[CVE-2026-46131][LOW] KVM: x86: check for nEPT/nNPT in slow flush hypercalls

[CVE-2026-46131][LOW] KVM: x86: check for nEPT/nNPT in slow flush hypercalls

[AL-KERNEL]

CVE: CVE-2026-46131
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: KVM: x86: check for nEPT/nNPT in slow flush hypercalls
Commit: 971f17f5d91045404e3914029ea57c3da90179a4
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=971f17f5d91045404e3914029ea57c3da90179a4
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46131
Analysis date: Thu, 28 May 2026 13:35:49 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A KVM x86 Hyper-V slow TLB flush hypercall used the wrong nested guest condition for L2 GPA translation, which can cause nested guest malfunction or DoS but does not show host memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-46131 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46131

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: LOW
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
======================================================================

CVE-2026-46131	LOW	CHECK	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:L/A:H';*CWE-20;CWE-754;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'A KVM x86 Hyper-V slow TLB flush hypercall used is_guest_mode to decide whether to translate an L2 GPA, but translate_nested_gpa() is only valid when nested EPT/NPT is active. In an L2 guest without nEPT/nNPT, this can apply the wrong address translation path and may make the hypercall fail or consume the wrong guest physical address, causing nested guest malfunction or DoS. For the CVSS the PR:L is used because triggering requires code execution in a guest or nested guest context able to issue the Hyper-V hypercall, but it does not require host privileges. The issue is not network reachable and no host memory corruption, information disclosure, or direct privilege escalation primitive is shown.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2) 	MAYBE	OOB VIRT SIMPLEFIX HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: **1**
* Paranoid score: **2**
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

* Local or guest-triggered interface: **+1**. Triggering requires code execution in a guest or nested guest context that can issue the relevant Hyper-V slow TLB flush hypercall.
* Availability impact realistic: **+1 paranoid**. Wrong GPA translation logic can cause hypercall failure, nested guest malfunction, or nested virtualization DoS.
* Rare and constrained configuration: **-1 conservative**. The affected path requires KVM x86, Hyper-V enlightenment handling, nested guest mode, slow flush hypercall path, and specifically the mismatch between guest mode and nEPT/nNPT state.
* No memory corruption: **+0**. The patch changes a condition from `is_guest_mode()` to `mmu_is_nested()` and does not show UAF, OOB write, rmap stale pointer, refcount misuse, or arbitrary write.
* No confidentiality signal: **+0**. No host memory disclosure or cross-VM data leak is shown.
* No integrity or LPE signal: **+0**. The patch does not support guest-to-host privilege escalation or host memory corruption.

## 3. Reachability analysis

The issue is reachable from a guest or nested guest that can issue the Hyper-V TLB flush hypercall. It is not network reachable. Namespaces and containers on the host do not materially affect reachability. The affected path is not default for ordinary workloads because it depends on nested virtualization and Hyper-V enlightened hypercall handling. Realistic exploitation appears limited to disrupting the nested guest or causing incorrect nested virtualization behavior, not compromising the host.

## 4. Severity interpretation

This behaves like a **Low-like or ordinary Moderate** correctness issue in a constrained KVM nested virtualization path. The theoretical concern is wrong address translation for an L2 GPA when nested EPT/NPT is not active. The realistic evidence supports nested guest malfunction or DoS only. There is no demonstrated memory corruption, no rmap/SPTE lifetime issue, no information leak, and no supported guest-to-host escalation primitive.

## 5. One-sentence report phrase

A KVM x86 Hyper-V slow TLB flush hypercall used the wrong nested guest condition for L2 GPA translation, which can cause nested guest malfunction or DoS but does not show host memory corruption or privilege escalation.

## 6. Manual review recommendation

**NO MANUAL CHECK NEEDED**

This appears to be a constrained nested virtualization correctness and DoS issue without a demonstrated host memory corruption, information disclosure, or guest-to-host privilege escalation primitive.

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

Patch: KVM: x86: check for nEPT/nNPT in slow flush hypercalls
Commit: 971f17f5d91045404e3914029ea57c3da90179a4
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=971f17f5d91045404e3914029ea57c3da90179a4

Commit description:

Checking is_guest_mode(vcpu) is incorrect, because translate_nested_gpa()
is only valid if an L2 guest is running *with nested EPT/NPT enabled*.
Instead use the same condition as translate_nested_gpa() itself.

Cc: stable@vger.kernel.org
Reviewed-by: Sean Christopherson <seanjc@google.com>
Fixes: aee7382 ("KVM: x86: Prepare kvm_hv_flush_tlb() to handle L2's GPAs", 2022-11-18)
Link: https://patch.msgid.link/20260503200905.106077-1-pbonzini@redhat.com/
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/hyperv.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=971f17f5d91045404e3914029ea57c3da90179a4

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

The original Linux kernel CVE announcement for CVE-2026-46131 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46131

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:L/A:H
  The Best / paranoid CVSS score: 5.3

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: 1
  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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).