[CVE-2026-43191][LOW] drm/amd/display: Adjust PHY FSM transition to TX_EN-to-PLL_ON for TMDS on DCN35 [ Upstream

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

CVE: CVE-2026-43191
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: drm/amd/display: Adjust PHY FSM transition to TX_EN-to-PLL_ON for TMDS on DCN35 [ Upstream
Commit: d1f7ceb00e8956ff6d183b7b45ef4e73c96f4c51
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d1f7ceb00e8956ff6d183b7b45ef4e73c96f4c51
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43191
Analysis date: Sat, 01 Aug 2026 13:07:04 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like. Autoclose candidate
Manual review required: NO

Summary:
AMD DCN35 TMDS link disable handling can leave the OTG stuck during PHY PLL transition, causing DCHVM invalidation ACK hangs and possible IOMMU watchdog timeout, with local PR:L reachability on affected display hardware and DoS-only impact.

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

The original Linux kernel CVE announcement for CVE-2026-43191 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43191

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-43191	LOW	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-362;CWE-667;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'AMD DCN35 display link disable handling can leave the OTG stuck when TMDS output is disabled and the PHY PLL is turned off during the transition. The stuck OTG can prevent DCHVM from ACKing invalidations when HUBP state and global sync are inconsistent, which may lead to an IOMMU watchdog timeout and a system hang. For the CVSS the PR:L is used for the paranoid score because a local user with access to DRM display control may be able to trigger display mode changes or output disable operations without full administrator privileges in desktop or seat managed environments. The issue is not network reachable and requires affected AMD DCN35 display hardware and a TMDS related path. Impact is denial of service through display or system hang. No evidence from the code change indicates information disclosure, arbitrary write, UAF, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like. Autoclose candidate (with actual score 2) 	MAYBE	DANGER HARDWARE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like. Autoclose candidate**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local desktop or seat-managed user may be able to trigger display mode changes, output disable, hotplug-like paths, or TMDS link transitions without full host-root privileges.

* Reliable kernel crash / strong DoS: +1
  The commit describes an OTG stuck condition that can lead to DCHVM invalidation ACK hang and IOMMU watchdog timeout.

* Availability impact realistic: +1 in paranoid score only
  The impact may be system-visible hang rather than a harmless display glitch, but this overlaps with the strong DoS signal, so it is not counted in the conservative score.

Subtracted signals:

* Physical-only or rare hardware-only condition: -2
  The issue is specific to AMD DCN35 display hardware and TMDS output handling.

* Hard or unreliable timing / transition window: -1 in conservative score
  The commit emphasizes that the PHY transition must be atomic because preemption can prevent completion before the IOMMU watchdog timeout. This suggests timing sensitivity.

Not triggered:

* Remote reachable: +0
  No network path is involved.

* Firmware-mediated external influence: +0
  The described trigger is display link state handling, not hostile firmware input.

* Generic memory corruption: +0
  No UAF, double free, OOB write, type confusion, stale pointer reuse, or arbitrary write is shown.

* Privilege escalation plausible: +0
  No concrete LPE primitive is supported by the patch.

* Confidentiality / integrity impact: +0
  The patch supports DoS only.

## 3. Reachability analysis

The likely trigger is local interaction with DRM/display control paths that disable TMDS output or change link state on affected AMD DCN35 systems. In typical desktop environments, a logged-in local user may have enough display-control authority through DRM master or compositor-mediated paths, so PR:L is more practical than PR:H for triage. Containers generally do not matter unless the container has access to the DRM device node. The issue is not default-reachable over the network and requires specific AMD display hardware plus a TMDS-related output path.

Call-site confidence: medium. The patch directly replaces the DCN35 disable_link_output hook and shows the relevant transition logic, but the full set of user-triggerable DRM call paths is not included.

## 4. Severity interpretation

This behaves like ordinary Moderate or Low-like hardware-specific DoS, not an Important-class kernel vulnerability. The realistic impact is display or system hang via IOMMU watchdog timeout. Theoretical escalation is not supported because the patch only changes PHY state sequencing and locking around the transition, with no evidence of memory lifetime corruption, attacker-controlled overwrite, page-cache corruption, or security-boundary bypass.

## 5. One-sentence report phrase

AMD DCN35 TMDS link disable handling can leave the OTG stuck during PHY PLL transition, causing DCHVM invalidation ACK hangs and possible IOMMU watchdog timeout, with local PR:L reachability on affected display hardware and DoS-only impact.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a hardware-specific local DoS with no demonstrated memory corruption, no confidentiality or integrity impact, no network reachability, and no plausible privilege-escalation primitive from the patch context.

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

Patch: drm/amd/display: Adjust PHY FSM transition to TX_EN-to-PLL_ON for TMDS on DCN35 [ Upstream
Commit: d1f7ceb00e8956ff6d183b7b45ef4e73c96f4c51
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d1f7ceb00e8956ff6d183b7b45ef4e73c96f4c51

Changed files:
  drivers/gpu/drm/amd/display/dc/hwss/dcn35/dcn35_hwseq.c
  drivers/gpu/drm/amd/display/dc/hwss/dcn35/dcn35_hwseq.h
  drivers/gpu/drm/amd/display/dc/hwss/dcn35/dcn35_init.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=d1f7ceb00e8956ff6d183b7b45ef4e73c96f4c51

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

The original Linux kernel CVE announcement for CVE-2026-43191 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-43191

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:L/UI:N/S:U/C:N/I:N/A:H
  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: 1
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).