[CVE-2026-80785][MODERATE 7.0] fbdev: serialize mode sysfs access with lock_fb_info() [ Upstream

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

CVE: CVE-2026-80785
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fbdev: serialize mode sysfs access with lock_fb_info() [ Upstream
Commit: 26135631ed8e487bc6aef70cc41934e258d09bbe
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=26135631ed8e487bc6aef70cc41934e258d09bbe
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80785
Analysis date: Fri, 04 Sep 2026 15:43:41 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in fbdev sysfs mode handling can allow concurrent access to freed modelist entries or store a stale freed pointer in fb_info mode, causing at least local kernel crash DoS and requiring manual review for possible UAF based privilege escalation in delegated framebuffer access deployments.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-80785 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80785

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-2026-80785	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-80785 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'fbdev sysfs mode handling has a race because show_mode, show_modes, and store_mode can access fb_info modelist and fb_info mode without holding lock_fb_info while store_modes can replace the modelist and free the old entries. A local actor who can trigger concurrent mode reads or writes and modelist replacement can cause a reader or writer to dereference freed memory, or store a stale freed mode pointer in fb_info mode. For the CVSS the PR:L is used for the paranoid score because some deployments may expose framebuffer control through delegated device access, reduced capability root, or service accounts rather than full host administrator access. The issue is not network reachable and requires local interaction with fbdev sysfs control paths. Impact is at least local denial of service via kernel crash. In the worst still defensible case, the stale pointer and use-after-free primitive may allow confidentiality or integrity impact and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 4)	YES	LOCK UAF HARDWARE LINUS KPANIC MEMORY  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Memory corruption, weak primitive: +1
  The patch describes a race where a pointer to an old `fb_info->modelist` entry can be loaded before `store_modes()` frees it, followed by dereference of freed memory or storing a stale freed pointer in `fb_info->mode`.
* Real lifetime corruption: +1
  This is a real object lifetime bug involving freed modelist entries and stale pointers.
* Reliable kernel crash / strong DoS: +1
  Dereferencing freed memory from fbdev sysfs paths can plausibly crash the kernel.
* Hard or unreliable race / special timing required: -1
  The trigger requires a concurrent `store_modes()` replacement/free operation racing with `show_mode()`, `show_modes()`, or `store_mode()`.
* Requires admin/root/CAP-like control in typical deployments: -1
  Replacing mode lists through fbdev sysfs is usually restricted. I use -1 rather than -2 because framebuffer control may be delegated to desktop/session services or reduced-privilege device-management contexts.

Paranoid additional signals:

* Local low-privilege trigger interpretation: +1
  In some deployments, framebuffer sysfs/device access may be exposed to a local seat user, container-like environment, service account, or reduced-capability root.
* Weak LPE concern: +1
  The stale freed pointer stored in `fb_info->mode` is more concerning than a simple NULL dereference because later access may operate on memory that has been reallocated for another object. No controlled reclaim is shown, so this remains weak.
* Paranoid cap applied: score capped at 5
  This is race-dependent UAF/stale-pointer behavior without demonstrated attacker-controlled reclaim, type confusion, callback control, or arbitrary write.

Call-site confidence: high. The affected sysfs functions and the freeing/replacement relationship are described directly in the patch context.

## 3. Reachability analysis

The bug is local and is not network reachable. A realistic trigger requires access to fbdev sysfs mode controls and concurrent operations against mode/modes attributes. In typical server deployments this is likely administrator-controlled, but on graphical systems or embedded products fbdev access may be delegated to local display/session components. Namespaces do not automatically make this reachable, but container or service configurations that expose framebuffer sysfs/device control could reduce the practical privilege requirement.

## 4. Severity interpretation

This is not an ordinary resource leak or validation-only bug. It is a race-dependent lifetime bug with a concrete UAF/stale-pointer candidate. Realistic exploitation evidence points mainly to local DoS through kernel crash. Theoretical escalation is plausible enough for manual review because a stale freed pointer may persist in `fb_info->mode`, but the patch does not show controlled reclaim, attacker-chosen object replacement, callback dispatch, or a direct write primitive. Therefore it is best treated as Actionable Moderate at minimum, with Strong Important candidate handling if affected products expose fbdev mode control to non-full-root local users.

## 5. One-sentence report phrase

A race in fbdev sysfs mode handling can allow concurrent access to freed modelist entries or store a stale freed pointer in fb_info mode, causing at least local kernel crash DoS and requiring manual review for possible UAF based privilege escalation in delegated framebuffer access deployments.

## 6. Manual review recommendation

YES REQUIRES MANUAL CHECK

MANUAL CHECK REQUIRED. This is a local race with a concrete UAF/stale-pointer primitive, and the practical PR value depends heavily on product-specific fbdev sysfs permissions and delegated framebuffer access policy.

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

Patch: fbdev: serialize mode sysfs access with lock_fb_info() [ Upstream
Commit: 26135631ed8e487bc6aef70cc41934e258d09bbe
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=26135631ed8e487bc6aef70cc41934e258d09bbe

Changed files:
  drivers/video/fbdev/core/fbsysfs.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=26135631ed8e487bc6aef70cc41934e258d09bbe

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

The original Linux kernel CVE announcement for CVE-2026-80785 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80785

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: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: 2
  Paranoid ActionableScore: 5

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).