[CVE-2026-64475][MODERATE 7.0] vfio/pci: Release the VGA arbiter client on register_device() failure [ Upstream

[CVE-2026-64475][MODERATE 7.0] vfio/pci: Release the VGA arbiter client on register_device() failure [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64475
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: vfio/pci: Release the VGA arbiter client on register_device() failure [ Upstream
Commit: 0f2a35a0c7ea7da347b814750eaa78adf3582381
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0f2a35a0c7ea7da347b814750eaa78adf3582381
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64475
Analysis date: Sat, 25 Jul 2026 09:51:45 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A VFIO PCI register_device() failure path could leave a VGA arbiter callback registered with a freed vdev cookie, creating a stale callback and UAF candidate that is local and privileged in typical deployments but should be manually reviewed for delegated VFIO environments.

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

The original Linux kernel CVE announcement for CVE-2026-64475 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64475

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-2026-64475	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:H/I:H/A:H';*CWE-416;CWE-404;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A stale VGA arbiter client registration in vfio/pci can remain after register_device() fails because the failure path did not unregister the VGA callback for the vfio PCI device. The stale registration can keep a freed vdev cookie and a later callback may dereference state associated with the freed vfio device, making this a use-after-free candidate rather than a simple resource leak. For the CVSS the PR:L is used in the paranoid score because some deployments delegate VFIO or PCI device management to reduced capability root, service accounts, or local users with device access rather than full host administration. The issue is not network reachable and requires local interaction with VFIO PCI device setup plus a specific registration failure window. Impact is at least local denial of service and in the worst still defensible case may include confidentiality and integrity impact due to UAF style memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	DANGER INIT UAF HARDWARE KPANIC 	YES	NO	checked

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

ActionableScore=7
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak/indirect corruption candidate: +1
  The failure path can leave a VGA arbiter callback registered with a freed `vdev` cookie, but the current commit notes that this is mostly benign in current kernels because the callback mostly uses `pci_dev` state.
* Real lifetime corruption: +1
  This is a stale registration after object cleanup, i.e. a lifetime/unwind bug with a potentially freed callback cookie.
* Reliable kernel crash / strong DoS concern: +1
  A later callback using the stale cookie can plausibly dereference freed state and crash the kernel.
* Privileged kernel/device-management memory corruption path: +1
  The bug is in VFIO PCI device registration and VGA arbiter callback cleanup, a trusted device-management path.
* Requires admin/root/CAP style device management in typical deployments: -2
  Triggering normally requires VFIO PCI binding/probe/register failure handling, which is usually controlled by host root or an equivalent device-management service.
* Special failure path / difficult reachability: -1
  The vulnerable state requires a `register_device()` failure after VGA arbiter registration.

Paranoid additional interpretation:

* Generic memory corruption, strong lifetime primitive: +2 instead of weak +1
  The patch context explicitly mentions a callback with a freed `vdev` cookie. A stale callback into freed device state is a UAF-class primitive.
* Weak LPE concern: +1
  There is no demonstrated reclaim or overwrite primitive, but stale callback dispatch into freed VFIO state is enough to require manual review.
* Confidentiality impact plausible: +1
  If freed `vdev` memory is reused and later interpreted through the stale callback path, limited disclosure or state confusion is technically defensible.
* Integrity impact plausible: +1
  The same UAF-style stale callback condition can plausibly corrupt or affect VFIO/device state, though no arbitrary write is shown.
* Reduced privilege / delegated VFIO interpretation: 0 instead of -2
  Some environments delegate VFIO or PCI device management to service accounts, containerized root, or reduced-capability management components, so treating this only as full host-root is too dismissive for paranoid triage.

Call-site confidence: medium. The patch shows the unwind site and the commit message identifies the stale callback/freed cookie consequence, but the full callback call chain is not included.

## 3. Reachability analysis

The issue is local, not network reachable. In typical deployments, triggering requires control over VFIO PCI device registration, binding, or a failure path during `vfio_pci_core_register_device()`, which usually implies host-root or a privileged device-management component. Namespace and container effects depend on whether VFIO device assignment is delegated. A normal unprivileged local user generally should not be able to trigger the registration path directly, but reduced-capability management services or containerized device orchestration can make the practical PR closer to PR:L than PR:H.

The path is not default-reachable for arbitrary users, but VFIO PCI is common in virtualization and passthrough environments. The realistic condition is a failed registration after VGA arbiter registration, followed by a later callback that observes stale state.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup because the commit explicitly references a callback with a freed `vdev` cookie. Conservative handling is Borderline / manual review recommended because the current callback behavior may be mostly benign and triggering depends on a specific failure path. Paranoid handling is Actionable Moderate to Strong Important candidate because stale callback registration with freed VFIO state is a UAF-class lifetime bug in a privileged device-management subsystem.

Theoretical memory corruption potential exists through stale callback use of freed device state. Realistic exploitation evidence is weaker because no attacker-controlled reclaim, overwrite primitive, or working LPE path is shown in the patch.

## 5. One-sentence report phrase

A VFIO PCI register_device() failure path could leave a VGA arbiter callback registered with a freed vdev cookie, creating a stale callback and UAF candidate that is local and privileged in typical deployments but should be manually reviewed for delegated VFIO environments.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because the patch fixes a real lifetime/unwind bug involving a stale callback and freed VFIO device cookie. The current impact may be mostly DoS, but the UAF-style callback pattern is strong enough to justify manual review.

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

Patch: vfio/pci: Release the VGA arbiter client on register_device() failure [ Upstream
Commit: 0f2a35a0c7ea7da347b814750eaa78adf3582381
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0f2a35a0c7ea7da347b814750eaa78adf3582381

Changed files:
  drivers/vfio/pci/vfio_pci.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=0f2a35a0c7ea7da347b814750eaa78adf3582381

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

The original Linux kernel CVE announcement for CVE-2026-64475 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64475

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:H/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 4
  Paranoid ActionableScore: 7

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).