CVE-2026-23221: Linux Kernel Use-After-Free Vulnerability

CVE-2026-23221 Overview

A use-after-free vulnerability has been identified in the Linux kernel's fsl-mc (Freescale Management Complex) bus driver. The driver_override_show() function reads the driver_override string without holding the device_lock, while the corresponding driver_override_store() function uses driver_set_override() to modify and free the string while holding the lock. This race condition can result in concurrent use-after-free if the string is freed by the store function while being read by the show function.

Critical Impact

A local attacker may be able to exploit this race condition to cause memory corruption, potentially leading to information disclosure, denial of service, or in some scenarios, privilege escalation on affected Linux systems.

Affected Products

• Linux kernel (versions with fsl-mc bus driver)
• Systems using Freescale/NXP QorIQ DPAA2 architecture
• Embedded Linux systems with fsl-mc subsystem enabled

Discovery Timeline

• 2026-02-18 - CVE CVE-2026-23221 published to NVD
• 2026-02-19 - Last updated in NVD database

Technical Details for CVE-2026-23221

Vulnerability Analysis

This vulnerability represents a classic Time-of-Check Time-of-Use (TOCTOU) race condition in kernel memory management. The fsl-mc bus driver exposes a sysfs interface allowing userspace to read and write the driver_override attribute. The core issue lies in the asymmetric locking strategy between the read and write operations.

The driver_override_show() function, responsible for displaying the current driver override value to userspace via sysfs, accesses the driver_override string pointer without first acquiring the device_lock mutex. Meanwhile, the driver_override_store() function properly holds the device_lock when calling driver_set_override(), which may free the existing string and allocate a new one. This creates a window where one thread reading the string may access memory that has been simultaneously freed by another thread performing a write operation.

Exploitation of this vulnerability requires local access to the system and the ability to interact with the sysfs interface. The impact includes potential kernel memory corruption, which could lead to system instability, information leakage from kernel memory, or in sophisticated attack scenarios, arbitrary code execution in kernel context.

Root Cause

The root cause is improper synchronization in the driver_override_show() function within the fsl-mc bus driver. The function fails to acquire the device_lock before reading the driver_override string pointer, creating a race window with concurrent write operations that properly hold the lock while modifying and freeing the same memory.

Attack Vector

The attack requires local access to a Linux system with the fsl-mc bus driver loaded. An attacker would need to create a race condition by:

1. Initiating a read operation on the driver_override sysfs attribute via /sys/bus/fsl-mc/devices/*/driver_override
2. Simultaneously triggering a write operation to the same attribute
3. Timing the operations to cause the read to access freed memory

The vulnerability affects the sysfs interface exposed by the fsl-mc subsystem. An attacker with sufficient privileges to access the device's sysfs entries could trigger the race condition to potentially corrupt kernel memory or leak sensitive information.

Detection Methods for CVE-2026-23221

Indicators of Compromise

• Kernel panic or oops messages referencing driver_override_show in the fsl-mc subsystem
• Unexpected system crashes on systems with fsl-mc hardware
• Memory corruption warnings in kernel logs related to slab allocator
• Suspicious concurrent access patterns to fsl-mc sysfs entries in audit logs

Detection Strategies

• Monitor kernel logs for use-after-free warnings using KASAN (Kernel Address Sanitizer) if enabled
• Implement audit rules for access to /sys/bus/fsl-mc/devices/*/driver_override paths
• Deploy kernel live patching solutions to detect and block exploitation attempts
• Use SentinelOne's Linux agent for real-time kernel exploit detection

Monitoring Recommendations

• Enable kernel debugging options such as CONFIG_DEBUG_SLAB and CONFIG_KASAN in development environments
• Configure syslog monitoring for kernel oops messages containing fsl-mc references
• Implement access control monitoring for sysfs interactions on production systems
• Review system logs for unusual patterns of concurrent sysfs access operations

How to Mitigate CVE-2026-23221

Immediate Actions Required

• Apply the latest kernel patches from the official Linux kernel repositories
• If patching is not immediately possible, restrict access to fsl-mc sysfs entries using filesystem permissions
• Monitor affected systems for signs of exploitation attempts
• Consider temporarily disabling the fsl-mc driver if not required for system operation

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability. The fix involves holding the device_lock around the read operation in driver_override_show() to ensure proper synchronization with concurrent write operations.

Patches are available in multiple kernel stable branches:

• Kernel Commit 1d6bd6183e72
• Kernel Commit a2ae33e1c636
• Kernel Commit b1983840287
• Kernel Commit c424e72cfa67
• Kernel Commit c71dfb7833db
• Kernel Commit dd8ba8c0c3f3

Workarounds

• Restrict access to /sys/bus/fsl-mc/devices/*/driver_override using filesystem permissions to limit exposure
• Implement SELinux or AppArmor policies to control access to fsl-mc sysfs entries
• Disable the fsl-mc driver module if the hardware is not in use on the affected system
• Use kernel lockdown mode where supported to limit userspace kernel interactions

# Configuration example
# Restrict access to fsl-mc driver_override sysfs entries
chmod 600 /sys/bus/fsl-mc/devices/*/driver_override

# Blacklist fsl-mc module if not required
echo "blacklist fsl-mc-bus" >> /etc/modprobe.d/blacklist-fsl-mc.conf

# Verify current module status
lsmod | grep fsl_mc