[CVE-2026-74424][LOW] fbcon: fix NULL pointer dereference for a console without vc_data

[CVE-2026-74424][LOW] fbcon: fix NULL pointer dereference for a console without vc_data

[AL-KERNEL]

CVE: CVE-2026-74424
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: fbcon: fix NULL pointer dereference for a console without vc_data
Commit: 8e9b8b008f4036df0318870c5d754134ff1b94cc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8e9b8b008f4036df0318870c5d754134ff1b94cc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74424
Analysis date: Sun, 16 Aug 2026 00:12:24 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
fbcon_new_modelist() can dereference a NULL vc_data pointer after console teardown leaves fb_display mode set, allowing a local framebuffer or console control path to cause a kernel crash, with no evidence of privilege escalation or memory corruption.

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

The original Linux kernel CVE announcement for CVE-2026-74424 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74424

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: LOW
Manual review required: NO

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
======================================================================

CVE-2026-74424	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-665;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'fbcon_new_modelist can dereference a NULL vc_data pointer when a framebuffer modelist changes while fb_display mode remains set for a console whose vc_data was already removed. A local user or process with access to framebuffer or virtual console control paths may be able to trigger the invalid state through console teardown and modelist update operations, causing a kernel crash. For the CVSS the PR:L is used for the paranoid score because reliable triggering requires local code execution or access to local device interfaces, but not necessarily full administrator privileges in all deployments. The issue is not network reachable. Impact is denial of service only because the patch indicates a NULL pointer dereference and does not show a UAF, OOB access, or arbitrary write primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	DANGER NULLPTR SIMPLEFIX IMPROVEONLY LINUS SYZBOT  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1 in paranoid scoring. A local process with access to framebuffer or virtual console control paths may be able to influence console teardown and modelist update behavior.
* Reliable kernel crash / strong DoS: +1. The described failure is a NULL pointer dereference in fbcon_new_modelist(), causing a kernel crash or oops.

Not triggered:

* Remote reachable: +0. No network path is involved.
* Generic memory corruption: +0. The patch and commit describe NULL pointer dereference only, not UAF, OOB write, double free, type confusion, or arbitrary write.
* Real lifetime corruption: +0. vc_data may be absent after teardown, but the actual dereference is NULL, not a stale freed object reuse.
* Privilege escalation plausible: +0. No reclaim, overwrite, callback control, type confusion, or refcount primitive is shown.
* Confidentiality / Integrity impact: +0. No data disclosure or write primitive is supported.
* Availability impact realistic: +0 additional. Crash impact is already counted via reliable DoS and should not be double counted here.

Potential negative signals:

* Conservative reachability uncertainty: local access to fbcon or framebuffer paths may depend on device permissions and active console configuration, so the lower score does not award local unprivileged trigger.

## 3. Reachability analysis

The bug is locally reachable through framebuffer console state changes, specifically a modelist update when fb_display[i].mode remains set while vc_cons[i].d is NULL. It is not remotely reachable and is not packet-triggered.

A fully unprivileged user may not always have the needed access, depending on framebuffer device permissions, virtual console access, seat management, and whether fbcon is active. Containers usually do not expose real framebuffer console control, so namespace-based reachability is limited unless the device nodes or console control are delegated. The path is common on systems using fbcon, but many server or container deployments will not expose it to untrusted users.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local DoS. The realistic impact is kernel crash from NULL pointer dereference. Theoretical memory corruption is not supported by the patch because the fix only prevents mode from remaining set when vc_data is absent and reorders the vc_data check before var_to_display() updates fb_display state.

There is no demonstrated UAF, attacker-controlled reclaim, writable stale object access, arbitrary write, or type confusion. Therefore this should not be treated as an Important-class kernel vulnerability.

## 5. One-sentence report phrase

fbcon_new_modelist() can dereference a NULL vc_data pointer after console teardown leaves fb_display mode set, allowing a local framebuffer or console control path to cause a kernel crash, with no evidence of privilege escalation or memory corruption.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed or handled in deferred mode unless the evaluated product exposes framebuffer or virtual console control to untrusted local users by default.

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

Patch: fbcon: fix NULL pointer dereference for a console without vc_data
Commit: 8e9b8b008f4036df0318870c5d754134ff1b94cc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8e9b8b008f4036df0318870c5d754134ff1b94cc

Commit description:

fbcon_new_modelist() runs when a framebuffer's modelist changes. For each
console mapped to it with fb_display[i].mode set, it reads vc_cons[i].d and
passes the vc_num to fbcon_set_disp(). This assumes a console with a mode
set has a vc_data, but it can be NULL. fbcon_set_disp() sets
fb_display[i].mode before it checks vc_data, and fbcon_deinit() leaves the
mode set after the vc_data is freed. fbcon_new_modelist() then dereferences
the NULL vc_data.

Keep fb_display[i].mode set only while the console has a vc_data. Check
vc_data before setting the mode in fbcon_set_disp(), and clear the mode in
fbcon_deinit(). The existing mode check in fbcon_new_modelist() then skips
such consoles.

Reported-by: syzbot+42525d636f430fd5d983@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=42525d636f430fd5d983
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Ian Bridges <icb@fastmail.org>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/video/fbdev/core/fbcon.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=8e9b8b008f4036df0318870c5d754134ff1b94cc

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

The original Linux kernel CVE announcement for CVE-2026-74424 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74424

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 1
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).