CVE-2026-31486: Linux Kernel Race Condition Vulnerability

CVE-2026-31486 Overview

CVE-2026-31486 is a race condition vulnerability in the Linux kernel's hardware monitoring (hwmon) subsystem, specifically within the PMBus core driver. The regulator operations pmbus_regulator_get_voltage(), pmbus_regulator_set_voltage(), and pmbus_regulator_list_voltage() access PMBus registers and shared data without proper mutex protection, creating potential race conditions that could lead to system instability or undefined behavior.

Critical Impact

Unprotected concurrent access to PMBus registers and shared data can result in race conditions, potentially causing system instability, data corruption, or unpredictable regulator behavior on affected Linux systems.

Affected Products

• Linux kernel (versions with vulnerable pmbus/core hwmon driver)
• Systems utilizing PMBus-based hardware monitoring and voltage regulators
• Embedded and server systems with PMBus power management ICs

Discovery Timeline

• 2026-04-22 - CVE CVE-2026-31486 published to NVD
• 2026-04-23 - Last updated in NVD database

Technical Details for CVE-2026-31486

Vulnerability Analysis

This vulnerability stems from missing synchronization primitives in the PMBus regulator operations within the Linux kernel's hwmon subsystem. The affected functions access PMBus registers and shared data structures concurrently without holding the update_lock mutex, which is designed to serialize access to these resources.

The complexity of addressing this issue is notable. Simply adding mutex protection to the voltage functions introduces a deadlock scenario: pmbus_regulator_notify() calls regulator_notifier_call_chain() while the mutex is already held (from functions like pmbus_fault_handler()). If a regulator callback then invokes one of the protected voltage functions, it attempts to acquire the same mutex, resulting in deadlock.

The fix implements a worker-based notification mechanism to safely decouple the notification path from mutex-protected critical sections. Events are stored as atomics in a per-page bitmask and processed asynchronously by a worker thread.

Root Cause

The root cause is the absence of proper mutex protection for regulator operations that access shared PMBus registers and data. The update_lock mutex, intended to synchronize access to these resources, was not being acquired by pmbus_regulator_get_voltage(), pmbus_regulator_set_voltage(), and pmbus_regulator_list_voltage() functions. This allows concurrent execution paths to simultaneously read or modify shared state, leading to race conditions.

Attack Vector

The vulnerability manifests through concurrent access to PMBus regulator operations. When multiple threads or interrupt contexts attempt to read or modify voltage settings simultaneously without synchronization, the following scenarios can occur:

1. Data Corruption: Partial reads or writes to multi-byte PMBus registers during concurrent access
2. Inconsistent State: Shared data structures may be modified while another operation is reading them
3. Race Between Check and Use: Voltage values read by one operation may become stale before use due to concurrent modifications

The attack vector is local, requiring access to trigger concurrent regulator operations on the affected system. While exploitation for arbitrary code execution is unlikely, the race condition could cause system instability or incorrect voltage regulation, which may have implications for hardware reliability and safety-critical systems.

Detection Methods for CVE-2026-31486

Indicators of Compromise

• Unexpected system instability or kernel panics related to PMBus or hwmon subsystems
• Erratic voltage regulator behavior or incorrect voltage readings from PMBus devices
• Kernel log messages indicating PMBus communication errors or timeouts
• Deadlock warnings in kernel logs when mutex debugging is enabled

Detection Strategies

• Monitor kernel logs (dmesg) for PMBus-related errors, warnings, or lockup reports
• Enable kernel lock debugging (CONFIG_PROVE_LOCKING, CONFIG_DEBUG_LOCKDEP) to detect potential deadlocks
• Review system hardware monitoring data for anomalous voltage readings or fluctuations
• Audit running kernel version against patched versions from the kernel.org stable branches

Monitoring Recommendations

• Implement continuous monitoring of kernel message buffers for hwmon and regulator subsystem anomalies
• Deploy system health checks that verify PMBus device communication integrity
• Configure alerts for unexpected kernel lockup or softlockup events
• Track kernel version deployments to ensure patched versions are running across the fleet

How to Mitigate CVE-2026-31486

Immediate Actions Required

• Update to a patched Linux kernel version that includes the mutex protection and worker-based notification fix
• Review kernel configurations on systems with PMBus hardware monitoring to assess exposure
• Prioritize patching for systems where PMBus regulator stability is critical (e.g., servers, embedded systems)
• Monitor affected systems for signs of instability until patches can be applied

Patch Information

The vulnerability has been addressed in the Linux kernel stable branches. The fix reworks pmbus_regulator_notify() to use a worker function that sends notifications outside of mutex protection, storing events as atomics in a per-page bitmask processed by the worker.

The following commits contain the fix:

• Kernel Git Commit 2c77ae3
• Kernel Git Commit 4e9d723
• Kernel Git Commit 754bd2b

Workarounds

• If immediate patching is not possible, reduce concurrent access to PMBus regulator interfaces where feasible
• Limit userspace applications or services that interact with hwmon PMBus interfaces to single-threaded operation
• Consider disabling PMBus regulator functionality temporarily on non-critical systems if hardware permits
• Apply kernel live patching if available for your distribution and the patch has been backported

# Check current kernel version
uname -r

# Verify if PMBus hwmon driver is loaded
lsmod | grep pmbus

# Monitor for PMBus-related kernel messages
dmesg | grep -i pmbus