[CVE-2026-63853][LOW] drm/amdgpu/vcn: set no_user_fence for VCN v4.0 enc ring [ Upstream

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

CVE: CVE-2026-63853
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: drm/amdgpu/vcn: set no_user_fence for VCN v4.0 enc ring [ Upstream
Commit: 1286b6872de0aee1feeeaa6dbac86369806de9a5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1286b6872de0aee1feeeaa6dbac86369806de9a5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63853
Analysis date: Sun, 19 Jul 2026 10:55:11 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-63853 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63853

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-2026-63853	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-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'The VCN v4.0 encoder ring could accept CS submissions with user fences even though this ring does not support 64 bit user fence writes. A local process that can submit AMDGPU command streams may request an unsupported user fence path and force an invalid GPU submission or recovery condition. The most defensible impact is local denial of service, such as failed command submission, GPU reset, or loss of acceleration for affected workloads. For the CVSS the PR:L because reliable triggering requires a local process with access to the AMDGPU render or DRM command submission interface. The issue is not network reachable and there is no concrete evidence of kernel memory corruption or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-63853 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

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

ActionableScore=2

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

2. Signal breakdown
* Local unprivileged trigger: +1. A local process with access to the AMDGPU DRM/render command submission interface may submit CS requests.
* Availability impact realistic: +1. Unsupported user fence submissions may plausibly cause failed submissions, GPU recovery, GPU reset, or loss of acceleration for affected workloads.
* No memory corruption signal. The patch only sets `no_user_fence = true` and does not show UAF, OOB access, double free, stale pointer reuse, type confusion, arbitrary write, or kernel data overwrite.
* No privilege escalation signal. There is no demonstrated primitive for kernel memory control, credential modification, or security boundary bypass.
* No remote reachability. This is a local DRM/GPU command submission path, not a network protocol path.
* No high-control API bonus. GPU CS submission is a rich device API, but this patch does not show attacker influence over kernel object lifetime, allocator layout, callback targets, refcounts, DMA mappings, or page ownership.

3. Reachability analysis
A local user or process that can access AMDGPU DRM/render nodes can potentially trigger the invalid CS submission path. In many desktop or compute deployments this means membership in a graphics/render-capable access group or execution under a service account with GPU access. Containers may matter if GPU devices are passed through, but this still requires local device access rather than network reachability. The issue is hardware and driver specific to AMDGPU VCN v4.0 encoder rings, so exposure is limited to affected AMD GPU systems with that IP block and driver path enabled.

4. Severity interpretation
This behaves like an ordinary Moderate or Low-like device-specific availability issue. The realistic impact is local DoS against GPU functionality, not host privilege escalation. Theoretical concern is limited because accepting unsupported 64-bit user fence writes sounds like invalid hardware command configuration, but the patch and commit do not demonstrate kernel memory corruption, page ownership violation, stale pointer lifetime, or an attacker-controlled overwrite. The validation-only nature of the fix keeps this below Actionable Moderate.

5. One-sentence report phrase
The VCN v4.0 encoder ring could accept user fence CS submissions despite lacking 64-bit user fence support, allowing a local GPU-capable process to trigger invalid submission or recovery behavior with denial-of-service impact only.

6. Manual review recommendation
NO MANUAL CHECK NEEDED
The issue is local, device-specific, validation-only, and has no concrete memory corruption or privilege escalation primitive in the patch context.

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

Patch: drm/amdgpu/vcn: set no_user_fence for VCN v4.0 enc ring [ Upstream
Commit: 1286b6872de0aee1feeeaa6dbac86369806de9a5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1286b6872de0aee1feeeaa6dbac86369806de9a5

Changed files:
  drivers/gpu/drm/amd/amdgpu/vcn_v4_0.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=1286b6872de0aee1feeeaa6dbac86369806de9a5

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

The original Linux kernel CVE announcement for CVE-2026-63853 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63853

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).