CVE-2025-71140: Linux Kernel Privilege Escalation Flaw

CVE-2025-71140 Overview

CVE-2025-71140 is a null pointer dereference vulnerability in the Linux kernel's MediaTek video codec (vcodec) driver. The vulnerability exists in the media subsystem's handling of encoder and decoder context lists, where improper synchronization using a mutex in hard IRQ context can lead to invalid context pointers and subsequent null pointer dereference in the IPI (Inter-Processor Interrupt) handler.

The issue was initially discovered in ChromeOS kernels and is reproducible on mainline Linux using Fluster with FFmpeg v4l2m2m decoders. Even when the actual capture format is not supported, the vulnerable code paths are still triggered, making this a broader concern for systems running MediaTek MT8173-based devices with video encoding/decoding capabilities.

Critical Impact
Local attackers could potentially trigger a denial of service condition by causing a kernel panic through the null pointer dereference, disrupting system availability on affected MediaTek-based devices.

Affected Products

Linux Kernel with MediaTek vcodec driver
Systems using MediaTek MT8173 SoC
ChromeOS devices with affected kernel versions

Discovery Timeline

2026-01-14 - CVE CVE-2025-71140 published to NVD
2026-01-14 - Last updated in NVD database

Technical Details for CVE-2025-71140

Vulnerability Analysis

The vulnerability stems from a concurrency control issue in the MediaTek vcodec driver's handling of context list operations. The driver uses a mutex to protect encoder and decoder context lists from unexpected changes originating from the SCP (System Control Processor) IP block. However, on the MT8173 platform, the VPU (Video Processing Unit) IPI handler is invoked from hard IRQ context, where mutex operations are prohibited.

When the IPI handler attempts to acquire the mutex in hard IRQ context, it triggers scheduler warnings and can lead to race conditions where the context pointer becomes invalid. This results in a null pointer dereference when the handler attempts to access the invalidated context, potentially causing a kernel panic and system crash.

The fix involves replacing the mutex with a spinlock, which is appropriate for hard IRQ context and sufficient for protecting the fast operations performed on the context list.

Root Cause

The root cause is the use of an inappropriate synchronization primitive (mutex) in a code path that can execute in hard IRQ context. Mutexes in the Linux kernel can sleep, which is forbidden in interrupt context. The VPU IPI handler on MT8173 devices runs in hard IRQ context, making the mutex usage incompatible with the execution environment. When context list operations occur simultaneously from different execution contexts, the race condition can cause the context pointer to become invalid, leading to a null pointer dereference.

Attack Vector

The vulnerability is locally exploitable on systems with MediaTek MT8173 hardware. An attacker with local access could potentially trigger the vulnerable code path by initiating video encoding or decoding operations that interact with the VPU IPI handler. The attack requires the ability to execute code that uses the v4l2m2m video codec interface, which typically requires appropriate device permissions.

The exploitation scenario involves triggering concurrent access to the encoder/decoder context lists while the IPI handler is active, causing the mutex/scheduler conflict that leads to the null pointer dereference. While this is primarily a denial of service vector, the kernel crash could potentially be leveraged in more sophisticated attack chains.

Detection Methods for CVE-2025-71140

Indicators of Compromise

Kernel panic logs showing null pointer dereference in the MediaTek vcodec IPI handler functions
Scheduler warnings in kernel logs related to mutex operations in IRQ context
System crashes or hangs during video encoding/decoding operations on MT8173 devices
Kernel oops messages referencing mtk_vcodec or VPU IPI handler code paths

Detection Strategies

Monitor kernel logs for scheduler warnings indicating mutex usage in hard IRQ context
Implement kernel crash dump analysis to identify null pointer dereferences in the vcodec subsystem
Use ftrace or eBPF probes to monitor the MediaTek vcodec driver's context list operations
Deploy kernel live patching detection to identify unpatched systems

Monitoring Recommendations

Enable kernel panic logging and crash dump collection on affected MediaTek-based systems
Monitor for unusual video codec driver behavior or repeated driver reloads
Track system stability metrics on devices using MT8173 hardware with video capabilities
Implement alerting for kernel oops messages related to media subsystem components

How to Mitigate CVE-2025-71140

Immediate Actions Required

Update the Linux kernel to a patched version containing the spinlock fix
On systems where immediate patching is not possible, consider disabling video codec functionality if not required
Monitor affected systems for signs of exploitation or stability issues
Review system exposure and prioritize patching for publicly accessible or critical systems

Patch Information

Multiple patches have been released to address this vulnerability across different kernel stable branches. The fix converts the mutex to a spinlock for protecting the encoder and decoder context lists, which is appropriate for the hard IRQ context in which the VPU IPI handler operates.

Patches are available at the following kernel.org commits:

Workarounds

Disable the MediaTek vcodec driver module (mtk_vcodec_dec and mtk_vcodec_enc) if video codec functionality is not required
Restrict access to video device nodes (/dev/video*) to trusted users only
Consider using software-based video encoding/decoding as a temporary alternative
Implement system monitoring to detect and respond to kernel crashes promptly

bash

# Disable MediaTek vcodec modules temporarily
sudo modprobe -r mtk_vcodec_dec
sudo modprobe -r mtk_vcodec_enc

# Blacklist modules to prevent automatic loading
echo "blacklist mtk_vcodec_dec" | sudo tee /etc/modprobe.d/disable-mtk-vcodec.conf
echo "blacklist mtk_vcodec_enc" | sudo tee -a /etc/modprobe.d/disable-mtk-vcodec.conf