[CVE-2026-89821][MODERATE REGULAR] drm/amd/display: avoid divide-by-zero in __is_lut_linear()

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

CVE: CVE-2026-89821
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: drm/amd/display: avoid divide-by-zero in __is_lut_linear()
Commit: 2320626425f3af3a25b8afb6876f205b1c6d4e64
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2320626425f3af3a25b8afb6876f205b1c6d4e64
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89821
Analysis date: Wed, 16 Sep 2026 11:03:49 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89821 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89821

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-2026-89821	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89821 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-369;*CWE-20;CWE-754;BEST CVSS score: '5.5';DESCR 'A divide by zero in __is_lut_linear() can occur when the AMD display color path receives a LUT with fewer than two entries and then computes the expected value using size - 1 as the divisor. A local process that can access the DRM/KMS color management interface may be able to submit such invalid LUT state and trigger a kernel divide error, causing a local denial of service. For the CVSS the PR:L is used because triggering requires a local process with access to the DRM KMS interface and the ability to submit color LUT state, but not necessarily full root privileges in desktop or seat based deployments. The issue is not network reachable and there is no indication of UAF, OOB access, or an arbitrary write primitive. Impact is denial of service via kernel crash only.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)  SKIP CVE-2026-89821 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	UAF HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3. No floor or cap changed the score.
* Raw paranoid score: 3
* Paranoid score: 3. No floor or cap changed the score.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive signals:

* +1. Concrete invalid input validation flaw. A LUT size below 2 reaches arithmetic that divides by size - 1.
* +1. Reliable kernel crash / strong DoS. The commit explicitly states that a single-entry LUT causes a kernel divide error (#DE).
* +1. Below-host-admin local reachability is plausible for the DRM/KMS color-management path. A DRM master, compositor, display service, or authorized local graphics client may be able to submit color LUT state without being host root.

Non-awarded signals:

* +0. No UAF, stale pointer, double free, refcount misuse, OOB access, or arbitrary write is established.
* +0. No confidentiality or integrity impact is awarded. The primitive is divide-by-zero only.
* +0. No strong LPE or weak LPE concern is awarded. There is no memory corruption or attacker-controlled write primitive.
* +0. Remote reachability is not established. This is not packet-triggered or network-facing.
* +0. Extra availability point is not awarded because the same divide error crash is already counted once.
* +0. Broad subsystem exposure is not separately awarded. AMDGPU/DRM is common, but the exact color LUT path requires a suitable display stack and access to KMS color-management operations.

Negative signals:

* No host-admin penalty is applied because host-admin-equivalent privilege is not positively established. The relevant path is a DRM/KMS control path, not a general host-root-only administrative operation.
* No race/timing penalty is applied because the bug is deterministic once size is below 2.
* No rare-hardware penalty is applied because AMD display hardware is hardware-specific but not inherently rare or physical-only in the scoring sense.

3. Reachability analysis

The likely trigger is local access to the DRM/KMS color-management interface that can submit or update a LUT reaching the AMD display color path. The most realistic actors are a DRM master, compositor, display server, delegated graphics service, or an authorized local session component. This supports below-host-admin reachability, but not arbitrary remote reachability.

NamespaceScopedPrivilege is NO because this is not a namespace-scoped network or user-namespace capability path. No prior host setup beyond having the AMD display driver and the relevant DRM/KMS path available is established. No special race is needed. The trigger appears to be an ordered invalid-parameter case: provide a LUT with fewer than two entries, then __is_lut_linear() computes with size - 1.

4. Primitive and severity interpretation

The exact primitive is divide-by-zero in kernel code due to insufficient validation of LUT size. This is CLASS_C because the demonstrated behavior is crash/DoS from invalid arithmetic state, not kernel memory corruption.

There is no active structured stale object, no fixed stale storage, and no concrete repeated release/refcount operation. The demonstrated consequence is a kernel divide error and local denial of service. The speculative consequence does not extend meaningfully beyond DoS because the patch and supplied context do not show UAF, OOB, arbitrary write, disclosure, callback influence, or credential/security-state corruption.

ImportantCandidate is NO because there is no established Class A or meaningful Class B primitive requiring Important-vs-Moderate exploitation review. AutoDowngradeAllowed is YES because the issue is a constrained crash-only Class C bug with no unresolved corruption or security-boundary primitive.

Likely operational severity is borderline Moderate: it can crash the kernel if reachable by a local graphics-control actor, but it does not show privilege escalation or data compromise.

5. One-sentence report phrase

A divide-by-zero in the AMDGPU display color LUT validation path can let a local DRM/KMS-capable process submit a single-entry LUT and trigger a kernel divide error, causing denial of service without evidence of memory corruption or privilege escalation.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: local kernel crash through a DRM/KMS control path may be reachable by non-root desktop or display-service actors, so deployment-specific access to KMS color-management operations should be checked, but the primitive itself is DoS-only.

7. Call-site confidence

Call-site confidence: medium

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

Patch: drm/amd/display: avoid divide-by-zero in __is_lut_linear()
Commit: 2320626425f3af3a25b8afb6876f205b1c6d4e64
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2320626425f3af3a25b8afb6876f205b1c6d4e64

Commit description:

__is_lut_linear() computes the expected value of each entry with

	expected = i * MAX_DRM_LUT_VALUE / (size - 1);

If it is ever called with a single-entry LUT, size - 1 is zero and the
kernel takes a divide error (#DE). A LUT with fewer than two entries
cannot describe a linear mapping anyway, so return false early instead
of dividing by zero.

Fixes: 086247a ("drm/amd/display: Use 4096 lut entries")
Cc: stable@vger.kernel.org
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.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=2320626425f3af3a25b8afb6876f205b1c6d4e64

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

The original Linux kernel CVE announcement for CVE-2026-89821 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89821

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: Not available
  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: unknown
  Paranoid ActionableScore: 3

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).