[CVE-2026-64042][IMPORTANT] vfio/pci: Check BAR resources before exporting a DMABUF [ Upstream

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

CVE: CVE-2026-64042
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: vfio/pci: Check BAR resources before exporting a DMABUF [ Upstream
Commit: 8443cd4497a4498c4b01058d76a92116244cb605
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8443cd4497a4498c4b01058d76a92116244cb605
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64042
Analysis date: Sun, 19 Jul 2026 18:20:13 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
VFIO PCI DMABUF export could expose unreserved PCI BAR MMIO resources to a local delegated VFIO user or VM service account, potentially allowing unauthorized device register access and platform-dependent confidentiality, integrity, or availability impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64042 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64042

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64042	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	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-284;CWE-862;CWE-668;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '7.8';DESCR 'VFIO PCI DMABUF export for MMIO BAR regions could expose a BAR resource even when the underlying PCI resource was not actually reserved. A local user or service with access to the VFIO device could create a DMABUF for such a region and then access MMIO that should not have been exported. For the CVSS the PR:L is used for the paranoid score because reliable triggering requires local access to a VFIO device file or a delegated VM service account rather than full host administrator privileges. The issue is not network reachable by itself. Impact may include unauthorized reads or writes to device MMIO and possible host or device denial of service depending on the affected BAR and platform resource layout.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-64042 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 5d74781ebc86c5fa9e9d6934024c505412de9b52 YES  NO NO unknown 	MAYBE	SIMPLEFIX DMAorINTERRUPT IMPROVEONLY LINUS KPANIC MEMORY  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local delegated trigger: +1. Triggering requires local access to a VFIO device or a VM service account with the relevant VFIO file descriptor, not remote network access.
* Security-boundary bypass with practical sensitive-object access: +2. The bug can expose PCI BAR MMIO resources that were not actually reserved for export, bypassing the intended VFIO resource ownership boundary.
* Confidentiality impact plausible: +1. BAR MMIO may expose device registers or device memory windows depending on hardware and platform layout.
* Integrity impact plausible: +1. MMIO access can potentially modify device state or registers outside the intended exported resource set.
* Availability impact realistic: +1. Incorrect MMIO access can plausibly destabilize the device, VM, driver, or host resource state.

Paranoid additional signal:

* Cross-boundary isolation or policy bypass: +1. In VFIO deployments, the affected path can cross a capability separated boundary between host resource ownership and a delegated VM or service account.

Not applied:

* Remote reachable: +0. No network-triggerable path is shown.
* Generic memory corruption: +0. The patch shows unauthorized BAR resource exposure, not UAF, OOB write, double free, or type confusion.
* Strong LPE plausibility: +0. Privilege escalation is not demonstrated as a memory corruption primitive, but the access-control bypass is still security relevant.
* Requires full admin/root penalty: 0. VFIO setup is privileged, but practical triggering may be by a reduced-privilege QEMU/libvirt service account or delegated local user, not necessarily full host root.

## 3. Reachability analysis

The bug is reachable through the VFIO PCI DMABUF export path for MMIO BAR regions. A typical arbitrary local user cannot access this path unless a VFIO device has been assigned or delegated. However, in virtualization deployments the effective attacker may be a local VM management process, QEMU process, libvirt service account, container root with delegated device access, or another reduced-privilege principal with access to the VFIO device.

Namespaces and containers matter because this should not be treated as full PR:H when the VFIO device is intentionally delegated to a less-privileged workload or service account. The issue is not default-enabled for all systems and depends on VFIO PCI plus DMABUF export support, but VFIO is common enough in virtualization and device passthrough environments that it should not be auto-closed as a niche correctness issue.

## 4. Severity interpretation

This behaves more like an Actionable Moderate with Strong Important candidate characteristics than an ordinary Moderate. The patch does not show direct kernel memory corruption or a proven LPE primitive, so it should not be scored as arbitrary code execution. The main risk is an access-control failure around PCI BAR MMIO export, where a delegated VFIO user may access unreserved device resources.

The conservative score is 6 because the issue is local and deployment-dependent, but it bypasses an intended resource boundary and may expose sensitive MMIO. The paranoid score is 7 because unauthorized BAR access can have high platform-dependent consequences for confidentiality, integrity, and availability.

## 5. One-sentence report phrase

VFIO PCI DMABUF export could expose unreserved PCI BAR MMIO resources to a local delegated VFIO user or VM service account, potentially allowing unauthorized device register access and platform-dependent confidentiality, integrity, or availability impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because impact depends heavily on the specific PCI device, BAR layout, IOMMU configuration, VFIO delegation model, and whether the exposed unreserved resource can affect host or device security boundaries.

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UPSTREAM PATCH SUMMARY
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Patch: vfio/pci: Check BAR resources before exporting a DMABUF [ Upstream
Commit: 8443cd4497a4498c4b01058d76a92116244cb605
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8443cd4497a4498c4b01058d76a92116244cb605

Changed files:
  drivers/vfio/pci/vfio_pci_dmabuf.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=8443cd4497a4498c4b01058d76a92116244cb605

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

The original Linux kernel CVE announcement for CVE-2026-64042 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64042

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:L
  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: 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: 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).