[CVE-2026-80578][MODERATE REGULAR] fbdev: core: Fix pointer desynchronization in fb_io_read()

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

CVE: CVE-2026-80578
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fbdev: core: Fix pointer desynchronization in fb_io_read()
Commit: 42bc07b4e5a3c8a02a433388f562a8f46d093e11
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=42bc07b4e5a3c8a02a433388f562a8f46d093e11
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80578
Analysis date: Wed, 26 Aug 2026 11:35:31 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended / not Strong Important by default
Manual review required: YES

Summary:
A local fbdev reader can trigger pointer desynchronization after partial copy_to_user in fb_io_read, causing out-of-bounds framebuffer I/O reads with possible limited information disclosure or device-specific denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-80578 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80578

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-80578	MODERATE	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:L/I:N/A:H';CWE-125;*CWE-200;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '5.3';DESCR 'fb_io_read can desynchronize the hardware src pointer from the remaining count after copy_to_user performs a partial copy into a faulty user buffer. The loop has already advanced src by the original chunk size, while count is reduced only by the bytes actually copied, so a later iteration may read past the originally bounded framebuffer I/O range. For the CVSS the PR:L is used because a local process must be able to read the fbdev device, commonly through /dev/fb access or a video-like device permission, but full administrator privileges are not inherently required. The issue is not network reachable. Impact is primarily local denial of service or device specific fault from out-of-bounds I/O reads, with possible limited confidentiality impact if data read past the intended framebuffer range can be returned to user space. No direct write primitive or privilege escalation primitive is shown by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended / not Strong Important by default (with actual score 4)	YES	WRITE OOB SIMPLEFIX HARDWARE LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  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 / not Strong Important by default**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local process with read access to `/dev/fb*` can potentially trigger the path by using a user buffer that causes partial `copy_to_user()`.

* Confidentiality impact plausible: +1
  The bug is an out-of-bounds I/O read. If data read beyond the intended framebuffer range is later copied back to userspace, limited information disclosure is plausible.

* Availability impact realistic: +1
  Out-of-bounds reads from framebuffer I/O or MMIO-like regions can cause device-specific faults, hangs, or kernel errors, though the reliability depends on the framebuffer driver and hardware mapping.

Paranoid additional signal:

* Reliable kernel crash / strong DoS: +1
  Counted only in the paranoid score because the commit explicitly says the loop can eventually perform out-of-bounds hardware reads, but it does not prove a universal host-wide crash on all systems.

Not awarded:

* No generic strong memory corruption: +0
  The primitive is read-only OOB I/O access, not UAF, double-free, OOB write, heap overwrite, arbitrary write, or type confusion.

* No privilege escalation plausible: +0
  The patch context does not show attacker-controlled overwrite, object replacement, callback control, refcount takeover, or another LPE-relevant primitive.

* No remote reachability: +0
  This is a local fbdev read path, not a network-exposed path.

* No admin/root penalty: +0
  Full root is not inherently required if the user has device access, although many deployments restrict `/dev/fb*` to root or a video-like group.

## 3. Reachability analysis

The bug is locally reachable through the fbdev read helper, typically via `/dev/fb*`. Triggering requires a process that can read the framebuffer device and can arrange a partial `copy_to_user()` result, for example by supplying a faulty or partially unmapped user buffer. This is not network reachable.

Namespaces do not automatically make this broadly reachable. Container exposure depends on whether the framebuffer device node is passed through into the container. On normal desktop, embedded, kiosk, or console systems, fbdev may be present, but device permissions often restrict access to root or a graphics/video group. The path is therefore practical only on systems where local users or service accounts have framebuffer read access.

Call-site confidence: medium. The patch shows the affected loop and the direct consequence of `src` and `count` desynchronization, but exact impact depends on the concrete fbdev driver and I/O mapping.

## 4. Severity interpretation

This behaves like a borderline Moderate issue rather than an Important-class vulnerability by default. The strongest supported primitive is an out-of-bounds I/O read after a partial `copy_to_user()`. That can plausibly cause limited information disclosure or device-specific availability impact, but the patch does not support a write primitive, UAF, object lifetime corruption, or privilege escalation path.

Theoretical risk is higher on hardware where reading past the intended framebuffer range can access sensitive adjacent device memory or cause a reliable system-wide fault. Realistic exploitation evidence is limited, so this should not be treated as Strong Important without manual confirmation of the affected hardware mappings.

## 5. One-sentence report phrase

A local fbdev reader can trigger pointer desynchronization after partial copy_to_user in fb_io_read, causing out-of-bounds framebuffer I/O reads with possible limited information disclosure or device-specific denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the bug is not a clear LPE or strong memory corruption issue, but it is an out-of-bounds I/O read in a hardware-facing path. Manual review should confirm whether affected framebuffer drivers map sensitive adjacent ranges or can fault reliably.

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

Patch: fbdev: core: Fix pointer desynchronization in fb_io_read()
Commit: 42bc07b4e5a3c8a02a433388f562a8f46d093e11
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=42bc07b4e5a3c8a02a433388f562a8f46d093e11

Commit description:

In fb_io_read(), if copy_to_user() performs a partial copy (e.g., due to
a faulty user buffer), the loop adjusts the chunk size 'c' and updates
the remaining 'count'. However, the hardware 'src' pointer has already
been eagerly advanced by the original chunk size.

If the loop is allowed to continue, the read will resume from an
incorrect, over-advanced offset. Since the remaining 'count' was only
decremented by the successful bytes, this desynchronization causes the
next iterations to execute more hardware reads than originally bounded,
eventually leading to out-of-bounds I/O reads.

Fix this by breaking out of the loop immediately upon a partial
copy_to_user(). A partial copy indicates a faulty user buffer, making
subsequent read attempts futile. Breaking out ensures we return the
number of successfully read bytes without risking out-of-bounds hardware
accesses in subsequent mismatched iterations.

Fixes: 6121cd9 ("fbdev: Move I/O read and write code into helper functions")
Cc: stable@vger.kernel.org
Signed-off-by: Mingyu Wang <25181214217@stu.xidian.edu.cn>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/video/fbdev/core/fb_io_fops.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=42bc07b4e5a3c8a02a433388f562a8f46d093e11

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

The original Linux kernel CVE announcement for CVE-2026-80578 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80578

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:H/PR:L/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 5.3

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).