[CVE-2026-98396][MODERATE REGULAR] powerpc: Do not restore KUAP in arch_exit_to_user_mode_prepare() [ Upstream

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

CVE: CVE-2026-98396
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: powerpc: Do not restore KUAP in arch_exit_to_user_mode_prepare() [ Upstream
Commit: c8e80c19c3b9c8db83a14fab00445a6064992442
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c8e80c19c3b9c8db83a14fab00445a6064992442
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98396
Analysis date: Wed, 14 Oct 2026 22:42:13 -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.

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

The original Linux kernel CVE announcement for CVE-2026-98396 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98396

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-2026-98396	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98396 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (independent Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC)	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-693;CWE-284;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'PowerPC restored the user AMR value too early during return to userspace, while platform specific kernel exit code could still run, enable interrupts, and retry work before RFI. This temporarily violated the KUAP invariant that kernel execution must keep user memory access blocked unless access is explicitly enabled. The demonstrated symptom is a KUAP debug warning, and systems configured with panic_on_warn may turn it into a kernel crash and local denial of service. No direct UAF, out of bounds access, information disclosure, or arbitrary write primitive is established by this change, so confidentiality and integrity impact are not assigned. For the CVSS the PR:L is selected because a local user process can exercise syscall and exception return paths, while reliable security impact requires reaching the narrow exit window and compatible platform behavior. AC:H reflects the timing and platform dependent conditions. The issue is not directly network reachable. Manual review is recommended because KUAP protects the kernel and user memory boundary, and the remaining exit code should be checked for user memory accesses or interrupt paths that could turn the temporary protection gap into a stronger primitive. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE 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_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH. The independent weaker Aegis shadow evaluated CVSS 0.0 and ActionableScore 1 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	MAYBE	DANGER INIT DMAorINTERRUPT HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	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
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

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 positive signals:

* +1 Local unprivileged trigger. An ordinary process can exercise the affected syscall and exception return paths without UID 0 or a capability.
* +1 Cross-boundary policy-state violation. User AMR is restored while PowerPC kernel exit code can still execute, temporarily violating the KUAP invariant. This scores the concrete protection-state error only, not unauthorized memory access.

Paranoid additional signal:

* +1 Kernel crash or strong DoS. The demonstrated WARN can become fatal on systems configured with panic_on_warn. This is configuration-dependent and is therefore included only in the paranoid score.

No negative points:

* No race penalty. The defect is an incorrect ordering of KUAP restoration, not an established narrow attacker-dependent race.
* No host-admin penalty. Exercising syscall or exception return does not require administrative privilege.
* No hardware rarity penalty. The issue is architecture-specific, but the supplied evidence does not establish rare physical hardware as a prerequisite.

Material non-awarded signals:

* No UAF, OOB read, OOB write, controlled reclaim, arbitrary write, or repeated lifetime mutation is established.
* No confidentiality or integrity point is awarded because no kernel access to attacker-selected user memory is shown during the exposed window.
* No Important-grade authorization bypass is awarded. There is no demonstrated DENY-to-ALLOW decision involving credentials, capabilities, or a protected resource.
* No extra Availability point is awarded for the same WARN or possible panic.
* B5-R does not apply because no retry-accumulated heap OOB write exists.
* B3-PENDING does not apply because no shared-backing write exists.
* B4-PENDING does not apply because no reachable authorization decision or protected resource is identified.

3. Reachability analysis

An ordinary local PowerPC process can enter the affected path by making a syscall or causing an exception that returns to userspace. The process controls neither the platform exit implementation nor the interrupt replay path, but no special privilege is required to reach the incorrect KUAP restoration.

The complete demonstrated trigger is local and does not depend on a user namespace, network namespace, delegated capability, prior privileged setup, device assignment, or network traffic. UID 0 is not required. The supplied path does not establish remote reachability.

The warning was reproduced on an emulated POWER10 pSeries system with CONFIG_PPC_KUAP_DEBUG. A fatal result additionally depends on panic_on_warn or equivalent policy. No attacker-dependent race is established.

4. Primitive and severity interpretation

The exact defect is premature restoration of the user AMR value. Kernel code may subsequently execute, enable interrupts, and retry exit work while KUAP is no longer in the expected blocked state.

This is an invalid security-state ordering condition rather than memory corruption or an authorization-decision bypass. The demonstrated consequence is a KUAP assertion warning. A kernel panic is possible under fatal-warning policy.

No concrete code path is shown reading from or writing to user memory during the exposed interval. Consequently, privilege escalation, information disclosure, arbitrary memory access, and integrity compromise remain speculative rather than patch-supported consequences.

PrimitiveClass is CLASS_C because the established behavior is invalid protection state with WARN or possible DoS, without an independently useful corruption or access primitive. ImportantCandidate is NO because no protected-resource access, memory-corruption primitive, or meaningful DENY-to-ALLOW transition is established. AutoDowngradeAllowed is YES because the unresolved concern is defense-state exposure rather than a concrete exploitable primitive.

5. One-sentence report phrase

PowerPC restored user AMR state before all kernel exit work completed, temporarily violating KUAP and causing a warning or possible panic without establishing unauthorized memory access or privilege escalation.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review the remaining interrupt and syscall exit work for any user-memory access while the user AMR value is active, because such a caller would materially change the primitive classification.

7. Call-site confidence

Call-site confidence: high

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

Patch: powerpc: Do not restore KUAP in arch_exit_to_user_mode_prepare() [ Upstream
Commit: c8e80c19c3b9c8db83a14fab00445a6064992442
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c8e80c19c3b9c8db83a14fab00445a6064992442

Changed files:
  arch/powerpc/include/asm/book3s/64/kup.h
  arch/powerpc/include/asm/entry-common.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=c8e80c19c3b9c8db83a14fab00445a6064992442

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

The original Linux kernel CVE announcement for CVE-2026-98396 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98396

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: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: 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).