[CVE-2026-68081][MODERATE 7.0] KVM: nVMX: Put vmcs12 pages if nested VM-Enter fails due to invalid guest state

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

CVE: CVE-2026-68081
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: nVMX: Put vmcs12 pages if nested VM-Enter fails due to invalid guest state
Commit: 7996013b85687034d2e820cef94d6404192e3a3d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7996013b85687034d2e820cef94d6404192e3a3d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68081
Analysis date: Sat, 08 Aug 2026 05:51:24 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
KVM nVMX can leak pinned vmcs12 pages or mappings when nested VM-Enter fails due to invalid guest state, allowing a malicious L1 guest with nested VMX access to repeatedly consume host resources and cause a denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-68081 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68081

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
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CVE-2026-68081	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H';*CWE-772;*CWE-401;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:L';BEST CVSS score: '6.5';DESCR 'KVM nVMX can leak pinned vmcs12 related pages or mappings when nested VM-Enter fails due to invalid guest state and KVM synthesizes a nested VM-Exit without releasing those references. A malicious L1 guest can repeatedly retry VMLAUNCH or VMRESUME with invalid guest state and gradually consume host pinned memory or mapping resources. For the CVSS the PR:L is used for the paranoid score because reliable triggering requires control of a guest or process that can execute nested VMX operations, but does not require administrative privileges on the host. The issue is not network reachable and is triggered through the virtualization control path. Impact is denial of service through resource exhaustion. No concrete UAF, OOB access, or arbitrary write primitive is indicated by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	VIRT SIMPLEFIX LEAK HARDWARE  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	NO	NO	guess

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

This is not a strong Important candidate from the patch alone because the bug is a resource leak, not demonstrated memory corruption or privilege escalation. It should not be auto-closed because it is guest-controlled KVM host resource exhaustion in the nested virtualization path.

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A malicious L1 guest can retry VMLAUNCH or VMRESUME with invalid guest state. From the host perspective this requires control of a guest or KVM workload, not host root.

* Reliable kernel crash / strong DoS: +1
  Repeated retries leak pinned vmcs12 pages and/or mappings, which can realistically exhaust host resources.

* Cross-boundary isolation/resource bypass: +1
  The leak is triggered from guest-controlled nested VMX state and consumes host-side pinned pages or mappings.

* Availability impact realistic: +1 conservative
  The impact is host resource exhaustion. It may persist until VM teardown or cleanup, so this is stronger than a transient warning.

* Broad/common subsystem exposure: +1 paranoid only
  KVM is widely deployed, and guest-to-host issues deserve manual review. This is only used in the paranoid score because nested VMX is not always enabled.

Subtracted signals:

* Legacy / non-default execution mode: -1
  The issue requires nested VMX/nested virtualization behavior, which is not universally enabled for all guests.

Not awarded:

* No generic memory corruption bonus. The patch shows leaked pinned pages/mappings, not UAF, OOB write, type confusion, or arbitrary write.
* No privilege escalation bonus. There is no demonstrated reclaim, overwrite, stale callback, refcount takeover, or object confusion primitive.
* No confidentiality or integrity impact. The supported impact is resource exhaustion DoS.

Call-site confidence: high. The patch directly adds nested_put_vmcs12_pages() on the failing nested VM-Enter path.

## 3. Reachability analysis

The bug is triggerable by an L1 guest that can use nested VMX and repeatedly attempts VMLAUNCH or VMRESUME with invalid guest state. It is not network reachable by itself. It requires KVM with nested virtualization exposed to the attacker-controlled guest. In cloud or virtualization environments where nested virtualization is offered to tenants, this becomes a meaningful guest-to-host DoS risk. If nested virtualization is disabled or only trusted guests can use it, practical exposure is much lower.

Namespaces and containers are not the main factor here. The relevant boundary is VM guest to host. Host privileges are not required by the guest attacker once nested VMX is available.

## 4. Severity interpretation

This behaves like an actionable Moderate KVM resource-exhaustion vulnerability. It is not an ordinary low-risk leak because the leaked objects are pinned host resources and the trigger is guest-controlled. It is also not clearly Important from the patch alone because there is no concrete memory corruption, sensitive data exposure, or privilege escalation primitive.

Theoretical worst case is host-wide availability loss through sustained resource exhaustion. Realistic evidence supports DoS, not LPE.

## 5. One-sentence report phrase

KVM nVMX can leak pinned vmcs12 pages or mappings when nested VM-Enter fails due to invalid guest state, allowing a malicious L1 guest with nested VMX access to repeatedly consume host resources and cause a denial of service.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a KVM guest-to-host resource exhaustion issue with a paranoid ActionableScore of 5, and real severity depends heavily on whether nested virtualization is exposed to untrusted guests.

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

Patch: KVM: nVMX: Put vmcs12 pages if nested VM-Enter fails due to invalid guest state
Commit: 7996013b85687034d2e820cef94d6404192e3a3d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7996013b85687034d2e820cef94d6404192e3a3d

Commit description:

Put all vmcs12 pages if KVM synthesizes a nested VM-Exit due to invalid
guest while emulating VMLAUNCH or VMRESUME.  The invalid guest state path
doesn't use nested_vmx_vmexit() as that API is intended to be used if and
only if L2 is active, and the open coded equivalent neglects to put the
vmcs12 pages.  Failure to put the vmcs12 pages leaks any pinned pages
(and/or mappings) if L1 retries VMLAUNCH/VMRESUME.

Note, the !from_vmenter scenario doesn't suffer the same problem, as
vmx_get_nested_state_pages() only gets/pins/maps the vmcs12 pages if L2 is
active, i.e. if a "full" VM-Exit is guaranteed before KVM will retry
getting vmcs12 pages.

Fixes: 96c66e8 ("KVM/nVMX: Use kvm_vcpu_map when mapping the virtual APIC page")
Fixes: 3278e04 ("KVM/nVMX: Use kvm_vcpu_map when mapping the posted interrupt descriptor table")
Fixes: fe1911a ("KVM: nVMX: Use kvm_vcpu_map() to get/pin vmcs12's APIC-access page")
Reported-by: Minh Nguyen <minhnguyen.080505@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.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=7996013b85687034d2e820cef94d6404192e3a3d

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

The original Linux kernel CVE announcement for CVE-2026-68081 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68081

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:L/PR:L/UI:N/S:C/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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: 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).