CVE-2026-68107: Linux Kernel Race Condition Vulnerability

CVE-2026-68107 Overview

CVE-2026-68107 is a Time-of-Check Time-of-Use (TOCTOU) race condition in the Linux kernel's AMD GPU driver, specifically within the drm/amdgpu/vcn4 Video Core Next (VCN) 4.0 encoder path. The vulnerability resides in how the driver processes Indirect Buffer (IB) parameter lengths during video encode command submissions. The function rereads the parameter length from the IB after it was already returned by vcn_v4_0_enc_find_ib_param(), creating a window where IB contents can change between reads. A local authenticated user with GPU access can exploit this race to affect confidentiality, integrity, and availability.

Critical Impact
A local low-privileged attacker can trigger a TOCTOU race in the amdgpu VCN4 encoder path to corrupt kernel state, leading to high impact on confidentiality, integrity, and availability with scope change.

Affected Products

Linux kernel branches shipping the drm/amdgpu VCN 4.0 driver
Distributions bundling affected upstream kernels with AMD GPU support
Systems using AMD GPUs with VCN 4.0 hardware video encode acceleration

Discovery Timeline

2026-08-10 - CVE-2026-68107 published to NVD
2026-08-13 - Last updated in NVD database

Technical Details for CVE-2026-68107

Vulnerability Analysis

The flaw exists in the AMD GPU driver's VCN 4.0 encoder IB validation logic. When the kernel validates an encode Indirect Buffer submitted from userspace, it calls vcn_v4_0_enc_find_ib_param() to locate a parameter and returns its length. The vulnerable code path then rereads the length field directly from the IB memory instead of reusing the value already returned. Because the IB resides in memory that userspace can modify concurrently, the second read can observe a different value than the first, breaking the validation invariant.

An attacker submits a crafted IB, races a second thread to modify the length field between the validate and use operations, and causes the driver to trust an already-validated length that no longer matches the buffer's actual contents. This constitutes a classic Time-of-Check Time-of-Use pattern [CWE-367] in kernel-mode code with scope change to affected components beyond the driver.

Root Cause

The root cause is redundant reading of untrusted IB memory. The parameter length is read once during discovery and again during use, but only the first read is validated. Because the IB is mapped to userspace-accessible memory, its contents can mutate between the check and the use, invalidating any assumptions the driver made about buffer geometry.

Attack Vector

Exploitation requires local access with GPU permissions, typically granted to any logged-in user through /dev/dri/renderD* device nodes. An attacker submits VCN encode commands through the standard DRM ioctl interface while a companion thread continuously rewrites the length field in shared IB memory. Winning the race causes the driver to process out-of-bounds data using a stale or attacker-controlled length, leading to kernel memory corruption or information disclosure.

No synthetic exploitation code is included here. Technical details of the fix are available in the upstream commits referenced under Kernel Git Commit.

Detection Methods for CVE-2026-68107

Indicators of Compromise

Unexpected kernel oops or WARN traces referencing vcn_v4_0_enc_find_ib_param or amdgpu in dmesg and /var/log/kern.log.
Repeated GPU reset events (amdgpu: GPU reset begin) originating from unprivileged local user processes.
Anomalous DRM ioctl volumes targeting /dev/dri/renderD* from non-graphical workloads or service accounts.

Detection Strategies

Enable kernel lockdown and KASAN in test environments to surface race-induced memory corruption during VCN encode operations.
Audit execve and open calls to DRM render nodes from processes that have no legitimate video encoding requirement.
Correlate amdgpu ring timeouts and hangs with the user context that submitted the offending command stream.

Monitoring Recommendations

Ingest auditd, dmesg, and journald GPU driver events into a centralized log pipeline for query and alerting.
Alert on kernel panics or GPU resets that repeat within short windows on the same host and user.
Track kernel package versions across the fleet to identify hosts still running unpatched amdgpu builds.

How to Mitigate CVE-2026-68107

Immediate Actions Required

Apply the upstream Linux kernel patch that reuses the length returned by vcn_v4_0_enc_find_ib_param() rather than rereading it from the IB.
Update to a distribution kernel that incorporates the fix from stable commit dbb02b4755f8c1f3773263f2d779872c1c0c073a.
Restrict access to /dev/dri/renderD* to trusted users on multi-tenant hosts until patches are deployed.

Patch Information

The fix is available in the upstream stable tree through multiple backport commits. Reference the following updates: 3b4082fabc67, bbbe6a2a8d8d, bd868c077f67, c309626bf91f, and ff6aa542d91d. Rebuild and reboot into the patched kernel to activate the fix.

Workarounds

Tighten group membership on the render and video groups to limit which local users can submit VCN encode commands.
Disable hardware-accelerated video encoding for untrusted workloads by unloading or blacklisting the amdgpu VCN encode paths where feasible.
Enforce mandatory access controls (SELinux, AppArmor) to constrain which processes may open DRM render nodes.

bash

# Verify kernel version and confirm the amdgpu VCN patch is applied
uname -r
grep -E 'vcn_v4_0_enc_find_ib_param' /proc/kallsyms

# Restrict render node access to a dedicated group
sudo chgrp render /dev/dri/renderD128
sudo chmod 0660 /dev/dri/renderD128