[CVE-2026-46209][IMPORTANT] drm/gem: Fix inconsistent plane dimension calculation in drm_gem_fb_init_with_funcs()

[CVE-2026-46209][IMPORTANT] drm/gem: Fix inconsistent plane dimension calculation in drm_gem_fb_init_with_funcs()

[AL-KERNEL]

CVE: CVE-2026-46209
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: drm/gem: Fix inconsistent plane dimension calculation in drm_gem_fb_init_with_funcs()
Commit: 6b992591e04f2cce813bcf239b354f375bbf84d3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6b992591e04f2cce813bcf239b354f375bbf84d3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46209
Analysis date: Thu, 28 May 2026 09:20:51 -0400
ActionableScore: 8
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
Inconsistent subsampled plane dimension calculation in `drm_gem_fb_init_with_funcs()` can let a local DRM user create an undersized GEM framebuffer object, leading to GPU out-of-bounds read or write beyond the object bounds.

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

The original Linux kernel CVE announcement for CVE-2026-46209 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46209

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: IMPORTANT
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-2026-46209	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-125;CWE-190;CWE-131;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'drm_gem_fb_init_with_funcs used truncating integer division for subsampled plane dimensions while framebuffer_check used rounded up dimensions. For formats such as NV12 with a very small framebuffer height, the chroma plane height could become zero, the height minus one calculation could underflow, and an undersized GEM object could pass the size check. A local user with access to a DRM device or render node can create such a framebuffer and cause the GPU to read or write beyond the GEM object bounds. For the CVSS the PR:L is used because render node or DRM device access is enough in many desktop, GPU compute, or container GPU passthrough setups. The issue is not network reachable. Impact is at least local denial of service or GPU memory corruption, and worst case may allow confidentiality or integrity impact through out-of-bounds access to adjacent GPU accessible memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8) 	MAYBE	HARDWARE LINUS  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local user with access to a DRM device or render node can submit framebuffer creation parameters.
* Memory corruption, strong corruption primitive: +2. An undersized GEM object can pass validation and later be accessed out of bounds by the GPU.
* Reliable kernel/GPU DoS: +1. OOB GPU access can fault, hang, reset the GPU, or destabilize the graphics stack.
* Local high-control kernel data-plane API: +1. DRM framebuffer creation exposes user-controlled dimensions, formats, handles, and offsets.
* Broad/default/common subsystem: +1. DRM/GEM framebuffer helpers and render/display paths are widely deployed.
* Constrained target/payload: -1. The exact OOB access pattern depends on GPU hardware and driver behavior, and the patch does not show a directly controlled CPU-side arbitrary write.

Paranoid additional signals:

* Privilege escalation plausible / strong LPE concern: +2. GPU OOB read/write beyond GEM BO bounds may access adjacent GPU-visible memory and can potentially corrupt or disclose data across processes or driver objects.
* Confidentiality and integrity impact plausible: +1. Adjacent GEM/TTM memory exposure or corruption is plausible in the worst case.

## 3. Reachability analysis

The bug is locally reachable through DRM framebuffer creation using specific subsampled formats such as NV12 and small or odd dimensions. A user needs access to a DRM device or render node, which is common on desktop systems and often delegated into GPU-enabled containers.

No host administrative privilege is required once the device node is accessible. The issue is not network reachable. Practical exploitability depends on whether the affected driver and hardware actually access memory beyond the undersized GEM object in a way that is observable or controllable.

## 4. Severity interpretation

This is at least an actionable Moderate and a strong Important candidate for manual triage. The realistic evidence supports GPU OOB access and likely DoS or GPU reset. The theoretical higher impact is confidentiality or integrity compromise of adjacent GPU-accessible memory.

This is not merely an integer overflow bug. The overflow directly weakens object size validation and permits an undersized GEM object to be used for a framebuffer plane.

## 5. One-sentence report phrase

Inconsistent subsampled plane dimension calculation in `drm_gem_fb_init_with_funcs()` can let a local DRM user create an undersized GEM framebuffer object, leading to GPU out-of-bounds read or write beyond the object bounds.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

The bug enables a plausible GPU OOB access primitive from a commonly exposed local DRM interface, and impact depends on driver memory layout, GPU fault behavior, and cross-process GEM memory isolation.

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

Patch: drm/gem: Fix inconsistent plane dimension calculation in drm_gem_fb_init_with_funcs()
Commit: 6b992591e04f2cce813bcf239b354f375bbf84d3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6b992591e04f2cce813bcf239b354f375bbf84d3

Commit description:

drm_gem_fb_init_with_funcs() computes sub-sampled plane dimensions
using plain integer division:

  unsigned int width  = mode_cmd->width  / (i ? info->hsub : 1);
  unsigned int height = mode_cmd->height / (i ? info->vsub : 1);

However, the ioctl-level framebuffer_check() in drm_framebuffer.c uses
drm_format_info_plane_width/height() which round up dimensions via
DIV_ROUND_UP(). This inconsistency corrupts the subsequent GEM object
size check for certain pixel format and dimension combinations.

For example, with NV12 (vsub=2) and a 1-pixel-tall framebuffer the
GEM size validation path sees height=0 instead of height=1. The
expression (height - 1) then wraps to UINT_MAX as an unsigned int,
causing min_size to overflow and wrap back to a small value. A tiny
GEM object therefore passes the size guard, yet when the GPU accesses
the chroma plane it will read or write memory beyond the object's
bounds.

Fix by replacing the open-coded divisions with drm_format_info_plane_width()
and drm_format_info_plane_height(), which use DIV_ROUND_UP() and match
the calculation already used in framebuffer_check().

Fixes: 4c3dbb2 ("drm: Add GEM backed framebuffer library")
Cc: stable@vger.kernel.org # v4.14+
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Ashutosh Desai <ashutoshdesai993@gmail.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260420013637.457751-1-ashutoshdesai993@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/drm_gem_framebuffer_helper.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=6b992591e04f2cce813bcf239b354f375bbf84d3

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

The original Linux kernel CVE announcement for CVE-2026-46209 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46209

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:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 6
  Paranoid ActionableScore: 8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).