CVE-2026-63856 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 v2.0 enc/dec rings
VCN encoder and decoder rings do not support 64-bit user fence writes, reject CS submissions with user fences.
(cherry picked from commit e2b5499fca55f1a32960a311bbb62e35891eaf73)
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability in question affects the Linux kernel's amdgpu driver implementation, specifically within the Video Core Next generation (VCN) subsystem version 2.0. This issue pertains to the handling of command submissions for video encoding and decoding operations on AMD graphics hardware. The problem stems from a fundamental mismatch between the hardware capabilities of VCN v2.0 rings and the software interface expectations, creating a potential denial-of-service condition that could compromise system stability.
The technical flaw manifests when the kernel attempts to submit command streams containing user fences to VCN v2.0 encoder and decoder rings. These particular hardware rings lack support for 64-bit user fence writes, which represents a critical architectural limitation in the video processing unit design. When user-space applications submit command submissions that include user fences, the driver fails to properly validate this compatibility before attempting the operation, resulting in rejection of such submissions. This behavior directly violates the expected interface contract between kernel drivers and hardware components, creating an inconsistent state where legitimate operations are rejected due to hardware constraints rather than software errors.
From an operational impact perspective, this vulnerability creates a denial-of-service scenario for applications relying on AMD graphics hardware for video processing tasks. The rejection of command submissions with user fences effectively prevents video encoding and decoding operations from completing successfully, potentially causing media playback failures, streaming interruptions, or application crashes. According to CWE classification, this represents a weakness in the interface between software components where insufficient validation leads to incompatible data handling. The issue particularly affects multimedia applications, video conferencing systems, and any software utilizing the AMD GPU's VCN v2.0 for hardware-accelerated video processing.
The mitigation strategy implemented in the fix involves setting the no_user_fence flag for VCN v2.0 enc/dec rings, which explicitly informs the driver subsystem that these particular rings cannot handle user fence operations. This approach aligns with the ATT&CK framework's concept of privilege escalation and system compromise through interface manipulation, as it prevents unauthorized or improperly validated operations from being submitted to incompatible hardware components. The cherry-picked commit e2b5499fca55f1a32960a311bbb62e35891eaf73 represents a targeted fix that addresses the specific hardware limitation while maintaining compatibility with other VCN versions that do support user fences. This solution follows industry best practices for handling hardware-specific constraints in kernel drivers, ensuring that system stability is maintained through proper interface validation rather than attempting to force incompatible operations. The fix demonstrates adherence to secure coding principles by properly identifying and handling hardware limitations at the driver level, preventing potential exploitation scenarios where malicious input could trigger system instability or resource exhaustion through malformed command submissions.