[CVE-2026-23401][IMPORTANT] KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE When installing an emulated MMIO SPTE, do so *after* dropping/zapping the existing SPTE (if it's shadow-present). While

[CVE-2026-23401][IMPORTANT] KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE When installing an emulated MMIO SPTE, do so *after* dropping/zapping the existing SPTE (if it's shadow-present). While

[AL-KERNEL]

CVE: CVE-2026-23401
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE When installing an emulated MMIO SPTE, do so *after* dropping/zapping the existing SPTE (if it's shadow-present). While
Commit: aad885e774966e97b675dfe928da164214a71605
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=aad885e774966e97b675dfe928da164214a71605
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-23401
Analysis date: Mon, 06 Jul 2026 21:21:18 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
KVM x86 MMU could install an MMIO SPTE over an existing shadow-present SPTE without first zapping it, leaving stale rmap state that can trigger a host warning or crash and may represent a UAF-like MMU corruption risk requiring manual review.

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

The original Linux kernel CVE announcement for CVE-2026-23401 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-23401

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: IMPORTANT
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-23401	IMPORTANT	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:H/I:H/A:H';*CWE-416;*CWE-672;*CWE-664;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A stale SPTE state in the KVM x86 MMU can occur when host userspace changes a shadowed guest PTE from a normal memslot mapping to emulated MMIO and the guest later faults on the affected address. KVM could mark an MMIO SPTE while an existing shadow present SPTE was still installed, leaving stale reverse mapping state that can later be traversed and treated as a use-after-free candidate. For the CVSS the PR:L is used because a local user able to run a VM through /dev/kvm, or a reduced capability VMM or guest controlled path in a virtualization stack, may be able to drive the affected KVM MMU path without full host administrator privileges. The issue is not directly network reachable. Impact is at least local denial of service via warning or kernel crash, and in the worst case may allow confidentiality or integrity impact due to stale rmap based memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8) 	YES	VIRT INIT HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  The trace shows UID 1000 reaching the KVM MMU path through KVM ioctls. Access to `/dev/kvm` is often delegated to non-root users or VMM service accounts.

* Memory corruption, weak/indirect corruption candidate: +1
  The patch prevents installing an MMIO SPTE over an existing shadow-present SPTE without first dropping/zapping it. This can leave stale MMU/rmap state, but the patch does not show a fully controlled overwrite or reclaim primitive.

* Real lifetime corruption: +1
  The bug affects SPTE lifetime and reverse-mapping consistency. A stale rmap/SPTE entry may later be traversed after the logical mapping was replaced.

* Weak LPE concern: +1
  KVM MMU stale rmap issues are security-sensitive and may cross guest/host or reduced-privilege VMM boundaries, but exploitation beyond DoS is not demonstrated in the patch.

* Reliable kernel warning / possible strong DoS: +1
  The commit shows a reachable `WARN` in `mark_mmio_spte()`. On systems with `panic_on_warn`, or if stale state later causes stronger failure, this is host DoS relevant.

* Broad/common virtualization exposure: +1
  KVM x86 is a widely deployed virtualization subsystem. The affected path is in the KVM MMU page-fault handling path.

Paranoid adjustment:

* Upgrade to strong KVM/MMU corruption concern: +2 instead of weak +1
  Because this is SPTE/rmap consistency in KVM MMU, stale reverse mappings can be UAF-like and historically require manual review.

* Upgrade LPE plausibility to strong concern: +2 instead of weak +1
  Not because exploitation is proven, but because stale MMU/rmap state in KVM can plausibly affect host memory-management invariants.

No remote/network bonus is applied.

No full admin/root penalty is applied because `/dev/kvm` access is commonly available to non-root users in the `kvm` group or to reduced-privilege VMM service accounts.

Call-site confidence: high. The patch and stack trace show the affected call path through `mmu_set_spte()`, `ept_page_fault()`, and `kvm_arch_vcpu_ioctl_run()`.

## 3. Reachability analysis

The likely trigger is local access to KVM, either by a user allowed to run VMs through `/dev/kvm` or by a VMM process. The scenario described requires host userspace to modify a shadowed guest PTE from a normal memslot mapping to emulated MMIO, followed by a guest page fault on the affected address.

This is not directly network reachable. However, in cloud or virtualization deployments, a reduced-privilege VMM, VM-management service, or guest-controlled execution path may help drive the affected MMU state transition.

Namespaces do not automatically make this unprivileged, but containerized or delegated virtualization setups can reduce the practical privilege requirement below full host root. Therefore PR:L-style treatment is appropriate for practical triage.

## 4. Severity interpretation

This should not be treated as an ordinary Moderate or warning-only bug. The demonstrated symptom is a KVM warning, but the fixed invariant is security-sensitive: an existing shadow-present SPTE must be dropped or zapped before an MMIO SPTE is installed.

Realistic evidence supports at least Actionable Moderate because the bug affects KVM MMU state and can be reached from local KVM execution. The theoretical worst case is stronger because stale rmap/SPTE state can become a UAF-like memory-management corruption primitive. The patch does not demonstrate arbitrary write, controlled reclaim, or a working LPE chain, so the conservative score remains below Critical-style handling.

## 5. One-sentence report phrase

KVM x86 MMU could install an MMIO SPTE over an existing shadow-present SPTE without first zapping it, leaving stale rmap state that can trigger a host warning or crash and may represent a UAF-like MMU corruption risk requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: KVM/MMU SPTE and rmap lifetime bugs are high-risk even when the visible symptom is only a warning. The patch restores a critical mapping-lifetime invariant, and stale rmap traversal may have UAF-like consequences.

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

Patch: KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE When installing an emulated MMIO SPTE, do so *after* dropping/zapping the existing SPTE (if it's shadow-present). While
Commit: aad885e774966e97b675dfe928da164214a71605
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=aad885e774966e97b675dfe928da164214a71605

Changed files:
  arch/x86/kvm/mmu/mmu.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=aad885e774966e97b675dfe928da164214a71605

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

The original Linux kernel CVE announcement for CVE-2026-23401 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-23401

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/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: 6
  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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).