[CVE-2026-63826][MODERATE 7.0] fbdev: fix use-after-free in store_modes()

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

CVE: CVE-2026-63826
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: fbdev: fix use-after-free in store_modes()
Commit: 5267eab88fa4c684459504b8be577ad64953b9a6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5267eab88fa4c684459504b8be577ad64953b9a6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63826
Analysis date: Sun, 19 Jul 2026 09:51:50 -0400
ActionableScore: 6
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A local use-after-free in fbdev `store_modes()` can leave framebuffer mode pointers referencing a freed modelist, allowing stale kernel heap reads or a kernel crash when fbcon or sysfs mode paths later dereference them.

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

The original Linux kernel CVE announcement for CVE-2026-63826 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63826

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-2026-63826	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-825;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free in the fbdev store_modes path can leave fb_display mode and fb_info mode pointing into an old modelist after that list is freed. A local attacker who can modify framebuffer modes and then trigger fbcon or fbdev mode handling can cause stale kernel heap reads through fb_mode_is_equal or mode_string. For the CVSS the PR:L is used for the paranoid score because some systems delegate framebuffer or console control to local users or service accounts even though the default sysfs write path is usually administrative. The issue is not network reachable and requires local interaction with fbdev or console control paths. Impact is at least local denial of service via kernel crash and may include information disclosure through stale mode data. Because this is a kernel UAF, the paranoid score keeps confidentiality and integrity impact high for manual-review sensitivity over autoclosed false negatives.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DANGER UAF HARDWARE LINUS SYZBOT KPANIC 	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Generic memory corruption: +2. The patch fixes a real stale pointer use-after-free in kernel fbdev state.
* Real lifetime corruption: +1. `fb_display[i].mode` and `fb_info->mode` can remain pointing into a freed modelist.
* Reliable kernel crash / strong DoS: +1. Dereferencing freed mode objects through `fb_mode_is_equal()` or `mode_string()` can crash or trigger KASAN/UAF reports.
* Confidentiality impact plausible: +1. `show_mode()` may format data from a freed and potentially reused heap object back to userspace.
* Requires admin/root in typical deployments: -2. Writing framebuffer sysfs mode lists and fbcon unbinding are normally administrative operations.

Paranoid adjustment:

* Local unprivileged trigger: +1. Some deployments expose framebuffer or console control to local users, video group members, service accounts, or reduced privilege root contexts.
* No strong LPE bonus. The patch shows UAF reads, but not attacker-controlled reclaim, write-after-free, callback control, refcount takeover, or type confusion.
* No integrity bonus in the main paranoid ActionableScore. Integrity impact is theoretically concerning for any kernel UAF, but this patch only demonstrates stale reads.

## 3. Reachability analysis

The bug is local only and is not network reachable. Triggering requires modifying fbdev mode lists from userspace and then reaching fbcon or fbdev mode handling paths such as `FBIOPUT_VSCREENINFO` with `FB_ACTIVATE_INV_MODE` or reading the mode sysfs attribute. In default deployments this is usually root or administrator controlled, but practical privilege may be lower on systems where framebuffer access is delegated to local users, display services, containers with device access, or video group members. The affected subsystem is legacy but still present on many console, embedded, VM, and compatibility configurations.

Call-site confidence: high. The commit message explicitly identifies stale pointers, the freeing site, and later dereference paths.

## 4. Severity interpretation

This is more than an ordinary Moderate correctness issue because it fixes a concrete kernel use-after-free. Realistic impact is most clearly local DoS and possible limited information disclosure through stale mode data. A practical privilege escalation path is not demonstrated by the patch, so it should not be treated as a strong Important candidate solely from the available evidence. The paranoid score is elevated because kernel lifetime bugs with user-triggerable stale object reads are historically under-triaged and deserve manual review when reachable by non-admin local users.

## 5. One-sentence report phrase

A local use-after-free in fbdev `store_modes()` can leave framebuffer mode pointers referencing a freed modelist, allowing stale kernel heap reads or a kernel crash when fbcon or sysfs mode paths later dereference them.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a concrete kernel UAF with identified stale dereference paths and possible local non-admin reachability in delegated framebuffer environments, even though the demonstrated primitive is read-oriented and not a proven LPE.

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

Patch: fbdev: fix use-after-free in store_modes()
Commit: 5267eab88fa4c684459504b8be577ad64953b9a6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5267eab88fa4c684459504b8be577ad64953b9a6

Commit description:

store_modes() replaces a framebuffer's modelist with modes from userspace.
On success it frees the old modelist with fb_destroy_modelist(). Two
fields still point into that freed list.

One pointer is fb_display[i].mode, the mode a console is using.
fbcon_new_modelist() moves these pointers to the new list. It only does so
for consoles still mapped to the framebuffer. An unmapped console is
skipped and keeps its stale pointer. Unbinding fbcon, for example, sets
con2fb_map[i] to -1 but leaves fb_display[i].mode set. An
FBIOPUT_VSCREENINFO ioctl with FB_ACTIVATE_INV_MODE later reaches
fbcon_mode_deleted(). That function reads the stale fb_display[i].mode
through fb_mode_is_equal(). The read is a use-after-free.

The other pointer is fb_info->mode, the current mode. It is set through
the mode sysfs attribute. store_modes() does not update fb_info->mode, so
it is left pointing into the freed list. show_mode(), the attribute's read
handler, dereferences the stale fb_info->mode through mode_string(). The
read is a use-after-free.

Clear both pointers before freeing the list. Commit a1f3058 ("fbcon:
Set fb_display[i]->mode to NULL when the mode is released") added the
helper fbcon_delete_modelist(). It clears every fb_display[i].mode that
points into a given list. So far it is called only from the unregister
path. Call it from store_modes() too, and set fb_info->mode to NULL.

Reported-by: syzbot+81c7c6b52649fd07299d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=81c7c6b52649fd07299d
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/all/ajjoDhAi2y4ArSlz@dev/
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/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=5267eab88fa4c684459504b8be577ad64953b9a6

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

The original Linux kernel CVE announcement for CVE-2026-63826 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63826

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:L/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: 3
  Paranoid ActionableScore: 6

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).