[CVE-2026-98865][LOW] sched_ext: Don't run ops.dequeue() with a DSQ lock held [ Upstream

[CVE-2026-98865][LOW] sched_ext: Don't run ops.dequeue() with a DSQ lock held [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98865
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: sched_ext: Don't run ops.dequeue() with a DSQ lock held [ Upstream
Commit: fe499ae9da46e297f578d4346c4c8e338c3118db
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fe499ae9da46e297f578d4346c4c8e338c3118db
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98865
Analysis date: Fri, 23 Oct 2026 21:00:01 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A sched_ext BPF scheduler can self deadlock when ops.dequeue() recursively accesses a user DSQ while the source DSQ lock is held, causing a possible CPU soft lockup or system DoS without a memory corruption or privilege escalation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-98865 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98865

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-2026-98865	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98865 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT LOW AEGIS_AS=1 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both 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-833;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '4.7';DESCR 'sched_ext can invoke ops.dequeue() while the source user DSQ lock is still held on task consume and move paths. A BPF scheduler that accesses or iterates a user DSQ from this callback can attempt recursive DSQ locking and self deadlock in scheduler context. This may produce a CPU soft lockup or a broader system availability failure, but the affected code does not provide a UAF, OOB access, information disclosure, or write primitive. For the CVSS the PR:L value is used for the paranoid score because a local user may trigger dequeue activity when an affected sched_ext BPF scheduler has already been installed by an administrator. Exploitation still requires the nondefault sched_ext configuration and a callback that locks a user DSQ, so AC:H is appropriate. The issue is not network reachable and its supported impact is denial of service only. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to LOW because the CVSS score, vector or bug class threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH. The independent weaker Aegis shadow evaluated CVSS 2.3 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	OOB DEADLOCK BPF RACE HARDWARE LINUS  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

## 1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap changed the score.
* Final recommended bucket: **Ordinary Moderate**::

## 2. Signal breakdown

* **+1 Reliable kernel DoS:** invoking `ops.dequeue()` while the source user DSQ lock remains held can make a BPF scheduler recursively acquire a DSQ lock, causing a deterministic self-deadlock and possible CPU soft lockup.
* **No additional availability point:** a broader scheduler stall is part of the same deadlock consequence, not an independent impact.
* **No memory-corruption signal:** the patch establishes neither UAF, OOB access, stale lifetime use, refcount corruption, nor attacker-directed write.
* **No active-state corruption signal:** DSQ state is protected and the callback placement is wrong, but no corrupted structured state is subsequently consumed.
* **No local-unprivileged bonus:** an ordinary task may cause dequeue activity after setup, but the evidence does not establish that such a user controls the required BPF scheduler behavior or complete trigger.
* **No privilege penalty:** loading sched_ext BPF schedulers is privilege-gated, but the supplied evidence does not resolve whether host-admin rights, delegated BPF rights, or prior administrator setup governs the complete evaluated trigger.
* **No race penalty:** this is a lock-ordering defect and deterministic recursive locking condition, not a narrow timing race.
* **No B5-R, B3-PENDING, V32, or V33 signal:** there is no heap OOB write, shared-backing write, checksum metadata corruption, or attacker-influenced active-state corruption.
* **No authorization signal:** no DENY-to-ALLOW decision or protected-resource access is involved.

## 3. Reachability analysis

The vulnerable condition requires sched_ext to be active and a BPF scheduler whose `ops.dequeue()` callback accesses or iterates a user DSQ. Loading and attaching such a scheduler is privilege-gated, but the supplied context does not establish whether the complete deployment requires host-admin-equivalent privilege or can be delegated.

After an affected scheduler is installed, ordinary scheduling activity may reach the consume or move paths. However, the evidence does not prove that an ordinary user can intentionally select the callback behavior or reliably construct the complete condition. No network trigger, user-namespace-complete path, special hardware, or timing race is established.

## 4. Primitive and severity interpretation

The exact primitive is recursive DSQ locking from `ops.dequeue()` while the source DSQ lock is already held. This is a deadlock and availability defect classified as **CLASS_C**.

There is no UAF, repeated release, active structured-object corruption, OOB write, authorization bypass, or useful primitive beyond the deadlock. The supported consequence is scheduler-context self-deadlock, potentially producing a CPU soft lockup or system availability loss. Privilege escalation, confidentiality loss, and integrity compromise are unsupported.

`ImportantCandidate=NO` because no Important-preserving corruption or security-boundary primitive exists. `AutoDowngradeAllowed=YES` because the defect is a constrained, configuration-dependent DoS with no unresolved memory-corruption leverage.

## 5. One-sentence report phrase

A sched_ext BPF scheduler can self deadlock when ops.dequeue() recursively accesses a user DSQ while the source DSQ lock is held, causing a possible CPU soft lockup or system DoS without a memory corruption or privilege escalation primitive.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

The demonstrated issue is a configuration-dependent recursive-locking DoS, and the patch provides no evidence of memory corruption, authorization bypass, cross-boundary impact, or exploitation leverage beyond availability loss.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: sched_ext: Don't run ops.dequeue() with a DSQ lock held [ Upstream
Commit: fe499ae9da46e297f578d4346c4c8e338c3118db
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fe499ae9da46e297f578d4346c4c8e338c3118db

Changed files:
  kernel/sched/ext/ext.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=fe499ae9da46e297f578d4346c4c8e338c3118db

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

The original Linux kernel CVE announcement for CVE-2026-98865 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98865

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:H
  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).