CVE-2026-31541: Linux Kernel Use-After-Free Vulnerability

CVE-2026-31541 Overview

CVE-2026-31541 is a Use-After-Free vulnerability in the Linux kernel's tracing subsystem, specifically affecting the trace_marker copy link list handling mechanism. The vulnerability exists in how the kernel manages RCU-protected link lists when the copy_trace_marker option is enabled for tracing instances.

When the copy_trace_marker option is enabled, data written to /sys/kernel/tracing/trace_marker is copied into the associated instance's buffer. The instance's trace_array descriptor is added to a marker_copies link list protected by RCU (Read-Copy-Update). The flaw occurs during instance deletion when flags are cleared in an incorrect order, leading to a race condition where synchronize_rcu() is never called, leaving stale pointers that can be dereferenced after the memory is freed.

Critical Impact
Local attackers with low privileges can exploit this Use-After-Free condition to potentially achieve arbitrary code execution with kernel privileges, leading to complete system compromise.

Affected Products

Linux Kernel versions prior to patched releases
Linux Kernel 7.0-rc1 through 7.0-rc4
Systems with tracing subsystem enabled and copy_trace_marker option in use

Discovery Timeline

2026-04-24 - CVE CVE-2026-31541 published to NVD
2026-04-28 - Last updated in NVD database

Technical Details for CVE-2026-31541

Vulnerability Analysis

The vulnerability stems from a race condition in the RCU synchronization logic within the Linux kernel's tracing subsystem. When a tracing instance is deleted, the kernel clears all enabled flags, including the copy_trace_marker flag. Subsequently, update_marker_trace() is called to handle the flag state change and remove the trace_array descriptor from the RCU-protected marker_copies link list.

The critical flaw lies in the ordering of operations: because the flag is cleared before update_marker_trace() is called, the function sees no state change (the flag is already cleared) and returns false. This prevents the necessary synchronize_rcu() call from executing, which would normally ensure all RCU readers have completed their read-side critical sections before the memory is freed.

Without proper RCU synchronization, other CPU cores may still hold references to the freed trace_array descriptor through the link list traversal, creating a classic Use-After-Free condition.

Root Cause

The root cause is an incorrect ordering of operations during instance cleanup in the tracing subsystem. The fix involves moving the clearing of all flags below the update_marker_trace() call, ensuring the function detects the actual state change and triggers proper RCU synchronization. Additionally, the fix modifies update_marker_trace() to check the flag state directly rather than relying on whether the list is empty.

Attack Vector

This vulnerability requires local access to the system with at least low-level privileges. An attacker would need the ability to interact with the tracing subsystem, typically through the /sys/kernel/tracing/ interface. The attack exploits the race condition by triggering instance deletion while RCU readers are still traversing the marker_copies link list.

The exploitation scenario involves:

Creating a tracing instance with the copy_trace_marker option enabled
Timing the deletion of the instance while another process is actively using trace_marker
Triggering access to the freed trace_array descriptor through the stale link list entry
Leveraging the Use-After-Free to corrupt kernel memory or achieve code execution

Detection Methods for CVE-2026-31541

Indicators of Compromise

Kernel panics or crashes with stack traces referencing trace_marker, update_marker_trace(), or RCU-related functions
Unexpected memory corruption errors in kernel logs related to the tracing subsystem
Suspicious activity involving rapid creation and deletion of tracing instances
KASAN (Kernel Address Sanitizer) reports indicating Use-After-Free in tracing code paths

Detection Strategies

Enable KASAN in kernel builds to detect Use-After-Free violations at runtime
Monitor system logs for kernel oops or panics with tracing subsystem references
Implement auditd rules to track access to /sys/kernel/tracing/ directories
Deploy endpoint detection solutions capable of identifying kernel-level memory corruption attacks

Monitoring Recommendations

Configure alerts for unexpected kernel crashes on production systems
Monitor for unusual patterns of tracing instance creation and deletion
Track privilege escalation attempts following tracing subsystem interactions
Implement file integrity monitoring for kernel modules related to tracing

How to Mitigate CVE-2026-31541

Immediate Actions Required

Apply the latest kernel security patches from your Linux distribution
Restrict access to /sys/kernel/tracing/ to only trusted users and processes
Consider disabling the tracing subsystem on production systems where it is not required
Monitor affected systems for signs of exploitation until patches are applied

Patch Information

The Linux kernel maintainers have released patches that correct the flag clearing order and improve the state checking logic in update_marker_trace(). The fix ensures synchronize_rcu() is properly called before memory is freed. Patches are available through the following commits:

Workarounds

Disable the copy_trace_marker option on all tracing instances if not explicitly required
Restrict access to the tracing interface using file permissions: chmod 700 /sys/kernel/tracing
Use SELinux or AppArmor policies to limit which processes can access the tracing subsystem
Consider compiling the kernel without CONFIG_TRACING if tracing functionality is not needed

bash

# Restrict tracing interface access
chmod 700 /sys/kernel/tracing
chown root:root /sys/kernel/tracing

# Disable copy_trace_marker on existing instances
echo 0 > /sys/kernel/tracing/instances/*/options/copy_trace_marker