CVE-2026-63849: Linux Kernel VCN User Fence Vulnerability

CVE-2026-63849 Overview

CVE-2026-63849 is a vulnerability in the Linux kernel's drm/amdgpu/vcn driver affecting the Video Core Next (VCN) v5.0.1 encoder ring. The VCN encoder and decoder rings do not support 64-bit user fence writes, but the driver previously accepted command submissions (CS) that included user fences. The fix rejects CS submissions with user fences by setting no_user_fence on the VCN v5.0.1 encoder ring. A local user with access to the graphics device can trigger the flaw through crafted command submissions, potentially impacting confidentiality, integrity, and availability of the affected system.

Critical Impact
Local attackers with access to the amdgpu device interface can submit malformed command streams that trigger kernel-level memory corruption on systems using AMD VCN v5.0.1 hardware.

Affected Products

Linux kernel branches containing the drm/amdgpu/vcn v5.0.1 encoder ring support prior to the fix
Systems using AMD GPUs with VCN v5.0.1 hardware blocks
Distribution kernels that have not backported the upstream fix

Discovery Timeline

2026-07-19 - CVE-2026-63849 published to NVD
2026-07-20 - Last updated in NVD database

Technical Details for CVE-2026-63849

Vulnerability Analysis

The issue resides in the AMD GPU Direct Rendering Manager (DRM) driver, specifically in the Video Core Next (VCN) v5.0.1 encoder ring implementation. The VCN block provides hardware-accelerated video encoding and decoding. User fences are 64-bit memory writes issued by the GPU on command completion to notify user space, allowing synchronization without a kernel round-trip.

The VCN v5.0.1 encoder and decoder rings do not support 64-bit user fence writes. Despite this hardware limitation, the driver accepted command submissions that requested user fences on these rings. When such a submission executed, the ring processed the user fence request against a ring that lacked support, producing undefined behavior at the driver and hardware interface.

Root Cause

The root cause is missing input validation on the ring capability flag. The no_user_fence field on the VCN v5.0.1 encoder ring was not set, so the command submission path did not reject user fence requests targeting an unsupported ring. This is a kernel driver validation flaw that a local, low-privileged user can reach through the standard DRM ioctl interface.

Attack Vector

An authenticated local user with permission to open the amdgpu render node crafts a command submission ioctl that specifies a user fence and directs it at the VCN v5.0.1 encoder ring. Because the driver did not reject the submission, the malformed request reached the ring, resulting in high impact to confidentiality, integrity, and availability as reflected in the CVSS vector. The attack requires no user interaction and operates at low complexity.

No public proof-of-concept code is available for this issue. Refer to the upstream kernel commits 081ef0e46c9c, 5a4bffd67e94, and 8f4954722eab for the technical fix.

Detection Methods for CVE-2026-63849

Indicators of Compromise

Unexpected amdgpu driver errors or GPU resets logged in dmesg referencing VCN rings or user fence handling
Local processes issuing repeated DRM_IOCTL_AMDGPU_CS calls that target VCN encoder rings with user fence flags set
System instability or kernel oops entries mentioning amdgpu_vcn on hosts with VCN v5.0.1 hardware

Detection Strategies

Audit kernel version and confirm whether the amdgpu commits fixing the VCN v5.0.1 no_user_fence flag have been applied
Monitor kernel logs for GPU hang, ring timeout, or fence-related warnings originating from the amdgpu subsystem
Correlate unusual GPU workload patterns from non-privileged users with kernel warnings on affected hardware

Monitoring Recommendations

Collect and centralize dmesg and journald kernel logs from Linux workstations and servers with AMD GPUs
Alert on repeated GPU resets, amdgpu_ring timeouts, or DRM scheduler errors on the same host
Track which users and processes issue command submissions to the amdgpu device to identify anomalous access

How to Mitigate CVE-2026-63849

Immediate Actions Required

Inventory Linux hosts using AMD GPUs with VCN v5.0.1 hardware blocks
Apply distribution kernel updates that include the upstream fix setting no_user_fence for the VCN v5.0.1 encoder ring
Restrict access to the amdgpu render node (/dev/dri/renderD*) to trusted users where feasible

Patch Information

The vulnerability is resolved in the Linux kernel by cherry-picking commit e16be95a2c3ee712b142cb27d2dca0b461181359, which sets no_user_fence on the VCN v5.0.1 encoder ring so that command submissions containing user fences are rejected. The fix has been merged into stable branches through commits 081ef0e46c9c, 5a4bffd67e94, and 8f4954722eab. Consult your Linux distribution's security advisories to confirm which packaged kernel builds contain the backport.

Workarounds

Limit local, interactive access on multi-user Linux systems with affected AMD GPUs until a patched kernel is deployed
Tighten permissions and group membership on /dev/dri/renderD* nodes to reduce the attack surface
Disable hardware-accelerated video encode workloads on VCN v5.0.1 hardware until patched kernels are installed where operationally acceptable

bash

# Verify running kernel and check for the amdgpu VCN fix
uname -r
grep -R "no_user_fence" /usr/src/linux-headers-$(uname -r)/include/drm/ 2>/dev/null

# Restrict render node access to a specific group
sudo chown root:render /dev/dri/renderD128
sudo chmod 660 /dev/dri/renderD128