[CVE-2026-53402][MODERATE REGULAR] fbdev: fbcon: fix out-of-bounds read in err_out of fbcon_do_set_font() [ Upstream

[CVE-2026-53402][MODERATE REGULAR] fbdev: fbcon: fix out-of-bounds read in err_out of fbcon_do_set_font() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53402
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fbdev: fbcon: fix out-of-bounds read in err_out of fbcon_do_set_font() [ Upstream
Commit: b5bb2c696e140c399cb874def2feedf61dee27d6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b5bb2c696e140c399cb874def2feedf61dee27d6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53402
Analysis date: Sun, 19 Jul 2026 11:14:33 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A local out-of-bounds read in fbcon font rollback can leave hi_font state inconsistent after fbcon_do_set_font failure, allowing later rendering to read beyond the reverted font array and potentially disclose kernel memory.

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

The original Linux kernel CVE announcement for CVE-2026-53402 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53402

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 REGULAR
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-2026-53402	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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:N/A:N';CWE-125;*CWE-200;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '4.7';DESCR 'An out-of-bounds read in the fbdev/fbcon font handling path can occur after fbcon_do_set_font fails during rollback. The error path restores the old font size and character count but can leave vc_hi_font_mask and the screen buffer in a mismatched state. A local user or process that can reach the virtual console font setting path may then cause inflated character indexes to be accepted and later rendered against a reverted 256 character font array. For the CVSS the PR:L is used because reliable triggering requires local access to the console font ioctl path and control over the local execution context, but full administrator privileges are not assumed for the conservative review model. The issue is not network reachable. Impact is primarily kernel memory disclosure through an out-of-bounds read, with no direct evidence of integrity impact or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4)	YES	OOB ERRORPATH LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

* Local unprivileged or low-privileged trigger: +1
  The issue is local and requires access to the VT or console font setting path. In typical hardened deployments this may require CAP_SYS_TTY_CONFIG, but the safer triage interpretation is PR:L when local console or reduced-privilege access can reach the path.

* Confidentiality impact plausible: +1
  The commit explicitly describes a deterministic out-of-bounds read from the reverted 256-character font array and potential kernel memory disclosure.

* Broad or common local kernel interface: +1
  fbcon and VT console paths are common local kernel interfaces on systems with virtual terminals or framebuffer console support, though they are less exposed on headless/server-only deployments.

* Practical disclosure path strongly implied: +1 paranoid only
  The described chain is concrete: failed rollback leaves vc_hi_font_mask inconsistent, userspace can place inflated character indexes in the screen buffer, and fbcon_putcs later renders using those indexes against a smaller font array.

No strong memory corruption, no OOB write, no UAF, no arbitrary write, and no privilege escalation primitive is shown.

## 3. Reachability analysis

A local user or process with access to the virtual console font manipulation path is needed. On many systems, changing console fonts is privileged or effectively restricted to local console users, so typical exposure is not broad remote attack surface. Containers usually do not expose real VT or fbcon font ioctls unless explicitly delegated. The bug is not network reachable. Realistic exploitation also depends on triggering the fbcon_do_set_font error path, for example by making vc_resize fail under memory pressure, which increases practical complexity.

Call-site confidence: high. The commit explains the rollback path, the state mismatch, and the later fbcon_putcs rendering consequence.

## 4. Severity interpretation

This behaves more like a borderline actionable Moderate than an ordinary low-risk cleanup. The concrete primitive is a local kernel out-of-bounds read with possible information disclosure, not a crash-only bug. However, there is no evidence of write corruption, UAF, object replacement, callback control, or privilege escalation. The practical severity is limited by local-only reachability, likely privilege or console-access requirements, and the need to hit an error rollback path.

## 5. One-sentence report phrase

A local out-of-bounds read in fbcon font rollback can leave hi_font state inconsistent after fbcon_do_set_font failure, allowing later rendering to read beyond the reverted font array and potentially disclose kernel memory.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue includes a deterministic local kernel OOB read and potential information disclosure, so it should not be auto-closed even though no LPE or write primitive is shown.

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

Patch: fbdev: fbcon: fix out-of-bounds read in err_out of fbcon_do_set_font() [ Upstream
Commit: b5bb2c696e140c399cb874def2feedf61dee27d6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b5bb2c696e140c399cb874def2feedf61dee27d6

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=b5bb2c696e140c399cb874def2feedf61dee27d6

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

The original Linux kernel CVE announcement for CVE-2026-53402 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53402

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:L/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 4.7

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: 3
  Paranoid ActionableScore: 4

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 REGULAR

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).