[CVE-2026-99186][MODERATE 7.0] x86/insn-eval: Fix shadowed segment index in get_seg_reg_override_idx() [ Upstream

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

CVE: CVE-2026-99186
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: x86/insn-eval: Fix shadowed segment index in get_seg_reg_override_idx() [ Upstream
Commit: 0efe927053f7eabd1e8658cbda96ee3d78bd1765
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0efe927053f7eabd1e8658cbda96ee3d78bd1765
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99186
Analysis date: Wed, 28 Oct 2026 22:02:18 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
An unprivileged local user can execute a crafted UMIP-emulated instruction that traps a CPU in an unkillable kernel loop, enabling host-wide CPU denial of service without a shown memory-corruption or privilege-escalation primitive.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-99186 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99186

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 7.0
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-99186	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99186 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC TEMP_DISTANCE_MERGE=LOW->MODERATE;AEGIS=MODERATE_KPANIC;GAP=2 (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-835;CWE-682;CWE-710;BEST CVSS score: '5.5';DESCR 'A shadowed loop variable in get_seg_reg_override_idx causes incorrect segment override handling during x86 instruction decoding. A local user can execute a CS prefixed SGDT, SIDT or SMSW memory instruction on a system using UMIP emulation and trap the CPU in an infinite kernel loop where even SIGKILL cannot be delivered. Multiple processes can tie up available CPUs until reboot, giving high availability impact. FS and GS overrides can also direct the emulated result to the wrong user space address, but there is no supported kernel memory corruption or privilege escalation primitive. For the CVSS the PR:L value is selected because an unprivileged local user context is required to execute the crafted instruction. The issue is not directly network reachable and requires affected x86 hardware and kernel behavior. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage LOW to MODERATE_KPANIC because the cross model evidence reconciliation 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 TEMP_DISTANCE_MERGE and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 5.5 and ActionableScore 4 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	MAYBE	LEAK HARDWARE LINUS BOOT TEMP_DISTANCE_MERGE KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=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 base score: 2
* Conservative score: 2; no floor or cap applied.
* Raw paranoid score: 2
* Paranoid score: 2; no stronger primitive is supported.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Local unprivileged trigger: the commit explicitly states that any user can execute a CS-prefixed SGDT, SIDT, or SMSW memory instruction and reach UMIP emulation through the user-space #GP path.
* +1 Reliable kernel strong DoS: loop-counter shadowing makes the CS-prefix loop nonterminating in kernel context; SIGKILL cannot be delivered, and multiple tasks can occupy CPUs until reboot.
* +0 FS/GS wrong-address write: the emulated result is written to an incorrect address in the triggering process’s user address space. No kernel-memory overwrite, protected victim, or cross-authority integrity effect is shown.
* No separate availability point: CPU exhaustion and reboot requirement are consequences of the same infinite-loop DoS.
* No remote reachability, UAF, OOB write, arbitrary write, authorization bypass, disclosure, or privilege escalation is shown.
* NEW52 B5-R does not apply: there is no retry-accumulated heap OOB write or later consumption of corrupted heap state.
* NEW53 B3-PENDING does not apply: no shared/file-backed storage write is involved.

3. Reachability analysis

An ordinary local user can directly execute the crafted instruction; no UID 0, capability, namespace privilege, prior privileged setup, or separate administrator action is identified. The affected path requires x86 UMIP emulation behavior, but the evidence does not justify a rare-hardware or host-admin penalty. No timing race is required.

4. Primitive and severity reading

The exact primitive is a kernel-context infinite loop caused by modification of the macro’s shadowing iterator. This is CLASS_C availability behavior, not memory corruption. The FS/GS case misdirects an emulated userspace write but does not establish modification of kernel or another principal’s state. ImportantCandidate is therefore NO, and automatic Important-to-Moderate downgrade is allowed.

5. One-sentence report phrase

An unprivileged local user can execute a crafted UMIP-emulated instruction that traps a CPU in an unkillable kernel loop, enabling host-wide CPU denial of service without a shown memory-corruption or privilege-escalation primitive.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm the affected x86/UMIP configurations and operational CPU-exhaustion exposure.

7. Call-site confidence

Call-site confidence: high

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

Patch: x86/insn-eval: Fix shadowed segment index in get_seg_reg_override_idx() [ Upstream
Commit: 0efe927053f7eabd1e8658cbda96ee3d78bd1765
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0efe927053f7eabd1e8658cbda96ee3d78bd1765

Changed files:
  arch/x86/include/asm/insn.h
  tools/arch/x86/include/asm/insn.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=0efe927053f7eabd1e8658cbda96ee3d78bd1765

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

The original Linux kernel CVE announcement for CVE-2026-99186 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99186

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: Not available
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).