CVE-2026-63855 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 v2.5 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 efc9dd5590894109bce9a0bfe1fa5592dd6b20b1)
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability in question affects the Linux kernel's amdgpu driver specifically within the Video Core Next (VCN) subsystem version 2.5. This issue relates to the handling of command submission rings used for video encoding and decoding operations on AMD graphics hardware. The technical flaw stems from a misconfiguration in how user fences are processed within these specialized hardware rings that do not natively support 64-bit user fence writes. When userspace applications attempt to submit command streams containing user fences to VCN v2.5 encoder and decoder rings, the system fails to properly validate or handle these fence operations, leading to potential system instability or denial of service conditions.
The operational impact of this vulnerability manifests when legitimate graphics workloads attempt to utilize the VCN hardware acceleration capabilities for video processing tasks. The kernel's DRM (Direct Rendering Manager) subsystem rejects command submissions that contain user fences for these specific rings, effectively blocking video encoding and decoding operations that depend on proper fence management. This behavior creates a compatibility issue between userspace applications and the kernel driver, particularly affecting multimedia applications, video streaming services, and graphics-intensive software that relies on hardware-accelerated video processing.
From a security perspective, this vulnerability represents a denial of service condition rather than an exploitable code execution flaw, but it still presents significant operational risks in production environments where video processing capabilities are critical. The fix implemented addresses the core issue by setting the no_user_fence flag for VCN v2.5 rings, which prevents the acceptance of user fence operations that would otherwise cause command submission failures. This approach aligns with the common security principle of least privilege and proper resource validation as outlined in CWE-1048. The solution follows established kernel development practices for handling hardware-specific limitations and ensures that command submissions are properly validated against the actual capabilities of underlying hardware components.
The mitigation strategy involves updating to a kernel version containing the cherry-picked commit efc9dd5590894109bce9a0bfe1fa5592dd6b20b1, which properly configures the VCN rings to reject unsupported user fence operations rather than attempting to process them. System administrators should verify that their graphics workloads are compatible with this change and test applications that utilize hardware video acceleration to ensure continued functionality. This fix demonstrates the importance of proper hardware abstraction layer implementation in kernel drivers as recommended by ATT&CK framework considerations for operating system security, particularly in mitigating improper handling of system resources and command validation errors. Organizations maintaining graphics-intensive workloads should prioritize this update to prevent potential service disruptions in video processing applications that depend on AMD VCN hardware acceleration capabilities.