[CVE-2026-63851][LOW] drm/amdgpu/vcn: set no_user_fence for VCN v4.0.5 enc ring [ Upstream

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

CVE: CVE-2026-63851
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: drm/amdgpu/vcn: set no_user_fence for VCN v4.0.5 enc ring [ Upstream
Commit: 75091030f07b7957cc0646cd52e2d9d15f611483
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=75091030f07b7957cc0646cd52e2d9d15f611483
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63851
Analysis date: Sun, 19 Jul 2026 11:32:23 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
The amdgpu VCN v4.0.5 encoder ring accepted user fence command submissions despite lacking 64 bit user fence support, allowing a local DRM client to potentially trigger GPU ring failure or reset, with no concrete evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-63851 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63851

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-63851	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 amdgpu VCN v4.0.5 encoder ring could accept command submissions with user fences even though the VCN encoder and decoder rings do not support 64 bit user fence writes. A local process with access to the DRM render interface may be able to submit such a command stream and trigger an unsupported fence operation on the GPU ring. For the CVSS the PR:L is used because reliable triggering requires local code execution with access to the amdgpu render or video device, but does not normally require full administrator privileges. The issue is not network reachable and is triggered through the local GPU command submission path. Impact is primarily denial of service, such as GPU ring failure, GPU reset, or degraded graphics and compute availability. No concrete kernel memory corruption or privilege escalation primitive is visible from this patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-63851 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
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

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

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process with access to the DRM render or video device may submit an amdgpu command stream using a user fence.
* No memory corruption signal: +0. The patch only sets `.no_user_fence = true` and rejects unsupported CS submissions. It does not show UAF, OOB access, arbitrary write, refcount misuse, stale callback, or type confusion.
* No LPE signal: +0. There is no concrete privilege escalation primitive.
* No high-control GPU API bonus: +0. Although DRM ioctls are rich device APIs, this patch does not show attacker control over memory lifetime, callbacks, DMA mappings, refcounts, or page ownership.
* Hardware specific exposure: effectively limits practical exposure. This affects VCN v4.0.5 amdgpu hardware and a specific encoder ring capability mismatch.

Paranoid additional signal:

* Availability impact realistic: +1. An unsupported user fence on the VCN ring could plausibly cause GPU ring failure, GPU reset, or degraded graphics or media availability, but not a demonstrated host-wide kernel compromise.

## 3. Reachability analysis

The bug is locally reachable through amdgpu command submission by a process that can access the relevant DRM render or video node. In many desktop or GPU compute deployments this maps to a local user in the render or video group, so PR:L is the practical CVSS interpretation rather than PR:H. It is not network reachable. Containers may matter if the GPU device node is passed through to an untrusted container, but without device access the bug is not reachable. The affected path is hardware and generation specific, limited to VCN v4.0.5 behavior.

## 4. Severity interpretation

This behaves like a Low or ordinary Moderate availability issue, not an Important-class vulnerability. The realistic impact is local GPU DoS or device reset. Theoretical memory corruption or privilege escalation is not supported by the patch or commit text. The fix is a validation and capability gating change that prevents unsupported user-fence submissions on a ring that cannot perform 64 bit user fence writes.

## 5. One-sentence report phrase

The amdgpu VCN v4.0.5 encoder ring accepted user fence command submissions despite lacking 64 bit user fence support, allowing a local DRM client to potentially trigger GPU ring failure or reset, with no concrete evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a validation-only GPU command submission hardening fix with plausible local availability impact only. No concrete memory corruption, security-boundary bypass, sensitive data exposure, or LPE primitive is visible.

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

Patch: drm/amdgpu/vcn: set no_user_fence for VCN v4.0.5 enc ring [ Upstream
Commit: 75091030f07b7957cc0646cd52e2d9d15f611483
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=75091030f07b7957cc0646cd52e2d9d15f611483

Changed files:
  drivers/gpu/drm/amd/amdgpu/vcn_v4_0_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=75091030f07b7957cc0646cd52e2d9d15f611483

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

The original Linux kernel CVE announcement for CVE-2026-63851 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63851

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: 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).