[CVE-2026-46032][MODERATE 7.0] KVM: nSVM: Triple fault if restore host CR3 fails on nested #VMEXIT

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

CVE: CVE-2026-46032
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: KVM: nSVM: Triple fault if restore host CR3 fails on nested #VMEXIT
Commit: 9a738cf170a4a2332ea3a15e23ec65b5757fe4a1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9a738cf170a4a2332ea3a15e23ec65b5757fe4a1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46032
Analysis date: Wed, 27 May 2026 12:35:07 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
KVM nSVM could ignore L1 CR3 restore failure on nested VMEXIT and continue with corrupted nested state instead of triple faulting, causing VM DoS and state-integrity risk.

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

The original Linux kernel CVE announcement for CVE-2026-46032 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46032

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-2026-46032	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-755;CWE-670;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:H';BEST CVSS score: '6.5';DESCR 'KVM nSVM could ignore failure to restore L1 CR3 on nested VMEXIT because nested_svm_vmexit() returned an error that most callers did not check. This could leave the vCPU continuing with corrupted nested state instead of following architectural shutdown behavior. For the CVSS the PR:L is used because triggering requires local KVM access or control of a VMM or nested guest state, but not host root. AC:H is used because the issue depends on nested SVM and a failed L1 CR3 restore path. The issue is not network reachable and does not involve UAF, out of bounds access, arbitrary write, or information disclosure. Impact is primarily VM or vCPU denial of service through forced triple fault behavior, with limited integrity concern from previously possible corrupted nested execution state.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DANGER VIRT INIT HARDWARE LINUS KPANIC 	NO	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 KVM/VMM trigger: +1
  Triggering requires local access to KVM or control of a VMM or nested guest state.

* Cross-boundary virtualization state concern: +1
  The bug affects nested VMEXIT handling between L2, L1, and L0 state transitions.

* Integrity impact plausible: +1
  Ignoring failure to restore L1 CR3 could let L1 continue with corrupted nested execution state.

* Availability impact realistic: +1
  Correct behavior is shutdown/triple fault. The visible impact is VM or vCPU DoS.

* Privileged kernel/virtualization state path: +1
  The affected code is trusted KVM nested virtualization state transition logic.

No signals apply for memory corruption, UAF, OOB access, arbitrary write, remote reachability, or information disclosure.

## 3. Reachability analysis

The issue is reachable from a local VMM workflow using nested SVM. The attacker needs control over nested virtualization state such that L1 CR3 restoration fails during nested VMEXIT.

Namespaces and containers matter only if `/dev/kvm` and nested virtualization are delegated to untrusted workloads. It is not network reachable. Practical exploitation requires specific nested SVM state and failure conditions.

## 4. Severity interpretation

This is an Actionable Moderate nested virtualization correctness and state-integrity issue. It is not a memory corruption or host escape bug based on the patch.

The realistic impact is guest or vCPU shutdown/DoS. The security concern is that pre-fix behavior could continue execution after failed host CR3 restoration, which is not architecturally valid and may corrupt nested state.

## 5. One-sentence report phrase

KVM nSVM could ignore L1 CR3 restore failure on nested VMEXIT and continue with corrupted nested state instead of triple faulting, causing VM DoS and state-integrity risk.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this affects nested virtualization state integrity and failed host-state restoration semantics, even though no memory corruption or direct host privilege escalation is shown.

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

Patch: KVM: nSVM: Triple fault if restore host CR3 fails on nested #VMEXIT
Commit: 9a738cf170a4a2332ea3a15e23ec65b5757fe4a1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9a738cf170a4a2332ea3a15e23ec65b5757fe4a1

Commit description:

If loading L1's CR3 fails on a nested #VMEXIT, nested_svm_vmexit()
returns an error code that is ignored by most callers, and continues to
run L1 with corrupted state. A sane recovery is not possible in this
case, and HW behavior is to cause a shutdown. Inject a triple fault
instead, and do not return early from nested_svm_vmexit(). Continue
cleaning up the vCPU state (e.g. clear pending exceptions), to handle
the failure as gracefully as possible.

From the APM:

  Upon #VMEXIT, the processor performs the following actions in order to
  return to the host execution context:

  ...

  if (illegal host state loaded, or exception while loading host state)
      shutdown
  else
      execute first host instruction following the VMRUN

Remove the return value of nested_svm_vmexit(), which is mostly
unchecked anyway.

Fixes: d82aaef ("KVM: nSVM: use nested_svm_load_cr3() on guest->host switch")
CC: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260303003421.2185681-10-yosry@kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  arch/x86/kvm/svm/nested.c
  arch/x86/kvm/svm/svm.c
  arch/x86/kvm/svm/svm.h

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=9a738cf170a4a2332ea3a15e23ec65b5757fe4a1

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

The original Linux kernel CVE announcement for CVE-2026-46032 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46032

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:H
  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).