[CVE-2026-74449][MODERATE REGULAR] drm/amd/display: Fix divide-by-zero in calculate_mcache_setting on zero viewport

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

CVE: CVE-2026-74449
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: drm/amd/display: Fix divide-by-zero in calculate_mcache_setting on zero viewport
Commit: 1f93537881fd22712732a60ad12572a3fd40c0ec
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1f93537881fd22712732a60ad12572a3fd40c0ec
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74449
Analysis date: Sat, 15 Aug 2026 15:17:03 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline Moderate / Manual review recommended
Manual review required: YES

Summary:
A local DRM/KMS plane or mode configuration on affected AMD display hardware can reach calculate_mcache_setting with a zero-area viewport, causing divide-by-zero kernel panic and denial of service, with no supported evidence of confidentiality impact, integrity impact, or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-74449 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74449

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-2026-74449	MODERATE	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;CWE-191;BEST CVSS score: '5.5';DESCR 'A divide-by-zero panic in the AMD display DML2 path can occur when calculate_mcache_setting processes a zero area viewport and leaves num_mcaches or mvmpg width or height equal to zero. A local user who can submit display mode or plane configuration through DRM/KMS may be able to trigger the invalid mode calculation and crash the kernel. For the CVSS the PR:L is used because triggering requires local access to the graphics control interface or an equivalent display management context, but not full administrative privileges in common desktop or seat based deployments. The issue is not network reachable and the impact is denial of service via kernel panic. No confidentiality or integrity impact is assigned because the primitive is divide-by-zero and integer underflow handling, not UAF, OOB access, or arbitrary write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate / Manual review recommended (with actual score 4)  SKIP CVE-2026-74449 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB DANGER UAF LOG HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline Moderate / Manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local user or local graphics session process that can submit DRM/KMS plane or mode configuration may reach the affected AMD display validation path.

* Reliable kernel crash / strong DoS: +1
  The commit explicitly describes a divide-by-zero panic from zero num_mcaches or zero mvmpg width/height.

* Availability impact realistic: +1
  The failure mode is a kernel panic, not only a rejected userspace request or warning.

Paranoid additional signal:

* Broad/default/common subsystem or broadly deployed exposure: +1
  AMDGPU display support is common on AMD desktop and workstation systems, although the exact DML2/DCN4 path is hardware and configuration dependent.

Signals not awarded:

* No generic memory corruption bonus. The patch shows divide-by-zero and possible integer underflow, but no UAF, OOB write, object lifetime bug, arbitrary write, or type confusion.
* No privilege escalation bonus. There is no supported LPE primitive beyond crashing the kernel.
* No confidentiality or integrity impact. The described primitive is DoS only.
* No remote/network signal. This is a local display control path, not packet or remote protocol parsing.

## 3. Reachability analysis

The likely trigger is local access to DRM/KMS mode setting or plane configuration on affected AMD display hardware. In many desktop deployments, direct modesetting is mediated by the compositor or DRM master, but local seat users or display management services may still exercise this path without full host root privileges. Containers usually do not get DRM/KMS control by default, but GPU device pass through or delegated /dev/dri access can make the path reachable from less trusted contexts. The bug is not network reachable and depends on an affected AMD display stack using this DML2 path.

Call-site confidence: high. The patch shows both calculate_mcache_setting callers and shows that failure is now propagated as unsupported mode instead of continuing into panic-prone calculations.

## 4. Severity interpretation

This behaves like a Borderline Moderate local DoS rather than an Important-class vulnerability. The realistic impact is kernel panic from invalid display configuration. Theoretical risk from the mentioned underflow is not enough to infer memory corruption because the patch does not show a write primitive, stale pointer, object confusion, or attacker-controlled overwrite target. Manual review is still reasonable because the trigger is a local GPU/display control path and the resulting failure is host-wide.

## 5. One-sentence report phrase

A local DRM/KMS plane or mode configuration on affected AMD display hardware can reach calculate_mcache_setting with a zero-area viewport, causing divide-by-zero kernel panic and denial of service, with no supported evidence of confidentiality impact, integrity impact, or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

This should not be treated as an Important candidate based on the provided patch, but it should receive manual review because the crash is kernel-wide and may be reachable by local non-root display users or delegated GPU contexts.

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

Patch: drm/amd/display: Fix divide-by-zero in calculate_mcache_setting on zero viewport
Commit: 1f93537881fd22712732a60ad12572a3fd40c0ec
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1f93537881fd22712732a60ad12572a3fd40c0ec

Commit description:

If a plane reaches calculate_mcache_setting with a zero-area viewport,
calculate_mcache_setting exits early with num_mcaches == 0 and
mvmpg_width/height == 0. This will cause a divide-by-zero panic and can
also cause an underflow on num_mcaches.

Fix this by changing calculate_mcache_setting to bool and adding guards
after each calculate_mcache_row_bytes call. If num_mcaches or
mvmpg_width/height is zero, return a false. Callers will propagate the
failure as a rejected mode, which prevents the panic.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/5302
Reviewed-by: Sun peng (Leo) Li <sunpeng.li@amd.com>
Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Fangzhi Zuo <jerry.zuo@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 29c0f7c )
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/gpu/drm/amd/display/dc/dml2_0/dml21/src/dml2_core/dml2_core_dcn4_calcs.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=1f93537881fd22712732a60ad12572a3fd40c0ec

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

The original Linux kernel CVE announcement for CVE-2026-74449 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74449

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: 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).