[CVE-2026-64562][MODERATE 7.0] KVM: nVMX: Hide shadow VMCS right after VMCLEAR

[CVE-2026-64562][MODERATE 7.0] KVM: nVMX: Hide shadow VMCS right after VMCLEAR

[AL-KERNEL]

CVE: CVE-2026-64562
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: nVMX: Hide shadow VMCS right after VMCLEAR
Commit: af56298e9d86e6098cd1d2e155cb2949b7c45412
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=af56298e9d86e6098cd1d2e155cb2949b7c45412
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64562
Analysis date: Tue, 04 Aug 2026 02:55:51 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A KVM nVMX race can leave `vmcs01.shadow_vmcs` pointing to a freed shadow VMCS so that vCPU migration may later trigger `VMCLEAR` on freed host memory, causing at least host DoS and requiring manual review for possible UAF based privilege impact.

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

The original Linux kernel CVE announcement for CVE-2026-64562 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64562

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-2026-64562	MODERATE	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-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in KVM nVMX can leave vmcs01 pointing to a freed shadow VMCS while loaded_vmcs_clear may still run on another CPU after vCPU migration. This can make __loaded_vmcs_clear execute VMCLEAR on a freed VMCS page, which is a use after free candidate in the host KVM VMX path. For the CVSS the PR:L is used because the attacker needs local KVM access or control of a guest context where nested VMX is exposed, but full host administrator privileges are not required in many virtualization deployments. The issue is not directly network reachable and requires a local or guest controlled execution context. Impact is at least denial of service via host kernel crash or instability. In the paranoid interpretation, the freed VMCS page may be reused before a later VMCLEAR, so confidentiality and integrity impact are kept at H for manual review sensitivity.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	VIRT INIT SIMPLEFIX HARDWARE  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  Triggering requires local KVM access or control of a guest where nested VMX is exposed. This is PR:L in many virtualization deployments, not full host root.

* Memory corruption, weak or indirect corruption candidate: +1
  The bug leaves `vmcs01.shadow_vmcs` pointing to a VMCS object that may already be freed. This is a stale pointer / UAF candidate, but the patch does not demonstrate attacker controlled reclaim or arbitrary target selection.

* Real lifetime corruption: +1
  The issue is explicitly a lifetime ordering bug: `free_nested()` frees the shadow VMCS while another async path may still use the pointer.

* Reliable kernel crash / strong DoS: +1
  Executing `VMCLEAR` on a freed VMCS page can plausibly crash or destabilize the host kernel.

* Hard or unreliable race / special timing required: -1
  The window depends on async interaction with `loaded_vmcs_clear()` and vCPU migration.

* Availability impact realistic: +1
  The impact is host-side KVM instability or crash, not just a guest-local failure.

Paranoid additional interpretation:

* Memory corruption, stronger primitive: +1 additional
  In the paranoid interpretation, `VMCLEAR` against a freed VMCS page may write CPU-derived VMCS state into a page that has been reallocated for another kernel object. Payload and target control are not demonstrated, so this is not treated as arbitrary write.

* Weak LPE concern: +1
  Because this is a UAF candidate in host KVM VMX code, privilege escalation cannot be excluded without manual review. This is weaker than a demonstrated reclaim or callback-control primitive.

* Privileged kernel / virtualization state corruption path: +1
  The affected object is VMCS state in the KVM nVMX path, which is a high-value host virtualization context.

## 3. Reachability analysis

The bug is not network reachable. It requires local access to KVM or guest control in an environment where nested VMX and shadow VMCS are enabled. In many systems `/dev/kvm` is accessible to users in a KVM-related group or to virtualization service accounts, so treating this as PR:L is more realistic than PR:H for triage. Containers usually cannot trigger this unless `/dev/kvm` is passed through or nested virtualization is delegated. The path is configuration dependent because it requires Intel VMX nested virtualization and shadow VMCS support, but it is not an obscure driver-only path.

Call-site confidence: medium. The patch provides the relevant lifetime sequence in `free_nested()` and identifies the async interaction with `loaded_vmcs_clear()`, but no reproducer or reclaim primitive is included.

## 4. Severity interpretation

This behaves above ordinary Moderate because it is a host KVM lifetime bug with a freed VMCS pointer and possible later hardware operation on the freed page. Realistic demonstrated impact is mainly host DoS or instability, with a hard race requirement. Theoretical memory corruption potential exists because the freed VMCS page may be reused before `VMCLEAR`, but the patch does not show attacker-controlled reclaim, arbitrary write, callback dispatch, or reliable object replacement. Therefore the conservative score stays borderline manual-review level, while the paranoid score reaches Actionable Moderate.

## 5. One-sentence report phrase

A KVM nVMX race can leave `vmcs01.shadow_vmcs` pointing to a freed shadow VMCS so that vCPU migration may later trigger `VMCLEAR` on freed host memory, causing at least host DoS and requiring manual review for possible UAF based privilege impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a KVM host-side lifetime/UAF candidate in nested VMX handling. Even though exploitability is race-dependent and no controlled reclaim is shown, host KVM memory lifetime bugs should not be auto-closed because guest-to-host or local KVM privilege impact may be underestimated.

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

Patch: KVM: nVMX: Hide shadow VMCS right after VMCLEAR
Commit: af56298e9d86e6098cd1d2e155cb2949b7c45412
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=af56298e9d86e6098cd1d2e155cb2949b7c45412

Commit description:

free_nested() frees the shadow VMCS while vmcs01 still points to it. But
because it is asynchronous with respect to loaded_vmcs_clear(), the vCPU
might migrate before the pointer is cleared and __loaded_vmcs_clear()
may then execute VMCLEAR.

The VMCS needs to stay attached until its explicit VMCLEAR completes, but
then it can be hidden and the page safely freed.

Fixes: 355f4fb ("kvm: nVMX: VMCLEAR an active shadow VMCS after last use")
Cc: stable@vger.kernel.org
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.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=af56298e9d86e6098cd1d2e155cb2949b7c45412

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

The original Linux kernel CVE announcement for CVE-2026-64562 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64562

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:L/I:L/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: 6

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).