CVE-2026-31584: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31584 Overview

A use-after-free vulnerability has been discovered in the Linux kernel's MediaTek video codec (vcodec) encoder release path. The vulnerability exists in the fops_vcodec_release() function, which frees the context structure (ctx) without first cancelling any pending or running work in ctx->encode_work. This creates a race condition where the workqueue handler (mtk_venc_worker) may still be accessing context memory after it has been freed, leading to potential privilege escalation or system crashes.

Critical Impact
Local attackers with low privileges can exploit this race condition to achieve arbitrary code execution in kernel context, potentially gaining complete system control on affected Linux systems with MediaTek hardware video encoding capabilities.

Affected Products

Linux Kernel (multiple versions with MediaTek vcodec support)
Systems utilizing MediaTek video encoding hardware
Embedded devices and mobile platforms with MediaTek SoCs

Discovery Timeline

April 24, 2026 - CVE-2026-31584 published to NVD
April 27, 2026 - Last updated in NVD database

Technical Details for CVE-2026-31584

Vulnerability Analysis

This vulnerability represents a classic race condition leading to a use-after-free (CWE-416) in the Linux kernel's MediaTek video codec driver. The core issue lies in the synchronization gap between the V4L2 memory-to-memory (m2m) framework's job lifecycle management and the actual workqueue execution lifecycle.

When a user closes a video encoding file descriptor, the kernel invokes fops_vcodec_release() to clean up resources. This function calls v4l2_m2m_ctx_release(), which waits for any in-progress m2m jobs to complete by monitoring the TRANS_RUNNING flag. However, the m2m framework considers a job "complete" as soon as v4l2_m2m_job_finish() is called—not when the worker function actually returns.

This creates a dangerous race window where CPU 0 (running the release path) observes the job as "done" and proceeds to free the context structure with kfree(ctx), while CPU 1 (running the workqueue) is still executing the worker function and accessing ctx members such as ctx->m2m_ctx and ctx->dev.

The vulnerability was confirmed using KASAN (Kernel Address Sanitizer), which detected a slab-use-after-free when the workqueue attempted to read from the already-freed context structure.

Root Cause

The root cause is insufficient synchronization between the context release path and the encode workqueue. The v4l2_m2m_ctx_release() function only synchronizes with the m2m job lifecycle (via the TRANS_RUNNING flag), not with the actual workqueue lifecycle. Work is queued during encode operations via queue_work(ctx->dev->encode_workqueue, &ctx->encode_work), and the worker function continues to access context fields even after calling v4l2_m2m_job_finish().

Attack Vector

The vulnerability requires local access to the system. An attacker would need to be able to open a video encoding device file and trigger encoding operations while simultaneously closing the file descriptor at a precise moment. The race window can be widened through various techniques to make exploitation more reliable. Successful exploitation could allow an attacker with low privileges to execute arbitrary code in kernel context, effectively gaining root privileges.

The race condition can be visualized as follows: while the release path on CPU 0 completes v4l2_m2m_ctx_release() and calls kfree(ctx), the workqueue on CPU 1 may still be dereferencing the now-freed context pointer, leading to memory corruption or controlled execution through heap manipulation techniques.

Detection Methods for CVE-2026-31584

Indicators of Compromise

KASAN reports showing slab-use-after-free in mtk_venc_worker with task kworker/u8:* in the call stack
Kernel crash dumps or oops messages referencing fops_vcodec_release or mtk_venc_worker functions
Workqueue traces showing mtk_vcodec_enc_wq with unexpected memory access patterns

Detection Strategies

Deploy kernel builds with KASAN enabled in testing environments to detect use-after-free conditions
Monitor for kernel oops or panic events related to the MediaTek vcodec driver components
Implement runtime integrity checking on systems with MediaTek video encoding hardware

Monitoring Recommendations

Enable kernel crash dump collection and analysis for systems running MediaTek hardware
Monitor for unusual workqueue behavior or timing anomalies in mtk_vcodec_enc_wq
Implement endpoint detection for suspicious patterns around video device file operations

How to Mitigate CVE-2026-31584

Immediate Actions Required

Update the Linux kernel to a patched version containing the fix
On systems where immediate patching is not possible, consider disabling MediaTek video encoding functionality if not required
Review system access controls to limit local user access to video device files

Patch Information

The fix adds a call to cancel_work_sync(&ctx->encode_work) before kfree(ctx) in the release path. This ensures the workqueue handler is both cancelled (if pending) and synchronized (waits for any running handler to complete) before the context is freed.

Linux kernel patches are available through the following commits:

Workarounds

Restrict access to /dev/video* device files associated with MediaTek encoders using file permissions or SELinux/AppArmor policies
Unload the mtk_vcodec_enc kernel module on systems where hardware video encoding is not required
Apply access control lists to limit which users can interact with video encoding devices

bash

# Restrict access to MediaTek vcodec devices
chmod 600 /dev/video*

# Optionally unload the driver if not needed
modprobe -r mtk_vcodec_enc

# Add module blacklist to prevent automatic loading
echo "blacklist mtk_vcodec_enc" >> /etc/modprobe.d/blacklist-mtk-vcodec.conf