[CVE-2026-98720][MODERATE 7.0] KVM: nVMX: Force MSR bitmap refresh if runtime eVMCS controls are modified

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

CVE: CVE-2026-98720
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: nVMX: Force MSR bitmap refresh if runtime eVMCS controls are modified
Commit: 2db11598b59f16ddb6420693f9a4ac4efc3d4789
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2db11598b59f16ddb6420693f9a4ac4efc3d4789
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98720
Analysis date: Sat, 24 Oct 2026 09:28:39 -0400
ActionableScore: 8
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
A stale nested-VMX MSR interception bitmap lets a nested guest read and modify the host’s physical TPR after runtime eVMCS controls change.

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

The original Linux kernel CVE announcement for CVE-2026-98720 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98720

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
======================================================================

CVE-2026-98720	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98720 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=DIFF TEMP_DISTANCE_MERGE=IMPORTANT->MODERATE_KPANIC;AEGIS=MODERATE;GAP=2 (final Legacy and Aegis differ; FINAL: MODERATE_KPANIC, Aegis: MODERATE)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H';CWE-284;CWE-863;*CWE-672;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:L/A:H';BEST CVSS score: '7.9';DESCR 'A stale vmcs02 MSR bitmap can remain after runtime eVMCS controls change, allowing L2 or L1 by proxy to read and write the physical L0 TPR when TPR virtualization is disabled. Writing L0 TPR can alter host interrupt priority and plausibly cause severe host availability loss, while reading it exposes only limited host CPU state. For the CVSS the PR:L value reflects the need to execute code in a nested guest or control its nested hypervisor configuration, lacking any requirement for host administrative privilege. Nested VMX and Hyper-V enlightened VMCS must be enabled, and the attacker must perform the required control transition, but no timing race or memory corruption primitive is demonstrated. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 7.9 and the production ActionableScore returned 8 with recommendation Strong Important candidate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. final reconciliation stage IMPORTANT to MODERATE_KPANIC because the cross model evidence reconciliation rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and TEMP_DISTANCE_MERGE. The independent weaker Aegis shadow evaluated CVSS 5.6 and ActionableScore 4 with primitive class CLASS_A and manual review REQUIRED. Its independently reconciled impact was MODERATE. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 8)	MAYBE	VIRT SIMPLEFIX HARDWARE  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_DISTANCE_MERGE	-	-	checked

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

ActionableScore=8
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=YES
ProtectedResourceImpact=CONFIRMED
B4PendingReview=NO
B4PendingConfirmed=YES

1. ActionableScore

* Raw base score: 8.
* Conservative score: 8; no floor or cap applied.
* Raw paranoid score: 8.
* Paranoid score: 8; no additional speculative effects scored.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

* +2 strong exploit leverage: stale `msr_bitmap02` gives L2, or L1 by proxy, direct read/write access to L0's host-owned TPR.
* +2 authorization/security-boundary bypass: nested VM-Enter consumes a stale interception bitmap, changing an intended trapped/denied direct MSR access into passthrough access to protected host state.
* +1 practical chain: runtime eVMCS control transition → omitted bitmap refresh → nested VM-Enter → guest TPR access reaches L0.
* +1 confidentiality: direct reading of L0 TPR exposes limited host CPU state.
* +1 integrity: the guest can directly modify L0 interrupt-priority state.
* +1 availability: hostile TPR writes can suppress or disrupt host interrupt delivery, plausibly causing serious host availability loss.

Not awarded: kernel-memory corruption, UAF, arbitrary memory write, credential access, LPE, or guaranteed host panic. No extra availability point was added for the same TPR disruption.

3. Reachability analysis

A malicious L1 controls the runtime eVMCS configuration and can execute L2 or induce L2 TPR accesses; the commit expressly identifies access by “L2 (or L1 by proxy).” The supplied evidence establishes a nested-guest actor rather than a host administrator or host UID 0 actor. Nested VMX and Hyper-V enlightened VMCS support must be enabled, but no timing race is required. This is a guest-to-host path, not a Linux user-namespace capability path.

4. Primitive and severity reading

The primitive is stale active authorization/translation state: `msr_bitmap02` remains configured for TPR passthrough after TPR virtualization is disabled, and hardware consumes that bitmap during later nested execution. This is not a UAF or heap overwrite. It is CLASS_A because the guest obtains a directed read/write operation against protected host CPU state across the guest/host boundary.

B4 is confirmed through the concrete path: runtime eVMCS control change → stale MSR interception policy → nested VM-Enter → guest RDMSR/WRMSR reaches L0 TPR. The fix restores protection by forcing bitmap recalculation. ImportantCandidate is therefore YES, and this concrete boundary primitive must not be automatically downgraded.

5. One-sentence report phrase

A stale nested-VMX MSR interception bitmap lets a nested guest read and modify the host’s physical TPR after runtime eVMCS controls change.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm affected nested-virtualization configurations and assess host interrupt disruption, while retaining Important classification for the proven guest-to-host state-access bypass.

7. Call-site confidence

Call-site confidence: high

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

Patch: KVM: nVMX: Force MSR bitmap refresh if runtime eVMCS controls are modified
Commit: 2db11598b59f16ddb6420693f9a4ac4efc3d4789
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2db11598b59f16ddb6420693f9a4ac4efc3d4789

Commit description:

Force a refresh of the vmcs02 MSR bitmap during nested VM-Enter if the
runtime eVMCS controls (pin, primary, secondary, etc.) are being updated.
If L1 isn't intercepting TPR writes, runs L2 with TPR virtualization, and
then runs the same L2 with TPR virtualization disabled, KVM will fail to
refresh msr_bitmap02 and leave TPR in passthrough mode even though TPR
virtualization is disabled.  I.e. failure to refresh the bitmap lets L2 (or
L1 by proxy) read and write L0's TPR.

Fixes: 502d2bf ("KVM: nVMX: Implement Enlightened MSR Bitmap feature")
Cc: stable@vger.kernel.org
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
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=2db11598b59f16ddb6420693f9a4ac4efc3d4789

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

The original Linux kernel CVE announcement for CVE-2026-98720 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98720

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:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:H
  The Best / paranoid CVSS score: 7.9

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: unknown
  Paranoid ActionableScore: 8

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).