[CVE-2026-89931][MODERATE REGULAR] KVM: nVMX: Ensure KVM_REQ_GET_NESTED_STATE_PAGES is cleared on VM-Exit

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

CVE: CVE-2026-89931
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: nVMX: Ensure KVM_REQ_GET_NESTED_STATE_PAGES is cleared on VM-Exit
Commit: ae190f2439ca30067927077bf570d4c5edf07a0f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ae190f2439ca30067927077bf570d4c5edf07a0f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89931
Analysis date: Wed, 16 Sep 2026 13:37:52 -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-2026-89931 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89931

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.

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

CVE-2026-89931	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89931 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:N/I:N/A:H';CWE-665;*CWE-672;*CWE-772;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'KVM nVMX may fail to clear KVM_REQ_GET_NESTED_STATE_PAGES when CONFIG_KVM_HYPERV is disabled because the request check was compiled only under the Hyper V path. A nested guest workflow can then leave the request pending across a nested VM Exit and later VMLAUNCH or VMRESUME, which may double map vmcs12 related pages. For the CVSS the PR:L is used because an attacker needs local ability to use KVM and run a nested virtualization workload, but full host administrative privileges are not necessarily required in common /dev/kvm group based deployments. The issue is not network reachable and is triggered through local KVM ioctls and guest execution rather than packet traffic. Impact is primarily denial of service or resource exhaustion from repeated page mapping or reference retention. No concrete arbitrary write, UAF, or information leak primitive is visible from this patch, so confidentiality and integrity remain none for the grounded score.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) 	MAYBE	DANGER VIRT SIMPLEFIX 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

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

Positive signals:

* +1. Stale KVM request state can survive VM Exit when CONFIG_KVM_HYPERV is disabled. The patch explicitly says KVM relies on kvm_check_request to clear KVM_REQ_GET_NESTED_STATE_PAGES.
* +1. Concrete KVM nested state mapping inconsistency is established. The uncleared request may lead to double mapping of vmcs12 pages.
* +1 paranoid only. Repeated double mapping can plausibly create resource or reference retention and host KVM instability, but the patch does not prove a stronger corruption primitive.

Negative or non-awarded signals:

* No RealUAF awarded. The supplied patch does not establish use after free or stale post lifetime object dereference.
* No concrete repeated release or refcount mutation is established. There is no visible first put followed by a second put on the exact same dangling reference.
* No controlled reclaim, arbitrary write, directed host memory corruption, callback control, or information leak is demonstrated.
* No KVM guest to host strong exploitation leverage awarded. The bug is in host KVM state, but the patch only establishes uncleared request state and possible double mapping.
* No remote reachability awarded. This is not packet reachable.
* No race penalty applied. The issue is an ordered stale request clearing bug, not a proven narrow timing race.
* No rare hardware penalty applied. Nested VMX and KVM are configuration dependent, but the supplied evidence does not prove rare physical hardware or physical access.

3. Reachability analysis

The evaluated attacker is a local VMM or local host user with access to /dev/kvm and ability to run nested virtualization workloads. A malicious guest may participate by causing nested VM Entry or VM Exit sequences, but the supplied evidence does not prove that a guest only attacker can complete the full trigger without cooperation from the VMM or host userspace.

Host administrator privilege is not positively required because KVM is commonly exposed to non root users through /dev/kvm group access. This is still not ordinary remote reachability and not namespace scoped privilege. No network namespace, CAP_NET_ADMIN, BPF, or container scoped capability path is involved.

Prior KVM availability and nested virtualization support are prerequisites. CONFIG_KVM_HYPERV=n is the relevant affected build condition. No special physical hardware beyond KVM capable x86 virtualization is established. Race repeatability is not the key issue.

4. Primitive and severity interpretation

The exact primitive is stale request state in KVM nVMX: KVM_REQ_GET_NESTED_STATE_PAGES may remain pending because the request check was compiled only under CONFIG_KVM_HYPERV. Since kvm_check_request also clears the request, skipping it can allow later nested VMLAUNCH or VMRESUME paths to process the same nested state page mapping request again.

This is best classified as CLASS_C invalid state or resource mapping behavior. It is not a demonstrated UAF, not a concrete double release, and not a proven active structured stale object reuse. The demonstrated consequence is possible double mapping of vmcs12 pages and likely DoS or resource instability. Stronger consequences such as guest to host memory corruption or privilege escalation are speculative from this patch alone.

ImportantCandidate=NO because no Class A primitive and no sufficiently strong Class B lifetime corruption primitive is established. AutoDowngradeAllowed=YES because the evidence supports borderline manual review for KVM state handling, but not an unresolved Important grade exploitation primitive.

5. One-sentence report phrase

KVM nVMX may fail to clear KVM_REQ_GET_NESTED_STATE_PAGES on nested VM Exit when CONFIG_KVM_HYPERV is disabled, allowing a local VMM attacker with /dev/kvm access to trigger possible double mapping of vmcs12 pages and host DoS or resource instability.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

KVM nested state page mapping is security sensitive and should be checked manually, but the supplied patch does not establish UAF, arbitrary host memory access, guest to host escape, or privilege escalation.

7. Call-site confidence

Call-site confidence: medium

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

Patch: KVM: nVMX: Ensure KVM_REQ_GET_NESTED_STATE_PAGES is cleared on VM-Exit
Commit: ae190f2439ca30067927077bf570d4c5edf07a0f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ae190f2439ca30067927077bf570d4c5edf07a0f

Commit description:

Always check and clear KVM_REQ_GET_NESTED_STATE_PAGES when emulating a
nested VM-Exit to ensure the request is cleared, even when KVM was built
with CONFIG_KVM_HYPERV=n, as KVM subtly relies on the "check" to clear
the flag and thus avoid double-mapping the vmcs12 pages, e.g. if KVM
manages to bail from VM-Enter without processing the request, and then
emulates VMLAUNCH or VMRESUME.

Fixes: b4f69df ("KVM: x86: Make Hyper-V emulation optional")
Cc: stable@vger.kernel.org
Reported-by: Yosry Ahmed <yosry@kernel.org>
Reviewed-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260724004757.131420-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/vmx/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=ae190f2439ca30067927077bf570d4c5edf07a0f

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

The original Linux kernel CVE announcement for CVE-2026-89931 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89931

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).