[CVE-2026-46076][MODERATE 7.0] KVM: nSVM: Raise #UD if unhandled VMMCALL isn't intercepted by L1

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

CVE: CVE-2026-46076
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: nSVM: Raise #UD if unhandled VMMCALL isn't intercepted by L1
Commit: 924d721fae95687acedbaf624a094ed0e8b67104
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=924d721fae95687acedbaf624a094ed0e8b67104
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46076
Analysis date: Wed, 27 May 2026 11:47:14 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
KVM nSVM could incorrectly handle an unhandled L2 VMMCALL in L0 instead of injecting `#UD`, allowing L2 to bypass the expected L1 VMMCALL interception model under specific Hyper-V nested TLB-flush conditions.

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

The original Linux kernel CVE announcement for CVE-2026-46076 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46076

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-2026-46076	MODERATE	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:L/I:H/A:H';CWE-284;CWE-863;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.5';DESCR 'KVM nSVM could process an L2 VMMCALL in L0 as a normal hypercall when L1 did not request VMMCALL interception and the call was not a supported Hyper V L2 TLB flush hypercall. Architecturally that unhandled VMMCALL should raise UD, so the old behavior let L2 bypass the expected L1 interception model and act partly as if it were L1 for this hypercall path. For the CVSS the PR:L is used because the attacker needs control of code inside the nested guest, typically L2 kernel or equivalent privileged guest context, but not host privileges. AC:H is used because the condition requires nested SVM, Hyper V L2 TLB flush enablement, and a specific L1 intercept configuration. The issue is not network reachable and does not involve UAF, out of bounds access, arbitrary write, information disclosure, or direct host privilege escalation. Impact is primarily nested virtualization policy bypass and possible guest or host side availability impact depending on which hypercall path is reached.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	VIRT INIT HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* Local trigger from nested guest context: +1
  The attacker needs code execution inside L2, typically privileged guest code, but not host privileges.

* Security-boundary / policy bypass: +2
  L2 can reach L0 hypercall handling even when L1 did not request VMMCALL interception, bypassing the expected nested virtualization control model.

* Cross-boundary isolation/policy bypass: +1
  The bug crosses the intended L2 to L1 to L0 virtualization boundary semantics.

* Integrity impact plausible: +1
  Incorrectly treating an L2 VMMCALL as an L1-like hypercall can affect nested virtualization state or hypercall semantics.

* Availability impact plausible: +1
  Mishandled hypercalls may affect guest or nested virtualization behavior, depending on the hypercall path.

Subtracting signals:

* Configuration-dependent / special state: -1
  Requires nested SVM, Hyper-V L2 TLB flush enablement, L1 not intercepting VMMCALL, and a non-supported Hyper-V hypercall.

No signals apply for memory corruption, UAF, arbitrary write, information disclosure as a demonstrated primitive, or direct host privilege escalation.

## 3. Reachability analysis

The issue is reachable from L2 guest code in a nested virtualization setup. It requires an L1 configuration where VMMCALL is not intercepted, and Hyper-V L2 TLB flush support causes L0 to intercept VMMCALL for its own handling.

Namespaces and containers are not directly relevant. This is not network reachable. The practical attacker is someone controlling code inside an L2 guest, usually with guest-kernel or equivalent privileges.

## 4. Severity interpretation

This is an actionable Moderate nested virtualization policy bug. It is not memory corruption and does not by itself demonstrate host privilege escalation.

The security concern is that L2 can bypass the expected L1 interception model and reach L0 hypercall handling when architectural behavior should be `#UD`. In environments relying on nested virtualization isolation or L1 policy enforcement, this deserves manual review.

## 5. One-sentence report phrase

KVM nSVM could incorrectly handle an unhandled L2 VMMCALL in L0 instead of injecting `#UD`, allowing L2 to bypass the expected L1 VMMCALL interception model under specific Hyper-V nested TLB-flush conditions.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a nested virtualization boundary and policy bypass issue. It is not memory corruption, but L2 bypassing L1 interception semantics can be security-relevant in nested virtualization deployments.

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

Patch: KVM: nSVM: Raise #UD if unhandled VMMCALL isn't intercepted by L1
Commit: 924d721fae95687acedbaf624a094ed0e8b67104
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=924d721fae95687acedbaf624a094ed0e8b67104

Commit description:

Explicitly synthesize a #UD for VMMCALL if L2 is active, L1 does NOT want
to intercept VMMCALL, nested_svm_l2_tlb_flush_enabled() is true, and the
hypercall is something other than one of the supported Hyper-V hypercalls.
When all of the above conditions are met, KVM will intercept VMMCALL but
never forward it to L1, i.e. will let L2 make hypercalls as if it were L1.

The TLFS says a whole lot of nothing about this scenario, so go with the
architectural behavior, which says that VMMCALL #UDs if it's not
intercepted.

Opportunistically do a 2-for-1 stub trade by stub-ifying the new API
instead of the helpers it uses.  The last remaining "single" stub will
soon be dropped as well.

Suggested-by: Sean Christopherson <seanjc@google.com>
Fixes: 3f4a812 ("KVM: nSVM: hyper-v: Enable L2 TLB flush")
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Kevin Cheng <chengkev@google.com>
Link: https://patch.msgid.link/20260228033328.2285047-5-chengkev@google.com
[sean: rewrite changelog and comment, tag for stable, remove defunct stubs]
Reviewed-by: Yosry Ahmed <yosry@kernel.org>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://patch.msgid.link/20260304002223.1105129-2-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/hyperv.h
  arch/x86/kvm/svm/hyperv.h
  arch/x86/kvm/svm/nested.c
  arch/x86/kvm/svm/svm.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=924d721fae95687acedbaf624a094ed0e8b67104

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

The original Linux kernel CVE announcement for CVE-2026-46076 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46076

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:L/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/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: unknown
  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).