[CVE-2026-63856][MODERATE REGULAR] drm/amdgpu/vcn: set no_user_fence for VCN v2.0 enc/dec rings [ Upstream

[CVE-2026-63856][MODERATE REGULAR] drm/amdgpu/vcn: set no_user_fence for VCN v2.0 enc/dec rings [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63856
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: drm/amdgpu/vcn: set no_user_fence for VCN v2.0 enc/dec rings [ Upstream
Commit: f264019be80de79f84f464846451445923bffea0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f264019be80de79f84f464846451445923bffea0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63856
Analysis date: Sun, 19 Jul 2026 11:32:18 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline Moderate. Manual review recommended but not Strong Important candidate
Manual review required: YES

Summary:
AMDGPU VCN v2.0 encoder and decoder rings accepted unsupported user fence submissions from local DRM clients, which could cause command failure, GPU ring hang, or GPU reset, resulting primarily in local denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-63856 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63856

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-2026-63856	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-20;CWE-754;CWE-693;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 v2.0 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 a command stream and trigger incorrect fence handling, which may lead to command failure, GPU ring hang, GPU reset, or local denial of service. For the CVSS the PR:L is selected because access to DRM render or card devices is often granted to non root local users in desktop and compute deployments. The issue is not network reachable and requires local device access. Impact is primarily availability loss. No direct UAF, OOB write, arbitrary kernel write, or information leak primitive is visible from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate. Manual review recommended but not Strong Important candidate (with actual score 4)  SKIP CVE-2026-63856 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=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline Moderate. Manual review recommended but not Strong Important candidate**

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. DRM render or card device access is commonly available to non-root local users through render, video, or similar groups.
* Availability impact realistic: +1. Unsupported user fence submission on VCN encoder or decoder rings can plausibly cause command failure, GPU ring hang, GPU reset, or local graphics or compute DoS.
* Broad/common subsystem exposure: +1. AMDGPU DRM is a common desktop and compute GPU driver path on affected AMD systems.

Paranoid additional signal:

* Reliable strong DoS concern: +1. If unsupported 64-bit user fence writes reliably wedge the VCN ring or force GPU reset, this becomes a stronger local availability issue.

Not awarded:

* No generic memory corruption. The patch only sets no_user_fence and does not show UAF, OOB write, arbitrary write, refcount misuse, type confusion, or stale pointer use.
* No privilege escalation signal. There is no concrete LPE primitive in the patch context.
* No confidentiality or integrity impact. The visible impact is rejection of unsupported command submissions, not data disclosure or attacker-controlled kernel memory modification.
* No network reachability. This is a local DRM command submission path.

## 3. Reachability analysis

A local process with access to the AMDGPU DRM device can potentially submit command streams to the affected VCN v2.0 encoder or decoder rings. In typical desktop or GPU compute deployments, this may not require root because DRM render nodes are often exposed to ordinary users or service accounts.

Containers can matter if the host passes through DRM render devices into the container. In that case, a containerized non-root or reduced-privilege workload may still reach the affected ioctl path. The issue is not network-triggerable by itself and requires affected AMDGPU hardware with VCN v2.0 support.

## 4. Severity interpretation

This behaves like a local availability issue rather than an Important-class memory corruption vulnerability. The patch is a validation hardening change that rejects unsupported user fence submissions on rings that cannot perform 64-bit user fence writes.

Theoretical device-state corruption is possible in the sense of invalid GPU command handling, but the patch does not support arbitrary kernel memory corruption, privilege escalation, or data disclosure. Conservative handling is ordinary to borderline Moderate. Paranoid handling reaches Actionable Moderate only if hardware-specific testing confirms reliable GPU hang or reset from unprivileged DRM access.

## 5. One-sentence report phrase

AMDGPU VCN v2.0 encoder and decoder rings accepted unsupported user fence submissions from local DRM clients, which could cause command failure, GPU ring hang, or GPU reset, resulting primarily in local denial of service.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED. YES REQUIRES MANUAL CHECK.

Manual review is recommended because non-root DRM access is common and the real severity depends on hardware-specific behavior, especially whether unsupported user fences cause only command rejection or a reliable GPU-wide hang/reset.

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

Patch: drm/amdgpu/vcn: set no_user_fence for VCN v2.0 enc/dec rings [ Upstream
Commit: f264019be80de79f84f464846451445923bffea0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f264019be80de79f84f464846451445923bffea0

Changed files:
  drivers/gpu/drm/amd/amdgpu/vcn_v2_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=f264019be80de79f84f464846451445923bffea0

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

The original Linux kernel CVE announcement for CVE-2026-63856 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63856

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: 3
  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).