CVE-2026-63853 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 v4.0 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 fd852c048b46f9825e904a4f3f4538fe9d8827d9)
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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 Core Next generation hardware acceleration subsystem. This issue impacts the VCN v4.0 encoder and decoder rings which are responsible for handling video encoding and decoding operations through hardware acceleration. The flaw stems from improper handling of user fence operations within the command submission process, creating a potential security risk that could be exploited by malicious actors.
The technical root cause involves the VCN v4.0 hardware rings not supporting 64-bit user fence writes as part of their command submission interface. When userspace applications attempt to submit command streams containing user fences, the kernel driver fails to properly validate or reject these submissions due to a missing configuration parameter. This creates an inconsistency between the hardware capabilities and the software handling, where the driver should explicitly disable user fence functionality for these specific rings but fails to do so. The vulnerability manifests as a denial of service condition or potential privilege escalation opportunity when malformed command submissions attempt to utilize unsupported fence mechanisms.
From an operational perspective, this vulnerability affects systems running Linux kernels with AMDGPU drivers that support VCN v4.0 hardware acceleration, particularly those implementing video encoding and decoding workloads. Attackers could potentially exploit this by crafting malicious command submissions that include user fences, causing the kernel to either crash or behave unpredictably during command processing. The impact extends beyond simple service disruption to potentially enabling privilege escalation scenarios where attackers might leverage the improper fence handling to gain elevated system privileges.
The mitigation strategy involves applying the patch that explicitly sets no_user_fence for VCN v4.0 encoder and decoder rings, effectively disabling user fence support for these hardware components. This aligns with security best practices for hardware abstraction layers where driver implementations must properly enforce hardware capabilities and limitations to prevent unexpected behavior. The fix follows established patterns in kernel security development where unsupported features are explicitly disabled rather than attempting to handle them through error recovery mechanisms. This approach prevents potential exploitation while maintaining full functionality for supported operations.
This vulnerability maps to CWE-122 Heap Buffer Overflow and CWE-674 Uncontrolled Resource Consumption, as the improper fence handling could lead to resource exhaustion or memory corruption scenarios within kernel space. The ATT&CK framework categorizes this under T1059 Command and Scripting Interpreter and T1068 Exploitation for Privilege Escalation, as attackers could leverage the hardware abstraction layer weakness to execute malicious code with elevated privileges. The fix demonstrates proper defensive programming practices by explicitly configuring hardware capabilities rather than relying on implicit behavior or incomplete validation logic that could be bypassed through crafted inputs.