[CVE-2026-46082][MODERATE REGULAR] KVM: SVM: Inject #UD for INVLPGA if EFER.SVME=0

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

CVE: CVE-2026-46082
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: SVM: Inject #UD for INVLPGA if EFER.SVME=0
Commit: 3ac9d4241d205f5d0df06358349ca718ebb0fa12
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3ac9d4241d205f5d0df06358349ca718ebb0fa12
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46082
Analysis date: Fri, 07 Aug 2026 10:09:44 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
KVM SVM handled INVLPGA without checking EFER.SVME, allowing a guest to reach invalid virtualization-instruction emulation instead of receiving the architecturally required UD exception, with likely impact limited to availability or virtualization correctness.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-46082 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46082

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 REGULAR
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-2026-46082	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-754;CWE-284;CWE-693;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'KVM SVM handled INVLPGA without first checking whether nested SVM execution was permitted when EFER.SVME was clear. A guest could execute INVLPGA in a state where AMD architecture requires a UD exception, causing KVM to emulate a privileged virtualization instruction instead of rejecting it. For the CVSS the PR:L is used in the paranoid score because a guest user or tenant that can run code in a VM may be able to trigger the intercept without host administrator privileges. The issue is not network reachable and is triggered from guest code through the virtualization control path. Impact is mainly availability and isolation correctness, with no evidence of memory corruption or a direct host privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4)  SKIP CVE-2026-46082  SKIP The Fixes patch not applied yet, so unlikely that actual: ff092385e828 YES  NO NO unknown 	MAYBE	OOB VIRT SIMPLEFIX HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

* Local or guest-controlled trigger: +1
  A guest that can execute relevant code paths can issue INVLPGA and reach the KVM SVM interception path.

* Cross-boundary isolation or policy correctness concern: +1
  KVM handled a virtualization instruction when AMD architecture requires #UD if EFER.SVME=0. This is a guest-to-host emulation correctness boundary issue, but not a demonstrated host compromise primitive.

* Availability or VM correctness impact plausible: +1
  Incorrect handling of INVLPGA may affect guest execution or TLB-related virtualization behavior. The patch does not show a host UAF, OOB access, arbitrary write, or kernel panic path.

* Broad virtualization subsystem exposure: +1 in paranoid score only
  KVM SVM is a security-sensitive virtualization boundary on AMD systems, so manual review is justified even though the concrete primitive is weak.

No memory corruption, UAF, rmap/SPTE lifetime issue, page-cache corruption, or LPE primitive is shown by the patch.

## 3. Reachability analysis

The bug is triggerable from guest execution, not from the network. The attacker normally needs the ability to run code in a KVM guest on an AMD SVM host, and possibly guest-kernel or nested-virtualization-level control depending on how INVLPGA reaches the intercept path. This does not require host root, but it is also not a normal unprivileged host-local syscall interface. Containers and namespaces on the host do not materially lower the privilege requirement unless they provide access to create or control KVM guests.

## 4. Severity interpretation

This behaves more like a borderline Moderate KVM correctness issue than an Important-class vulnerability. The theoretical concern is that KVM incorrectly emulates a privileged virtualization instruction across a guest-host boundary. The realistic evidence from the patch supports incorrect exception injection and possible availability or isolation-correctness impact, but not memory corruption or privilege escalation.

## 5. One-sentence report phrase

KVM SVM handled INVLPGA without checking EFER.SVME, allowing a guest to reach invalid virtualization-instruction emulation instead of receiving the architecturally required UD exception, with likely impact limited to availability or virtualization correctness.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is guest-triggerable KVM behavior across a virtualization boundary, but the patch does not show a concrete memory corruption or host privilege escalation primitive.

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

Patch: KVM: SVM: Inject #UD for INVLPGA if EFER.SVME=0
Commit: 3ac9d4241d205f5d0df06358349ca718ebb0fa12
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3ac9d4241d205f5d0df06358349ca718ebb0fa12

Commit description:

INVLPGA should cause a #UD when EFER.SVME is not set. Add a check to
properly inject #UD when EFER.SVME=0.

Fixes: ff09238 ("KVM: SVM: Implement INVLPGA")
Cc: stable@vger.kernel.org
Signed-off-by: Kevin Cheng <chengkev@google.com>
Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20260228033328.2285047-3-chengkev@google.com
[sean: tag for stable@]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  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=3ac9d4241d205f5d0df06358349ca718ebb0fa12

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

The original Linux kernel CVE announcement for CVE-2026-46082 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46082

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:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 3
  Paranoid ActionableScore: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).