CVE-2026-23127 Overview
A use-after-free vulnerability has been identified in the Linux kernel's performance monitoring subsystem (perf). The flaw occurs when calling refcount_inc(&event->mmap_count) inside perf_mmap_rb(), triggering a reference count warning indicating a potential use-after-free condition. This vulnerability manifests when creating a group member event with the PERF_FLAG_FD_OUTPUT flag after the group leader has been mmap-ed.
Critical Impact
The vulnerability allows manipulation of kernel reference counts, potentially leading to use-after-free conditions that could enable privilege escalation or denial of service on affected Linux systems.
Affected Products
- Linux Kernel (versions with affected perf subsystem)
Discovery Timeline
- 2026-02-14 - CVE CVE-2026-23127 published to NVD
- 2026-02-18 - Last updated in NVD database
Technical Details for CVE-2026-23127
Vulnerability Analysis
This vulnerability resides in the Linux kernel's perf (performance events) subsystem, specifically within the perf_mmap_rb() function. The root issue involves improper reference count management when handling memory-mapped performance event buffers.
When a performance event is created with the PERF_FLAG_FD_OUTPUT flag, it copies the output event configuration through perf_event_set_output(). This causes the event->rb (ring buffer) to be set. Subsequently, when mmap is called on the event, perf_mmap_rb() attempts to increment the event->mmap_count reference counter. However, if event->mmap_count is 0 at this point, the kernel's refcount protection mechanism detects this as an addition on a zero reference count—a classic indicator of use-after-free behavior.
The vulnerability poses risks related to memory corruption and could potentially be exploited for privilege escalation or system destabilization. Two events updating the same user_page simultaneously is also prevented by the implemented fix.
Root Cause
The root cause is improper handling of the mmap_count reference counter when an event has inherited its ring buffer from another event via PERF_FLAG_FD_OUTPUT. The perf_event_set_output() function sets event->rb but does not initialize event->mmap_count, leading to a scenario where refcount_inc() is called on a zero counter.
Attack Vector
The attack vector requires local access to the system with the ability to invoke the perf_event_open syscall. An attacker can trigger this vulnerability by:
- Creating a performance event using perf_event_open syscall
- Memory-mapping the event's file descriptor
- Creating a second performance event with PERF_FLAG_FD_OUTPUT flag, referencing the first event as its group leader
- Memory-mapping the second (victim) event's file descriptor, triggering the refcount warning
The vulnerability is triggered locally through specific syscall sequences that manipulate perf event relationships and memory mappings.
Detection Methods for CVE-2026-23127
Indicators of Compromise
- Kernel warning messages containing refcount_t: addition on 0; use-after-free in system logs
- Suspicious perf_event_open syscall patterns with PERF_FLAG_FD_OUTPUT flag usage
- Unusual kernel crash dumps or oops messages related to the perf subsystem
Detection Strategies
- Monitor kernel logs (dmesg) for refcount warnings from lib/refcount.c
- Implement syscall auditing for perf_event_open with flag analysis
- Deploy kernel-level security monitoring for anomalous perf subsystem behavior
Monitoring Recommendations
- Enable comprehensive audit logging for perf-related syscalls
- Configure alerting on kernel warning messages related to refcount violations
- Review system stability logs for unexplained kernel warnings in production environments
How to Mitigate CVE-2026-23127
Immediate Actions Required
- Apply the kernel patch from the official Linux kernel git repository
- Review and restrict access to performance monitoring capabilities using perf_event_paranoid sysctl
- Monitor systems for signs of exploitation attempts targeting the perf subsystem
Patch Information
The Linux kernel maintainers have released patches to address this vulnerability. The fix disallows the case when event->mmap_count = 0, preventing two events from updating the same user_page. Patches are available through the official kernel git repository:
Workarounds
- Restrict access to the perf subsystem by setting /proc/sys/kernel/perf_event_paranoid to 3 or higher
- Limit perf capabilities to trusted users only via kernel capabilities management
- Monitor for and block suspicious syscall patterns involving PERF_FLAG_FD_OUTPUT
# Restrict perf_event_open access to privileged users only
echo 3 > /proc/sys/kernel/perf_event_paranoid
# Make the setting persistent across reboots
echo "kernel.perf_event_paranoid = 3" >> /etc/sysctl.conf
sysctl -p
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
