[CVE-2026-89915][MODERATE 7.0] KVM: arm64: Remove VM-wide VNCR mapping counter [ Upstream

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

CVE: CVE-2026-89915
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: arm64: Remove VM-wide VNCR mapping counter [ Upstream
Commit: 5453b85c7ebb605febac3df42021f9471663f051
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5453b85c7ebb605febac3df42021f9471663f051
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89915
Analysis date: Wed, 16 Sep 2026 10:48:22 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
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-2026-89915 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89915

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: YES

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
======================================================================

CVE-2026-89915	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89915 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-664;CWE-362;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'KVM arm64 nested virtualization can miss VNCR related TLB invalidations because a VM wide VNCR mapping counter may say that no L0 fixmap mapping is active even though the VNCR pseudo TLB still needs invalidation. An L1 guest that changes or invalidates its own VNCR backing state can leave stale translations visible to L0, causing incorrect reads or writes of guest controlled state and possible VM or host side instability. For the CVSS the PR:L is used because an attacker generally needs local ability to run or control a KVM VM with nested virtualization enabled, but does not need full host administrator privileges if /dev/kvm access is delegated. The issue is not network reachable and is triggered through KVM virtualization control paths rather than packet traffic. Impact is at least denial of service and, for the paranoid interpretation, may include confidentiality or integrity impact from stale hypervisor translation use, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) 	MAYBE	VIRT HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4. No floor or cap applied.
* Raw paranoid score: 5
* Paranoid score: 5. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2. CLASS_B active translation lifetime and consistency violation. The VM-wide VNCR mapping counter can cause KVM arm64 nested virtualization to skip required VNCR pseudo-TLB invalidation while stale translation state may still be operational.
* +1. Active translation synchronization signal. The fix forces the affected E2H and TGE TLBI case into the slow path instead of relying on an opportunistic mapping counter, which indicates that stale TLB state may survive outside the immediate software object.
* +1. Weak indirect corruption or unsafe mapping-selection candidate. A stale VNCR pseudo-TLB entry can make L0 use an obsolete L1-provided VNCR backing page, creating incorrect hypervisor-side state access, but without a demonstrated arbitrary host read or write primitive.

Paranoid additional signal:

* +1. Active semantic translation-state transition concern. The stale VNCR translation may survive across L1 invalidation or backing-state changes, so the old active translation can be reused after the semantic state it represented should have been retired.

Non-awarded signals:

* RealUAF not awarded. The patch establishes missed invalidation of active translation state, not a heap object use-after-free.
* Strong guest-to-host exploitation leverage +2 not awarded. The evidence does not establish guest-controlled host memory reclaim, arbitrary host read/write, callback control, or demonstrated guest-to-host escape.
* Reliable kernel crash / strong DoS +1 not awarded. The commit describes missed invalidations and broken guarantees, but does not show an observed host panic or deterministic crash.
* Remote reachable +2 not awarded. The issue is reached through KVM virtualization control and guest execution paths, not network packet processing.
* Local unprivileged trigger +1 not awarded. Below-host-admin /dev/kvm access is plausible, but ordinary unprivileged reachability for all required setup is not established.
* Concrete repeated release/refcount +1 not awarded. No repeated put, release, free, or refcount mutation on the same dangling reference is present.
* Confidentiality and integrity points not awarded as direct ActionableScore signals. Possible C/I impact remains a paranoid review concern from stale hypervisor translation use, not a demonstrated disclosure or write primitive.

3. Reachability analysis

The evaluated attacker model is a local VMM or a user with access to `/dev/kvm`, running or controlling an arm64 KVM VM with nested virtualization enabled. This is below host-administrator privilege when `/dev/kvm` is delegated to a user or service, so HostAdminRequired is NO for the interface semantics. It is not namespace-scoped.

A malicious L1 guest can perform TLBI operations and may influence the VNCR state being invalidated, but the complete setup of nested virtualization and VM execution is controlled by the VMM or local KVM user. Therefore this is not scored as a pure guest-only exploit chain unless additional evidence shows the guest can independently trigger the complete sequence. No network trigger is established. No special rare physical hardware is established beyond arm64 KVM nested virtualization support. The issue is not a timing race in the usual UAF sense, so no race penalty is applied.

4. Primitive and severity interpretation

The exact primitive is a missed invalidation of active VNCR pseudo-TLB state in KVM arm64 nested virtualization. This is an active translation-state consistency bug, not fixed stale storage and not a concrete UAF. The stale state participates in hypervisor-side VNCR handling and can cause L0 to use stale L1-provided VNCR translation state after invalidation should have occurred.

This fits CLASS_B because it is a meaningful active-state lifetime / synchronization violation in a security-sensitive KVM MMU/TLB path, but the supplied evidence does not establish CLASS_A. There is no demonstrated controlled host memory corruption, no arbitrary read/write, no controlled reclaim, no callback hijack, and no proven guest-to-host privilege escalation.

ImportantCandidate is YES because stale hypervisor translation state in nested KVM has unresolved exploitation and isolation implications beyond an ordinary warning-only bug. AutoDowngradeAllowed is NO because automatically closing this as ordinary Moderate would lose a concrete active translation synchronization primitive that needs manual KVM/MMU review. Operationally, this should be treated as Actionable Moderate at minimum, with manual review to decide whether the stale VNCR translation can produce stronger confidentiality, integrity, or guest-to-host effects.

5. One-sentence report phrase

KVM arm64 nested virtualization may skip required VNCR pseudo-TLB invalidation because a VM-wide mapping counter can incorrectly suppress slow-path TLBI handling, leaving stale hypervisor translation state active and requiring manual review for possible VM or host impact.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not a proven arbitrary host read/write or LPE, but it is a concrete missed-invalidation bug in KVM nested virtualization translation state, and the security impact of stale VNCR pseudo-TLB entries cannot be safely auto-downgraded from the patch alone.

7. Call-site confidence

Call-site confidence: high

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

Patch: KVM: arm64: Remove VM-wide VNCR mapping counter [ Upstream
Commit: 5453b85c7ebb605febac3df42021f9471663f051
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5453b85c7ebb605febac3df42021f9471663f051

Changed files:
  arch/arm64/include/asm/kvm_host.h
  arch/arm64/kvm/hyp/vhe/switch.c
  arch/arm64/kvm/nested.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=5453b85c7ebb605febac3df42021f9471663f051

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

The original Linux kernel CVE announcement for CVE-2026-89915 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89915

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:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 4
  Paranoid ActionableScore: 5

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).