CVE-2026-63851 Overview
CVE-2026-63851 is a Linux kernel vulnerability in the AMD GPU (amdgpu) driver, specifically in the Video Core Next (VCN) v4.0.5 encoder ring implementation. The VCN encoder and decoder rings do not support 64-bit user fence writes, yet the driver previously accepted command submissions (CS) containing user fences. The fix rejects CS submissions with user fences on affected rings by setting the no_user_fence flag. A local, authenticated attacker with the ability to submit GPU commands can trigger memory corruption or undefined behavior in kernel context.
Critical Impact
Local users with GPU submission access can trigger kernel-level memory integrity issues, leading to potential privilege escalation, data corruption, or system denial of service on hosts with affected AMD VCN v4.0.5 hardware.
Affected Products
- Linux kernel builds shipping the drm/amdgpu/vcn driver for VCN v4.0.5 hardware
- Systems using AMD GPUs with VCN v4.0.5 video encoder/decoder blocks
- Distributions relying on the affected upstream kernel commits prior to backport
Discovery Timeline
- 2026-07-19 - CVE-2026-63851 published to NVD
- 2026-07-20 - Last updated in NVD database
Technical Details for CVE-2026-63851
Vulnerability Analysis
The vulnerability resides in the amdgpu Direct Rendering Manager (DRM) driver's handling of the VCN v4.0.5 encoder ring. AMD GPU command submissions can optionally include a user fence, a 64-bit value written by the GPU to a user-provided address to signal completion. The VCN encoder and decoder rings on v4.0.5 hardware lack support for 64-bit user fence writes. Before the fix, the driver accepted amdgpu_cs_ioctl submissions containing user fences targeting these rings, resulting in improper ring programming and undefined write behavior in kernel-adjacent memory regions.
The attack requires local access with permissions to open /dev/dri/renderD* or /dev/dri/card* and issue GPU command submissions, which is typically granted to any logged-in user on a graphical desktop. Successful exploitation impacts confidentiality, integrity, and availability at the kernel boundary.
Root Cause
The root cause is missing input validation in the ring capability descriptor. The VCN v4.0.5 encoder ring initialization failed to set the no_user_fence flag, so the command submission ioctl path did not reject requests that attached user fences. The upstream patches, referenced by commits 589a254, 6d9a98c, 7509103, and 7f23b5c, explicitly mark these rings as unsupported for user fence writes and cause the CS ioctl to return an error for such submissions.
Attack Vector
An attacker with local access crafts an amdgpu_cs_ioctl command submission targeting the VCN v4.0.5 encoder or decoder ring and populates the user fence structure with an attacker-controlled address and value. Because the kernel does not reject the submission, the resulting GPU-side write path operates on an unsupported ring, producing incorrect memory writes. No user interaction is required beyond issuing the ioctl from an unprivileged process on the affected system.
No public proof-of-concept exploit is available at the time of publication, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. The exploitation code path involves direct DRM ioctl interaction with the amdgpu driver; see the referenced kernel commits for the exact function-level changes.
Detection Methods for CVE-2026-63851
Indicators of Compromise
- Unexpected amdgpu kernel log entries referencing VCN ring faults, GPU resets, or invalid fence writes on VCN v4.0.5 hardware
- Kernel oops or general protection faults originating from amdgpu_vcn_* symbols in stack traces
- Repeated amdgpu_cs_ioctl submissions from unprivileged processes targeting VCN rings with user fence flags set
Detection Strategies
- Audit installed kernel package versions against the fixed commit hashes (589a254, 6d9a98c, 7509103, 7f23b5c) using uname -r and distribution changelogs
- Monitor dmesg and journalctl -k for amdgpu errors correlated with user-space video encode/decode workloads outside expected media applications
- Correlate DRM ioctl telemetry from eBPF-based sensors to identify anomalous VCN command submissions from non-media processes
Monitoring Recommendations
- Enable kernel audit rules on /dev/dri/renderD* and /dev/dri/card* access, alerting on unusual process lineage
- Track GPU reset counters exposed through sysfs under /sys/class/drm/card*/device/ for unexplained increments
- Forward kernel ring-buffer logs to a centralized SIEM for retention and cross-host anomaly analysis
How to Mitigate CVE-2026-63851
Immediate Actions Required
- Upgrade to a Linux kernel build that includes the upstream fix commits 589a254bf3e88204c8402b9cbccd5e23a0af990f, 6d9a98c5ed65ba92a09e4ca5a5f6941448145529, 75091030f07b7957cc0646cd52e2d9d15f611483, or 7f23b5c420b9f68a210c29c5123bace670aa8cc9
- Inventory hosts equipped with AMD GPUs that expose VCN v4.0.5 hardware to identify prioritized patch targets
- Restrict interactive local access on multi-tenant systems until patched kernels are deployed
Patch Information
The fix is applied in the upstream Linux kernel drm/amdgpu/vcn driver, which sets no_user_fence on the VCN v4.0.5 encoder ring and rejects command submissions that include user fences. Refer to Kernel Git Commit 589a254, Kernel Git Commit 6d9a98c, Kernel Git Commit 7509103, and Kernel Git Commit 7f23b5c for the exact source changes. Apply the vendor-backported kernel package from your distribution as soon as it becomes available.
Workarounds
- Limit membership in the video and render groups to trusted users on shared systems to reduce the attacker pool with GPU submission access
- Unload the amdgpu module on servers that do not require GPU acceleration by blacklisting it in /etc/modprobe.d/ until a patched kernel is installed
- Deploy Linux Security Module policies (SELinux, AppArmor) to constrain which processes can open DRM render nodes
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
