CVE-2026-23248: Linux Kernel Use-After-Free Vulnerability

CVE-2026-23248 Overview

A use-after-free vulnerability has been identified in the Linux kernel's performance monitoring subsystem (perf/core). The flaw exists in the perf_mmap function where a race condition between a failing mmap() setup and a concurrent mmap() on a dependent event can lead to reference count corruption and potential memory safety violations.

The vulnerability was discovered through Syzkaller fuzzing, which reported a "refcount_t: addition on 0; use-after-free" warning in the perf_mmap function. This class of vulnerability can potentially be exploited to achieve local privilege escalation or cause denial of service conditions on affected Linux systems.

Critical Impact
Race condition in Linux kernel perf subsystem allows use-after-free through concurrent mmap operations, potentially enabling local privilege escalation or system crashes.

Affected Products

Linux kernel (versions with vulnerable perf/core implementation)
Systems using perf subsystem with output redirection or inherited events
Linux distributions running unpatched kernel versions

Discovery Timeline

March 18, 2026 - CVE-2026-23248 published to NVD
March 18, 2026 - Last updated in NVD database

Technical Details for CVE-2026-23248

Vulnerability Analysis

This vulnerability is a classic Time-of-Check Time-of-Use (TOCTOU) race condition combined with a use-after-free memory corruption issue. The root cause lies in the improper synchronization of the mmap_mutex lock during ring buffer initialization in the perf_mmap function.

During normal operation, when perf_mmap() is invoked, a ring buffer (rb) is allocated and assigned to event->rb while holding the mmap_mutex. However, the mutex is released before calling map_range() to perform the actual memory mapping operation. This creates a critical window where the ring buffer is visible to other threads but not yet fully initialized.

If map_range() fails, the cleanup path via perf_mmap_close() is triggered. During this cleanup, the reference count on the ring buffer is decremented. However, because the mutex was released, a concurrent thread performing an mmap() operation (via inherited events or output redirection) can observe the valid event->rb pointer and attempt to increment its reference count. If the cleanup path has already dropped the reference count to zero and freed the memory, this results in either a use-after-free condition or a refcount saturation warning.

Root Cause

The vulnerability stems from insufficient mutex scope in the perf_mmap function. The mmap_mutex is released prematurely—before the map_range() call completes—creating a race window where concurrent threads can access a partially initialized or dying ring buffer. This violates the atomicity requirements for ring buffer lifecycle management.

The fix extends the scope of mmap_mutex to cover the entire map_range() operation, ensuring that ring buffer initialization and mapping (or cleanup on failure) occurs atomically, preventing other threads from accessing a half-initialized or deallocated ring buffer.

Attack Vector

An attacker with local access to the system could exploit this vulnerability through the following mechanism:

Create a perf event and initiate an mmap() operation on it
Trigger a failure condition in the map_range() function
Concurrently attach to the same event via inherited events or output redirection from another thread
Race the cleanup path to access the freed ring buffer, potentially gaining control over kernel memory

The exploitation scenario involves racing the cleanup routine during ring buffer initialization. The vulnerability is triggered through the perf event interface, specifically targeting the memory mapping functionality when events use output redirection or inheritance mechanisms.

Detection Methods for CVE-2026-23248

Indicators of Compromise

Kernel log messages containing "refcount_t: addition on 0" warnings related to perf subsystem
Unexpected system crashes or kernel panics originating from perf_mmap or perf_mmap_close functions
Anomalous perf event creation patterns with rapid concurrent mmap operations

Detection Strategies

Monitor kernel logs for refcount warnings and use-after-free indicators in the perf subsystem
Deploy kernel crash dump analysis to identify exploitation attempts targeting perf_mmap
Implement syscall monitoring for unusual patterns of perf_event_open and mmap combinations
Use kernel runtime integrity checking tools to detect memory corruption

Monitoring Recommendations

Enable CONFIG_REFCOUNT_FULL for enhanced refcount validation in development/testing environments
Configure kernel crash reporting to capture detailed information on perf-related failures
Monitor for processes repeatedly triggering perf subsystem errors as potential exploitation attempts
Review audit logs for users with CAP_PERFMON or CAP_SYS_ADMIN capabilities making unusual perf calls

How to Mitigate CVE-2026-23248

Immediate Actions Required

Update to a patched Linux kernel version as soon as available from your distribution
Restrict access to the perf subsystem by setting kernel.perf_event_paranoid to 2 or higher
Review and limit CAP_PERFMON and CAP_SYS_ADMIN capabilities to essential users only
Monitor systems for signs of exploitation while awaiting patch deployment

Patch Information

The Linux kernel maintainers have released patches to address this vulnerability by extending the mmap_mutex scope to cover the map_range() call. This ensures atomic handling of ring buffer initialization and cleanup.

Patches are available through the following kernel git commits:

Organizations should apply these patches through their Linux distribution's package management system or by building a patched kernel from source.

Workarounds

Increase kernel.perf_event_paranoid sysctl value to restrict perf access to privileged users
Disable unprivileged access to perf events by setting kernel.perf_event_paranoid=3 where supported
Use SELinux or AppArmor policies to restrict access to the perf subsystem
Consider disabling perf entirely on systems where performance monitoring is not required

bash

# Configuration example
# Restrict perf access to root only
echo 2 > /proc/sys/kernel/perf_event_paranoid

# Make the setting persistent across reboots
echo "kernel.perf_event_paranoid = 2" >> /etc/sysctl.conf
sysctl -p

# Verify the current setting
sysctl kernel.perf_event_paranoid