[CVE-2026-46065][IMPORTANT] fbdev: defio: Disconnect deferred I/O from the lifetime of struct fb_info [ Upstream

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

CVE: CVE-2026-46065
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: fbdev: defio: Disconnect deferred I/O from the lifetime of struct fb_info [ Upstream
Commit: 2a40f8bc9bb713329f1c35ffc199ee961a7135b0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2a40f8bc9bb713329f1c35ffc199ee961a7135b0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46065
Analysis date: Wed, 27 May 2026 15:06:57 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local user with framebuffer mmap access may retain a deferred I/O mapping across fbdev hot-unplug, causing stale `fb_info` use in fault or page_mkwrite handling and creating a kernel UAF with DoS impact and plausible LPE concern.

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

The original Linux kernel CVE announcement for CVE-2026-46065 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46065

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-2026-46065	IMPORTANT	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:H/I:H/A:H';*CWE-416;*CWE-672;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'fbdev deferred I/O used a mapping-private pointer to fb_info while user space could keep an active mmap after device hot-unplug. If fb_info was freed before the last VMA was closed, later fault or page_mkwrite handling could operate on stale kernel state and trigger a use-after-free. For the CVSS the PR:L is selected because practical triggering requires local access to the framebuffer device and an active mmap, but this does not necessarily require full host root in systems where /dev/fb access is delegated to a local user or graphics group. The issue is not network reachable and normally depends on a hot-unplug or device removal event, so AC:H is appropriate. Impact is at least local denial of service via kernel crash. In the paranoid view, privilege escalation cannot be excluded because this is a real lifetime UAF in a memory mapping fault path, although no reliable overwrite primitive is demonstrated by the patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	READ LOCK DANGER INIT NOMEMCHK HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	NO	NO	checked

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

ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local user with access to `/dev/fb*` can keep an active mmap of framebuffer memory.
* Generic memory corruption, strong primitive class: +2. The bug is a real use-after-free of `struct fb_info`.
* Real lifetime corruption: +1. The mapping lifetime can outlive the `fb_info` lifetime after hot-unplug.
* Reliable kernel crash / strong DoS: +1. Later `fault` or `page_mkwrite` on the stale mapping can access freed state and crash.
* Availability impact realistic: +1. The affected path can cause local kernel DoS.
* Weak LPE concern: +1. UAF occurs in an mmap fault/write path, but no controlled reclaim or overwrite primitive is demonstrated.
* Hard / special triggering condition: -1. Reliable triggering normally requires a retained mmap plus hot-unplug or device removal timing.
* Rare / configuration-dependent exposure: -1. Requires fbdev deferred I/O and a removable or removable-like framebuffer device.

Conservative total: 5

Paranoid additional interpretation:

* Privilege escalation plausible: upgraded from weak +1 to strong +2. UAF in a VMA fault/page_mkwrite path is a historically sensitive class because freed objects may be reclaimable and later dereferenced in kernel context.
* Privileged kernel/device-management memory corruption path: +1. The corruption is tied to hot-unplug cleanup and device lifetime management, which is a trusted kernel teardown path.

Paranoid total: 7

Not counted:

* No remote/network score. The affected path is local framebuffer mmap and device removal.
* No Dirty-Pipe-like or page-cache corruption score. The patch does not show attacker-controlled writes into shared file-backed cache.
* No demonstrated arbitrary write or reliable object confusion primitive.

## 3. Reachability analysis

A realistic attacker needs local access to a framebuffer device using deferred I/O and the ability to keep an mmap active while the device is removed or hot-unplugged. On desktops, embedded systems, graphical appliances, or systems where `/dev/fb*` is exposed to a local user or graphics group, PR:L is a defensible practical assumption. If only root can open the framebuffer and trigger removal, the practical risk is lower.

Namespaces and containers usually do not expose fbdev devices by default. If a container or sandbox is granted `/dev/fb*`, the bug may cross an intended device-access boundary, but the hot-unplug/removal condition may still require host action or special hardware behavior.

The path is not default-reachable on headless servers and is not network-triggerable. Exploitation conditions are realistic mainly on systems with fbdev deferred I/O, local device access, and removable or dynamically removed framebuffer devices.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup because the patch fixes a real lifetime bug where user mappings can outlive `struct fb_info`. The demonstrated impact is local DoS through use-after-free after hot-unplug. The theoretical higher impact is local privilege escalation through UAF reclaim in a memory mapping fault/write path, but the patch does not provide evidence of a reliable overwrite primitive.

Overall this should be handled as Actionable Moderate at minimum, with Strong Important candidate treatment for manual triage because UAF plus mmap fault handling is a sensitive kernel exploitation class.

## 5. One-sentence report phrase

A local user with framebuffer mmap access may retain a deferred I/O mapping across fbdev hot-unplug, causing stale `fb_info` use in fault or page_mkwrite handling and creating a kernel UAF with DoS impact and plausible LPE concern.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real UAF in an mmap fault/page_mkwrite path, and although reliable exploitation depends on hot-unplug or removal timing, the bug class and lifetime context are strong enough to require manual review.

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

Patch: fbdev: defio: Disconnect deferred I/O from the lifetime of struct fb_info [ Upstream
Commit: 2a40f8bc9bb713329f1c35ffc199ee961a7135b0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2a40f8bc9bb713329f1c35ffc199ee961a7135b0

Changed files:
  drivers/video/fbdev/core/fb_defio.c
  include/linux/fb.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=2a40f8bc9bb713329f1c35ffc199ee961a7135b0

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

The original Linux kernel CVE announcement for CVE-2026-46065 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46065

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:N/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: 5
  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).