CVE-2026-63850 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
drm/amdgpu/vcn: set no_user_fence for VCN v5.0.0 enc ring
VCN encoder and decoder rings do not support 64-bit user fence writes, reject CS submissions with user fences.
(cherry picked from commit 49b1fbbb5a071197ee71e2d70959b1cb29bdc317)
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability in question relates to the AMDGPU driver implementation within the Linux kernel, specifically affecting the Video Codec Unit v5.0.0 hardware components. This issue impacts the VCN encoder and decoder rings which are responsible for handling video encoding and decoding operations in AMD graphics processing units. The core problem manifests when the system attempts to submit command submissions containing user fences to these specific hardware rings that lack native support for 64-bit user fence writes. This architectural limitation creates a fundamental incompatibility between the software command submission mechanism and the hardware capabilities of the VCN v5.0.0 implementation.
The technical flaw stems from the absence of proper validation and handling within the drm/amdgpu/vcn subsystem where the driver fails to correctly identify that certain VCN rings do not support user fence operations. When command submissions containing user fences are processed, the system does not appropriately reject these requests before they reach the hardware level, leading to potential system instability or denial of service conditions. The vulnerability is particularly significant because it affects the core video processing capabilities of AMD GPUs and represents a clear mismatch between software expectations and hardware limitations.
Operationally, this vulnerability could result in system crashes, application failures, or complete denial of service for video processing applications that rely on the affected VCN rings. The impact extends beyond simple functionality loss as it affects the reliability of graphics-intensive workloads including video encoding, decoding, and streaming applications. Systems utilizing AMD GPUs for professional video processing, gaming, or multimedia applications would be particularly vulnerable to disruptions caused by this issue. The vulnerability essentially prevents proper command submission to specific hardware rings, forcing applications to either fail gracefully or potentially cause system-wide instability when attempting to use these video processing capabilities.
The mitigation strategy involves implementing proper fencing validation within the driver layer to prevent command submissions containing user fences from being sent to unsupported VCN rings. This approach aligns with the established practice of maintaining hardware abstraction layers that properly handle device-specific capabilities and limitations. The fix implemented in commit 49b1fbbb5a071197ee71e2d70959b1cb29bdc317 addresses this by setting no_user_fence for the affected VCN v5.0.0 encoder and decoder rings, effectively disabling user fence support for these specific hardware components. This solution follows industry best practices for device driver development and adheres to the principle of least privilege where software components only utilize features supported by underlying hardware. The vulnerability classification aligns with CWE-691, which addresses insufficient protection of code against uncontrolled inputs, particularly in the context of device driver interfaces where proper input validation is critical for system stability and security.
The fix demonstrates proper adherence to the ATT&CK framework's concept of privilege escalation through driver manipulation by ensuring that command submission mechanisms respect hardware capabilities rather than attempting operations that could lead to system compromise. This vulnerability highlights the importance of maintaining proper abstraction layers between software interfaces and hardware implementations, particularly in graphics drivers where complex interactions between multiple subsystems can create security and stability concerns. The resolution represents a defensive programming approach that prevents potentially harmful command sequences from reaching hardware components that cannot properly handle them, thereby maintaining system integrity while preserving functional capabilities for supported operations.