[CVE-2026-80579][MODERATE 7.0] fbdev: clear fb_info->mode before deleting a videomode

[CVE-2026-80579][MODERATE 7.0] fbdev: clear fb_info->mode before deleting a videomode

[AL-KERNEL]

CVE: CVE-2026-80579
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fbdev: clear fb_info->mode before deleting a videomode
Commit: ce7fef961c63229b22dec415fb988899f479d61f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ce7fef961c63229b22dec415fb988899f479d61f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80579
Analysis date: Wed, 26 Aug 2026 13:58:36 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local fbdev lifetime bug can leave `fb_info->mode` pointing to a deleted videomode, allowing later sysfs mode reads to dereference freed memory and potentially cause limited information disclosure or kernel crash.

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

The original Linux kernel CVE announcement for CVE-2026-80579 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80579

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.

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

CVE-2026-80579	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H';*CWE-416;CWE-825;*CWE-664;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '6.1';DESCR 'A stale pointer use-after-free can occur in the fbdev mode handling path because fb_info mode may still reference a videomode entry that was deleted from the modelist through FBIOPUT_VSCREENINFO with FB_ACTIVATE_INV_MODE. Later sysfs mode reads through show_mode can dereference the freed object and read fields from memory that is no longer owned by the videomode list. For the CVSS the PR:L reflects that a local user or process with access to the fbdev ioctl and mode sysfs interface is needed. The issue is not network reachable and is triggered through a local framebuffer control interface. Impact is at least a local denial of service or limited information disclosure via stale kernel heap reads. No direct arbitrary write primitive is visible from this patch context, so integrity impact is not raised in the selected scoring.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	SIMPLEFIX IMPROVEONLY LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged or low-privileged trigger: +1
  Trigger is via local fbdev ioctl `FBIOPUT_VSCREENINFO` with `FB_ACTIVATE_INV_MODE`, followed by local sysfs mode read. This is not network reachable.

* Memory corruption, weak or indirect corruption candidate: +1
  `fb_info->mode` can remain as a stale pointer to a deleted `fb_videomode` entry. The shown primitive is stale pointer dereference, not arbitrary write or callback control.

* Real lifetime corruption: +1
  This is a real object lifetime bug: a pointer survives deletion of the object it references.

* Confidentiality impact plausible: +1
  `show_mode()` may read fields through a freed object. If the freed slab object is reused, limited kernel heap disclosure through sysfs is plausible.

Paranoid additional signal:

* Reliable kernel crash or strong DoS: +1
  A stale kernel pointer dereference through sysfs can plausibly cause an Oops or crash depending on allocator reuse, debug options, poisoning, and object lifetime timing. This justifies the paranoid score but is not strong enough to claim LPE.

Not awarded:

* Privilege escalation plausible: +0
  No reclaim control, write-after-free, callback dispatch, type confusion, refcount takeover, or arbitrary write is shown.

* Integrity impact plausible: +0
  The patch context shows read-side stale dereference, not a write primitive.

* Remote reachable: +0
  No network path.

* Requires admin/root penalty: 0
  Access may depend on `/dev/fb*` permissions or video group membership, but full host root is not inherently required by the described path.

Call-site confidence: high. The commit message describes both the delete path and later sysfs `show_mode()` dereference.

## 3. Reachability analysis

A local user or local process with access to the framebuffer device can attempt to delete a videomode through `FBIOPUT_VSCREENINFO` using `FB_ACTIVATE_INV_MODE`. The later dereference is through local sysfs mode reads. Containers and namespaces usually do not expose real fbdev devices by default, but systems that delegate `/dev/fb*` to graphical sessions, kiosks, embedded devices, or video-group users may make this reachable without full root. The path is not remotely reachable and is deployment dependent, but fbdev remains common enough in graphics, console, and embedded environments that it should not be treated as purely theoretical.

## 4. Severity interpretation

This behaves like an actionable Moderate lifetime bug rather than an ordinary low-risk cleanup. The realistic impact is limited information disclosure or local DoS from a stale pointer dereference. The theoretical memory corruption concern exists because this is a real UAF-style lifetime violation, but the patch does not show attacker-controlled reclaim, object replacement, write access, callback control, or another primitive that would support treating it as an Important LPE candidate.

## 5. One-sentence report phrase

A local fbdev lifetime bug can leave `fb_info->mode` pointing to a deleted videomode, allowing later sysfs mode reads to dereference freed memory and potentially cause limited information disclosure or kernel crash.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This should not be auto-closed because it is a real stale-pointer UAF class issue reachable from a local device interface, even though the demonstrated primitive appears read-side only and does not currently justify privilege-escalation scoring.

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

Patch: fbdev: clear fb_info->mode before deleting a videomode
Commit: ce7fef961c63229b22dec415fb988899f479d61f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ce7fef961c63229b22dec415fb988899f479d61f

Commit description:

fb_set_var() can delete a mode from info->modelist when userspace
passes FB_ACTIVATE_INV_MODE through FBIOPUT_VSCREENINFO. The code
checks that the mode being deleted is not the current info->var and
that fbcon is not using it, but it does not check fb_info->mode.

fb_info->mode may still point into the modelist entry being deleted.
If the entry is freed, later mode sysfs reads through show_mode() can
dereference a stale pointer.

Clear fb_info->mode before calling fb_delete_videomode() when it
matches the mode being removed.

Cc: stable@vger.kernel.org # v7.1+
Signed-off-by: Melbin K Mathew <mlbnkm1@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/video/fbdev/core/fbmem.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=ce7fef961c63229b22dec415fb988899f479d61f

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

The original Linux kernel CVE announcement for CVE-2026-80579 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80579

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:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 6.1

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: 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: NO
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).