[CVE-2026-63855][LOW] drm/amdgpu/vcn: set no_user_fence for VCN v2.5 enc/dec rings [ Upstream

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

CVE: CVE-2026-63855
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: drm/amdgpu/vcn: set no_user_fence for VCN v2.5 enc/dec rings [ Upstream
Commit: 2c6fb056567efb49f8674108b86088a1cfaa86d0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2c6fb056567efb49f8674108b86088a1cfaa86d0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63855
Analysis date: Sun, 19 Jul 2026 11:32:21 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
AMDGPU VCN v2.5 encoder and decoder rings incorrectly accepted command submissions with user fences despite not supporting 64 bit user fence writes, allowing a local DRM user to cause GPU command failure, hang, or reset without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-63855 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63855

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-2026-63855	LOW	CHECK WITH IMPACT FROM ORIG NN LOW	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-20;CWE-754;CWE-755;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'AMDGPU VCN v2.5 encoder and decoder rings could accept command submissions with user fences even though these rings do not support 64 bit user fence writes. A local process with access to the AMDGPU DRM device could submit such work and trigger incorrect fence handling, failed command completion, GPU hang, or device reset. For the CVSS the PR:L is used because triggering requires local access to the GPU device interface rather than remote network access. The issue is not network reachable and is limited to the local graphics or compute stack. Impact is primarily denial of service against GPU availability, with no clear evidence of kernel memory corruption, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-63855 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	SIMPLEFIX HARDWARE  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process with access to the AMDGPU DRM device can submit command streams. This is usually PR:L rather than PR:H because render or video group access is commonly delegated to non-root users.

* Availability impact realistic: +1
  Unsupported user fence submissions on VCN v2.5 encoder or decoder rings can plausibly cause failed command completion, GPU hang, or device reset.

Not triggered:

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

* Firmware-mediated external influence: +0
  The trigger is a local DRM command submission, not firmware input.

* Generic memory corruption: +0
  The patch only rejects unsupported user fence CS submissions by setting no_user_fence. It does not show UAF, OOB write, double free, type confusion, arbitrary write, or refcount misuse.

* Privilege escalation plausible: +0
  There is no demonstrated LPE primitive.

* Reliable kernel crash or strong host DoS: +0
  The likely impact is GPU availability degradation or reset, not a demonstrated kernel panic or host-wide crash.

* Rare hardware condition: 0
  This is hardware specific to AMDGPU VCN v2.5, but it is reachable through a normal local GPU interface, so I would not apply a strong rarity penalty.

## 3. Reachability analysis

The bug is reachable by a local process that can access the AMDGPU DRM device and submit command streams to VCN v2.5 encoder or decoder rings. In typical desktop or compute deployments, this may be available to users in render or video related groups, so PR:L is the practical CVSS interpretation. Containers only matter if the GPU device node is passed through to the container. The path is not network reachable and requires affected AMDGPU hardware plus VCN v2.5 support.

## 4. Severity interpretation

This behaves like ordinary Moderate or Low-like local device DoS rather than an Important kernel vulnerability. The patch is a validation or capability-declaration fix: VCN v2.5 rings do not support 64 bit user fence writes, so CS submissions with user fences must be rejected. There is no concrete memory corruption primitive, no evidence of attacker-controlled overwrite, no information disclosure, and no privilege escalation path. A paranoid interpretation can raise availability to high for repeated GPU hangs or resets, but that still does not make it an Important-class issue.

## 5. One-sentence report phrase

AMDGPU VCN v2.5 encoder and decoder rings incorrectly accepted command submissions with user fences despite not supporting 64 bit user fence writes, allowing a local DRM user to cause GPU command failure, hang, or reset without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed or handled as a low-priority local GPU DoS unless the affected product treats GPU availability as security critical or a vendor advisory later documents a concrete memory corruption or privilege escalation primitive.

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

Patch: drm/amdgpu/vcn: set no_user_fence for VCN v2.5 enc/dec rings [ Upstream
Commit: 2c6fb056567efb49f8674108b86088a1cfaa86d0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2c6fb056567efb49f8674108b86088a1cfaa86d0

Changed files:
  drivers/gpu/drm/amd/amdgpu/vcn_v2_5.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=2c6fb056567efb49f8674108b86088a1cfaa86d0

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

The original Linux kernel CVE announcement for CVE-2026-63855 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63855

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:N/I:N/A:L
  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: 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).