[CVE-2026-63850][MODERATE REGULAR] drm/amdgpu/vcn: set no_user_fence for VCN v5.0.0 enc ring [ Upstream

[CVE-2026-63850][MODERATE REGULAR] drm/amdgpu/vcn: set no_user_fence for VCN v5.0.0 enc ring [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63850
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: drm/amdgpu/vcn: set no_user_fence for VCN v5.0.0 enc ring [ Upstream
Commit: 9f5a1459ecc3195282be617639d710b54779c9dc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9f5a1459ecc3195282be617639d710b54779c9dc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63850
Analysis date: Sun, 19 Jul 2026 11:32:35 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended, not Strong Important
Manual review required: YES

Summary:
A local user with AMDGPU render-node access may submit VCN v5 command streams with unsupported user fences, causing GPU synchronization failure or GPU hang, with impact limited to denial of service and no demonstrated memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-63850 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63850

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.

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

CVE-2026-63850	MODERATE	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-754;CWE-693;**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 'AMDGPU VCN v5 encoder and decoder rings do not support 64 bit user fence writes, but command submissions with user fences could still be accepted on the affected ring. A local user with access to the DRM render device could submit a VCN command stream with a user fence and trigger unsupported fence handling, which can break synchronization or cause a GPU hang and driver recovery. For the CVSS the PR:L is used because reliable triggering requires a local process able to submit AMDGPU command streams through the render node, but this access is often available to non root desktop or compute users. The issue is not network reachable. Impact is primarily denial of service affecting GPU media workloads or possibly GPU availability, with no concrete evidence of kernel memory corruption or privilege escalation from this patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended, not Strong Important (with actual score 3)  SKIP CVE-2026-63850 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	SIMPLEFIX HARDWARE  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: **2**
* Paranoid score: **4**
* Final recommended bucket: **Borderline / manual review recommended, not Strong Important**::

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged trigger: +1**
  A local process with access to the AMDGPU DRM render node can submit command streams. This is commonly available to non-root desktop or compute users.

* **Reliable device-level DoS concern: +1**
  Unsupported 64-bit user fence handling on a VCN ring can plausibly break synchronization, hang the GPU ring, or force GPU recovery.

Not counted conservatively:

* **No generic memory corruption: +0**
  The patch only sets `.no_user_fence = true` to reject unsupported CS submissions. It does not show UAF, OOB write, double free, refcount misuse, type confusion, arbitrary write, or page ownership violation.

* **No privilege escalation plausible: +0**
  There is no visible attacker-controlled memory corruption primitive or kernel object lifetime issue.

* **No remote/network trigger: +0**
  This is a local DRM command submission path, not a network-reachable path.

Paranoid additional signals:

* **Availability impact realistic: +1**
  A GPU hang or reset may affect all GPU clients on the system, especially graphics, media, or compute workloads.

* **Broad local exposure on affected systems: +1**
  AMDGPU render nodes are commonly exposed to local users on desktop and GPU compute systems, although this specific issue depends on VCN v5.0.0 hardware.

## 3. Reachability analysis

A local user or process with access to `/dev/dri/renderD*` can potentially trigger the affected path by submitting AMDGPU command streams that use user fences on the VCN v5 encoder ring. This does not normally require full root privileges, but it does require access to the GPU render device, usually controlled by device permissions such as the `render` or `video` group.

Containers may matter if the host passes through the AMDGPU render node into the container. In that case, a container user with device access may be able to reach the same ioctl path, but this is a device exposure decision rather than a generic namespace bypass.

The affected path is not network reachable. It is also hardware dependent: systems without AMDGPU VCN v5.0.0 support are not affected.

## 4. Severity interpretation

This behaves more like a **local device DoS / Actionable Moderate borderline case**, not an Important-class vulnerability.

The realistic impact is GPU ring failure, GPU reset, media workload disruption, or local GPU availability loss. The patch context does not support confidentiality impact, integrity impact, kernel memory corruption, or privilege escalation.

The paranoid score is higher only because local render-node access is commonly available and GPU hangs may affect other users or workloads on the same host.

## 5. One-sentence report phrase

A local user with AMDGPU render-node access may submit VCN v5 command streams with unsupported user fences, causing GPU synchronization failure or GPU hang, with impact limited to denial of service and no demonstrated memory corruption or privilege escalation.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

Manual review is recommended because the failure mode is hardware and driver-path dependent, and practical impact depends on whether unsupported user fence handling causes a recoverable CS rejection, GPU ring hang, full GPU reset, or broader workload disruption.

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

Patch: drm/amdgpu/vcn: set no_user_fence for VCN v5.0.0 enc ring [ Upstream
Commit: 9f5a1459ecc3195282be617639d710b54779c9dc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9f5a1459ecc3195282be617639d710b54779c9dc

Changed files:
  drivers/gpu/drm/amd/amdgpu/vcn_v5_0_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=9f5a1459ecc3195282be617639d710b54779c9dc

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

The original Linux kernel CVE announcement for CVE-2026-63850 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63850

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: 2
  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).