[CVE-2026-64284][MODERATE REGULAR] KVM: x86: Ensure vendor's exit handler runs before fastpath userspace exits

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

CVE: CVE-2026-64284
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: KVM: x86: Ensure vendor's exit handler runs before fastpath userspace exits
Commit: b3436d9b9b1affe1c3191ac9831308923f5f03c3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b3436d9b9b1affe1c3191ac9831308923f5f03c3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64284
Analysis date: Sat, 25 Jul 2026 08:37:57 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline. Manual review recommended
Manual review required: YES

Summary:
KVM x86 fastpath userspace exits could skip vendor exit handling before returning from KVM_RUN, allowing VMX PML dirty log updates from the final run to be missed and potentially corrupting migration or checkpoint state integrity.

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

The original Linux kernel CVE announcement for CVE-2026-64284 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64284

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.

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

CVE-2026-64284	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:L';CWE-703;CWE-754;CWE-693;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.3';DESCR 'KVM x86 fastpath userspace exits could return from KVM_RUN before the VMX or SVM vendor exit handler performed required state handling. On VMX with PML enabled, this can skip the PML buffer flush and cause memory written by the final KVM_RUN to never be marked dirty. This can corrupt the integrity of dirty logging used by live migration, checkpoint, backup, or other VMM workflows that depend on accurate guest memory tracking. For the CVSS the PR:L value is used because triggering requires local access to KVM or control of a VM process rather than full host administrator privileges. The issue is not directly network reachable. Impact is primarily guest state integrity loss and possible operational disruption, not host privilege escalation or kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline. Manual review recommended (with actual score 4)	YES	DANGER VIRT INIT SIMPLEFIX HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline. Manual review recommended**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged or low-privileged trigger: +1
  Triggering requires access to KVM or control of a VMM process using KVM_RUN. This is typically local PR:L rather than full host root.

* Integrity impact plausible: +1
  Missed PML flushing can cause guest memory written during the final KVM_RUN to not be marked dirty, affecting dirty logging correctness.

* Broad or common subsystem exposure: +1
  KVM x86 is a widely deployed virtualization subsystem, and dirty logging is important for migration, checkpointing, and backup workflows.

Paranoid-only signal:

* Availability impact realistic: +1
  If dirty logging corruption affects live migration or checkpoint restore, the result may be guest state loss, service disruption, or failed recovery. This is operationally significant, but not a direct host crash.

Not applied:

* No generic memory corruption bonus.
  The patch does not show UAF, OOB write, stale rmap traversal, arbitrary write, or host memory corruption.

* No privilege escalation bonus.
  The described effect is dirty log integrity loss, not host privilege escalation.

* No remote/network bonus.
  The bug is triggered through KVM control flow, not directly by network packets.

* No KVM/MMU rmap UAF rule.
  This patch concerns fastpath exit ordering and PML buffer flushing, not SPTE overwrite, rmap stale pointers, or page table lifetime corruption.

Call-site confidence: high.
The patch shows the relevant vcpu_enter_guest path and the VMX/SVM vendor handle_exit placement.

## 3. Reachability analysis

A local user or service with access to /dev/kvm and control over a VM can plausibly reach this path through KVM_RUN. In many deployments this is not full root, because access may be delegated to a qemu or kvm group, a virtualization service account, or a containerized VMM environment.

The issue is not directly network reachable. A remote attacker would need an additional path to control the VMM, guest behavior, migration workflow, or management plane.

The most relevant deployment condition is VMX with PML enabled and workflows depending on KVM dirty logging, such as live migration, checkpoint, backup, or incremental memory tracking. This is not every KVM workload, but it is common enough in virtualization environments to avoid auto-closing without review.

## 4. Severity interpretation

This behaves more like a borderline actionable Moderate than an Important memory corruption issue.

Realistic impact is integrity degradation of guest dirty logging and possible operational disruption. The patch does not support host kernel memory corruption, LPE, guest-to-host escape, or direct host DoS. The paranoid score is raised only because missed dirty pages can materially affect VM migration or recovery correctness.

## 5. One-sentence report phrase

KVM x86 fastpath userspace exits could skip vendor exit handling before returning from KVM_RUN, allowing VMX PML dirty log updates from the final run to be missed and potentially corrupting migration or checkpoint state integrity.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is not a memory corruption or direct LPE issue, but KVM dirty logging correctness affects migration and backup integrity in real virtualization deployments, so it should not be automatically closed as a trivial Moderate.

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

Patch: KVM: x86: Ensure vendor's exit handler runs before fastpath userspace exits
Commit: b3436d9b9b1affe1c3191ac9831308923f5f03c3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b3436d9b9b1affe1c3191ac9831308923f5f03c3

Commit description:

Move the handling of fastpath userspace exits into vendor code to ensure
KVM runs vendor specific operations that need to run before userspace gains
control of the vCPU.  E.g. for VMX (and soon to be for SVM as well), KVM
needs to flush the PML buffer prior to exiting to userspace, otherwise any
memory written by the final KVM_RUN might never be flagged as dirty.

Note, waiting to snapshot CR0 and CR3 until svm_handle_exit() is flawed in
general, as that risks consuming stale state in a fastpath handler.  That
will be addressed in a future change.

Fixes: f7f39c5 ("KVM: x86: Exit to userspace if fastpath triggers one on instruction skip")
Cc: stable@vger.kernel.org
Cc: Nikunj A. Dadhania <nikunj@amd.com>
Reviewed-by: Nikunj A. Dadhania <nikunj@amd.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20260423162628.490962-2-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/svm/svm.c
  arch/x86/kvm/vmx/vmx.c
  arch/x86/kvm/x86.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=b3436d9b9b1affe1c3191ac9831308923f5f03c3

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

The original Linux kernel CVE announcement for CVE-2026-64284 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64284

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:N/I:H/A:L
  The Best / paranoid CVSS score: 5.3

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).