion, or a fatal system condition.
* No confidentiality or integrity points. No disclosure, unauthorized modification, authorization bypass, or privilege escalation primitive is established.

3. Reachability analysis

The affected condition requires a workload containing at least two RT or deadline scheduling tasks, with a queued migrate-disabled task retained at the head of the pushable task list and another task running on the same runqueue. The supplied trace uses two equal-priority SCHED_FIFO tasks on an isolated CPU.

The patch and commit do not establish who can create the complete workload in the evaluated environment. SCHED_FIFO and SCHED_DEADLINE use is commonly restricted through CAP_SYS_NICE, RLIMIT_RTPRIO, cgroup or service policy, but the exact applicable privilege boundary is not included in the supplied context. Therefore HostAdminRequired is UNKNOWN rather than YES or NO.

No namespace-aware capability path is demonstrated. The fact that real time scheduling rights can potentially be delegated to a service or container does not establish that all required operations, including the relevant migrate_disable state and RT balancing scenario, are available in an attacker-controlled namespace. NamespaceScopedPrivilege is UNKNOWN.

There is no network path, special hardware requirement, firmware dependency, boot option, or prior host setup demonstrated by the patch. The issue is local scheduler behavior and its effect is limited by the CPU placement and workload duration. The loop ends when the running task sleeps or otherwise yields.

4. Primitive and severity interpretation

The exact primitive is a scheduler logic and resource-consumption defect. A migrate-disabled RT or deadline task remains advertised as pushable even though it cannot be migrated. The RT or deadline balancer repeatedly attempts failed migrations and can repeatedly invoke stopper threads.

This is not an active-object lifetime bug, fixed stale storage bug, UAF, double release, refcount mutation, structured stale-object reuse, or security-boundary bypass. The patch does not show a freed object, a stale reference, a post-free operation, a controlled replacement, an arbitrary write, or an attacker-controlled memory-access primitive.

The demonstrated consequence is degraded CPU availability and impaired real time scheduling on an affected CPU. The commit documents repeated failed push attempts and significant CPU time consumed by stopper activity. A host-wide crash, persistent resource exhaustion, confidentiality impact, integrity impact, privilege escalation, and remote exploitation are speculative and unsupported.

PrimitiveClass is CLASS_C because the demonstrated behavior is an invalid scheduler state leading to local availability degradation. ImportantCandidate is NO because there is no unresolved kernel-corruption or boundary-bypass primitive. AutoDowngradeAllowed is YES because the issue is a constrained local scheduling DoS without meaningful exploitation leverage beyond the demonstrated resource consumption.

5. One-sentence report phrase

A scheduler logic error can repeatedly attempt impossible migrations of migrate-disabled RT or deadline tasks, causing excessive stopper activity and CPU starvation on the affected processor.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch establishes a local scheduler performance and availability defect only. It does not establish memory corruption, privilege escalation, remote reachability, security-boundary bypass, or a meaningful unresolved exploitation primitive.

7. Call-site confidence

Call-site confidence: medium

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

Patch: sched/rt,dl: Skip migrate-disabled tasks when picking a push candidate [ Upstream
Commit: 755476d7ba92a1f3fe2a376c515d710bc6baebc3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=755476d7ba92a1f3fe2a376c515d710bc6baebc3

Changed files:
  kernel/sched/deadline.c
  kernel/sched/rt.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=755476d7ba92a1f3fe2a376c515d710bc6baebc3

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

The original Linux kernel CVE announcement for CVE-2026-98087 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98087

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:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1

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).