[CVE-2026-72287][MODERATE 7.0] KVM: nVMX: Move vTPR vs. TPR Threshold consistency check into "normal" checks

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

CVE: CVE-2026-72287
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: nVMX: Move vTPR vs. TPR Threshold consistency check into "normal" checks
Commit: 7d066368f72e6192af7e21c5817626f6da666991
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7d066368f72e6192af7e21c5817626f6da666991
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72287
Analysis date: Sat, 15 Aug 2026 10:12:21 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A late KVM nVMX consistency check can fail after partial VMCS state loading and, with EPT disabled, may leave `vmcs01.GUEST_CR3` controlled by L1 instead of KVM, creating a guest to host isolation risk and requiring manual review.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72287 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72287

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-2026-72287	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H';CWE-693;CWE-269;CWE-754;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'KVM nVMX can reach a dangerous nested VM entry failure path when the late vTPR versus TPR threshold consistency check fails after some VMCS state has already been loaded. With EPT disabled, the failure path may leave vmcs01.GUEST_CR3 using an L1 controlled CR3 instead of the KVM controlled CR3, which can break the shadow paging isolation boundary and can lead to host memory corruption or guest to host privilege escalation. For the CVSS the PR:L is used for the paranoid score because an attacker needs the ability to run or control a nested virtualization workload from a guest or local KVM context, but full host administrative privileges should not be assumed. The issue is not directly network reachable. Impact is at least a denial of service and in the paranoid interpretation may affect confidentiality and integrity because wrong CR3 selection in non EPT mode can undermine KVM memory isolation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-72287 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 1100e4910ad207bc00aedc8dfdb228dd1b81f310 YES  NO NO unknown 	MAYBE	VIRT WARNONLY HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A guest or local KVM user controlling nested VMX state can influence `vmcs12` and the virtual APIC state. This is not network reachable.
* Weak LPE concern: +1
  The patch explicitly says KVM may run L1 with an L1 controlled CR3 instead of KVMs CR3 when EPT is disabled. This is a serious isolation invariant failure, but the patch does not show a direct arbitrary write primitive.
* Weak or indirect corruption candidate: +1
  Wrong CR3 selection in KVM shadow paging is a high value MMU state corruption candidate.
* Cross boundary isolation or policy bypass: +1
  The bug crosses the guest to host virtualization boundary in the affected mode.
* Reliable kernel crash or strong DoS: +1
  Bad VMCS or MMU state can plausibly lead to host instability or KVM failure.
* Legacy or non default execution mode: -1
  The dangerous condition depends on EPT being disabled.
* Rare and difficult to reach configuration: -1
  It also depends on nested VMX and an off by default `warn_on_missed_cc` consistency check path.

Paranoid additional interpretation:

* Strong LPE plausibility instead of weak LPE: +2
  Running L1 with an L1 controlled CR3 in non EPT shadow paging can undermine the KVM memory isolation boundary and is a plausible guest to host escalation primitive.
* Confidentiality impact plausible: +1
  Broken MMU isolation may expose memory mappings outside the intended guest context.
* Integrity impact plausible: +1
  Wrong CR3 usage may allow unauthorized memory influence or corruption in the host virtualization context.
* Availability impact realistic: +1
  Host wide KVM crash or corrupted execution state is plausible.

Call-site confidence: high. The patch includes both the late check removal in `nested_vmx_enter_non_root_mode()` and the earlier check in `nested_vmx_check_tpr_shadow_controls()`.

## 3. Reachability analysis

The bug is local or guest-originated, not remotely network reachable. A realistic attacker needs the ability to run or control a nested virtualization workload or otherwise influence L1 nested VMX state. Full host root should not be assumed, so PR:L is appropriate for the paranoid interpretation.

The issue is configuration dependent. It requires nested VMX, EPT disabled, and the off by default `warn_on_missed_cc` check path. These constraints reduce broad exposure, but they do not make the bug low risk because the affected state is KVM MMU and VMCS isolation state.

Namespaces and containers do not directly make this reachable unless they provide access to KVM device interfaces or nested virtualization control.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup. It is a KVM isolation invariant failure involving `vmcs01.GUEST_CR3` and shadow paging. The conservative view is Actionable Moderate because exploitation depends on non default conditions and no concrete arbitrary write primitive is demonstrated.

The paranoid view is a Strong Important candidate because an L1 controlled CR3 in a host KVM execution path can plausibly break guest to host isolation and may lead to privilege escalation or memory corruption. Theoretical impact is high. Realistic exploitability is constrained by EPT disabled and the off by default check path.

## 5. One-sentence report phrase

A late KVM nVMX consistency check can fail after partial VMCS state loading and, with EPT disabled, may leave `vmcs01.GUEST_CR3` controlled by L1 instead of KVM, creating a guest to host isolation risk and requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug affects KVM MMU and CR3 isolation state, has plausible guest to host privilege escalation implications, and should not be auto-closed despite non default triggering conditions.

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

Patch: KVM: nVMX: Move vTPR vs. TPR Threshold consistency check into "normal" checks
Commit: 7d066368f72e6192af7e21c5817626f6da666991
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7d066368f72e6192af7e21c5817626f6da666991

Commit description:

Move the off-by-default consistency check for vmcs12.tpr_threshold vs.
the virtual APIC vTPR into the "normal" controls checks, as waiting until
KVM has loaded some amount of state is unnecessary and actively dangerous.
Specifically, failure to unwind vmcs01.GUEST_CR3 to KVM's value when EPT
is disabled results in KVM running L1 with an L1-controlled CR3, not with
KVM's CR3!

Alternatively, KVM could simply reset the MMU to force a reload of
vmcs01.GUEST_CR3, but the _only_ reason the check was shoved into a "late"
flow was to wait until the vmcs12 pages were retrieved.  Rather than build
up more crusty code, simply access vTPR using a regular guest memory access
(performance isn't a concern).  To circumvent the restrictions that led to
KVM deferring nested_get_vmcs12_pages(), (a) use a VM-scoped API to read
guest memory so that it always hits non-SMM memslots (for RSM), and (b)
skip the check (since its off-by-default anyways) when the vCPU doesn't
want to run, i.e. when userspace is restoring/stuffing state.

If reading guest memory fails, simply skip the consistency check, as KVM's
de facto ABI is that VMX instruction accesses to non-existent memory get
PCI Bus Error semantics, where reads return 0xFFs.  And if vTPR=0xFF, then
the vTPR is guaranteed to be greater than or equal to TPR_THRESHOLD.

Fixes: 1100e49 ("KVM: nVMX: Add an off-by-default module param to WARN on missed consistency checks")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260612145642.452392-2-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=7d066368f72e6192af7e21c5817626f6da666991

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

The original Linux kernel CVE announcement for CVE-2026-72287 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72287

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:C/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 5
  Paranoid ActionableScore: 7

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).