[CVE-2026-89917][MODERATE 7.0] KVM: arm64: Handle VNCR TLB invalidation race with vcpu_put() VNCR unmapping [ Upstream

[CVE-2026-89917][MODERATE 7.0] KVM: arm64: Handle VNCR TLB invalidation race with vcpu_put() VNCR unmapping [ Upstream

[AL-KERNEL]

CVE: CVE-2026-89917
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: arm64: Handle VNCR TLB invalidation race with vcpu_put() VNCR unmapping [ Upstream
Commit: 5cbd8761d001c11ada73b1753d772c4745a70e72
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5cbd8761d001c11ada73b1753d772c4745a70e72
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89917
Analysis date: Wed, 16 Sep 2026 10:44:11 -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.

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

The original Linux kernel CVE announcement for CVE-2026-89917 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89917

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89917	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89917 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:C/C:L/I:L/A:H';CWE-362;CWE-367;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in KVM arm64 nested virtualization can occur between VNCR TLB invalidation and vcpu_put unmapping of the VNCR fixmap. The race can make both paths observe stale mapping state and can lead to a BUG_ON or a call to vncr_fixmap with cpu -1, causing a host kernel crash or unstable fixmap state. For the CVSS the PR:L is used because a local user or guest workload with access to KVM is needed to create and run the affected vCPU state. The issue is not network reachable and requires an affected arm64 KVM nested virtualization configuration. Impact is at least denial of service via host kernel crash and in the paranoid case may include limited confidentiality or integrity impact because the bug affects host fixmap unmapping state across racing CPUs. Privilege escalation is not proven from the diff, but the KVM race and fixmap state issue justify manual review. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)  SKIP CVE-2026-89917 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	VIRT RACE HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

======================================================================
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:

* +1 Reliable kernel crash / strong DoS. The commit explicitly reports a BUG_ON path and potentially lethal effects from vncr_fixmap(-1).
* +1 Active host mapping synchronization violation. VNCR fixmap unmapping can race between TLBI handling and vcpu_put(), and the fix makes the cpu ownership transition atomic.
* +1 Weak / indirect corruption candidate. The stale cpu value can lead to invalid fixmap selection, including cpu -1, affecting host fixmap state rather than only passive metadata.
* +1 KVM host boundary relevance. The bug is in host KVM arm64 nested virtualization state and can cause host-side failure from VM execution context reachable by a local VMM / KVM user.

Paranoid additional signal:

* +1 Unresolved active host translation-state hazard. The race can leave stale VNCR mapping state or clear the wrong fixmap entry, which is more concerning than a simple WARN-only path, but still does not establish arbitrary host read/write or guest escape.

Non-awarded signals:

* RealUAF not awarded. The patch shows a race in active mapping ownership and unmapping, not a freed object being dereferenced after lifetime end.
* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Confidentiality and integrity bonuses not awarded as independent ActionableScore signals.
* Strong guest-to-host exploitation leverage +2 not awarded. The evidence supports host crash and active fixmap-state corruption concern, but not a concrete guest-directed host memory primitive.
* Local ordinary unprivileged trigger +1 not awarded. Access to KVM normally requires /dev/kvm permission or equivalent local VMM role.
* Hard race penalty not applied. The patch establishes a race, but does not prove that the timing is one-shot or externally uncontrollable.
* Nested virtualization rarity penalty not applied. The affected path is nested KVM arm64 VNCR handling, but the input does not prove rare hardware or an unusually inaccessible deployment condition.

3. Reachability analysis

The evaluated attacker model is primarily a local host user or VMM process with access to /dev/kvm and the ability to run affected arm64 nested virtualization state. This is below full host-admin in many deployments, so HostAdminRequired=NO, but it is not ordinary unprivileged reachability.

A guest-only trigger is not fully established from the patch alone. The guest can influence VNCR and TLBI-related activity, but vcpu_put() and scheduling/lifecycle behavior are host/VMM-controlled. Therefore the complete trigger should be treated as local VMM/KVM-user reachable, with guest-only reachability unresolved.

The issue is not network reachable. Namespace-scoped privilege does not apply to this KVM path. No prior host setup beyond availability of KVM arm64 nested virtualization is proven as a strict security prerequisite. The race appears repeatable under VM execution and vCPU scheduling, but exact reliability is not established.

4. Primitive and severity interpretation

The exact primitive is a race/TOCTOU in host KVM VNCR fixmap unmapping state. The stale state is not a heap UAF, but it affects active host mapping state and can cause both BUG_ON failure and invalid fixmap unmapping using cpu -1.

This is an active structured mapping-state bug rather than fixed stale storage. It is not a cleanup-only stale dereference. There is no concrete repeated release/refcount operation. There is no evidence that a first lifetime operation frees an object and a second operation mutates the same freed object.

PrimitiveClass=CLASS_B because the bug corrupts or desynchronizes active host mapping/unmapping state in KVM, but does not establish a Class A primitive such as controlled reclaim, arbitrary write, callback control, demonstrated LPE, or guest-to-host escape.

The demonstrated consequence is host kernel crash / DoS. The speculative consequence is limited host fixmap-state corruption beyond the immediate crash, which justifies manual review but not Strong Important scoring by itself.

ImportantCandidate=YES because this is KVM host active mapping state with unresolved fixmap corruption implications. AutoDowngradeAllowed=NO because automatically closing it as ordinary Moderate would discard a concrete active-state synchronization primitive in a virtualization boundary path.

Likely operational severity is actionable Moderate, with Strong Important consideration only if further analysis shows guest-only reachability, controlled stale mapping effects, or a concrete host memory corruption primitive.

5. One-sentence report phrase

A race in KVM arm64 nested VNCR handling can desynchronize VNCR fixmap ownership during concurrent TLBI invalidation and vcpu_put() unmapping, leading to host kernel crash and requiring manual review because invalid fixmap state such as vncr_fixmap(-1) may have effects beyond a simple crash.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch does not prove UAF or arbitrary memory corruption, but it does establish an active host KVM mapping synchronization bug with invalid fixmap unmapping and virtualization-boundary relevance.

7. Call-site confidence

Call-site confidence: medium

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

Patch: KVM: arm64: Handle VNCR TLB invalidation race with vcpu_put() VNCR unmapping [ Upstream
Commit: 5cbd8761d001c11ada73b1753d772c4745a70e72
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5cbd8761d001c11ada73b1753d772c4745a70e72

Changed files:
  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=5cbd8761d001c11ada73b1753d772c4745a70e72

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

The original Linux kernel CVE announcement for CVE-2026-89917 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89917

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:C/C:L/I:L/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).