CVE-2026-68177: Linux Kernel Use-After-Free Vulnerability

CVE-2026-68177 Overview

CVE-2026-68177 is a use-after-free vulnerability in the Linux kernel's tracing subsystem. The flaw resides in the enable_event trigger logic, where the module reference count is released too early relative to the trigger's actual lifetime. A local attacker with the ability to configure event triggers can exploit the race between module removal and trigger invocation to read freed memory. Successful exploitation compromises kernel confidentiality, integrity, and availability. The issue has been resolved upstream by moving trace_event_put_ref() into the delayed private_data_free() callback so the module cannot be unloaded while active references remain.

Critical Impact
Local attackers can trigger a kernel use-after-free by removing a module while an enable_event trigger still references its event data.

Affected Products

Linux kernel (tracing subsystem, enable_event trigger)
Distributions shipping kernels prior to the upstream fix commits 159fdc3e01dc and e091351b3881
Systems where unprivileged or semi-privileged users can configure tracing triggers

Discovery Timeline

2026-08-10 - CVE-2026-68177 published to NVD
2026-08-13 - Last updated in NVD database

Technical Details for CVE-2026-68177

Vulnerability Analysis

The Linux kernel tracing subsystem supports event triggers, including the enable_event trigger, which enables or disables another trace event when a condition fires. To prevent freeing trigger data while it is still in use, the kernel defers cleanup until after an RCU grace period through a private_data_free() callback.

The defect stems from an inconsistent teardown sequence. While the enable_event trigger data itself was correctly deferred, the associated module reference count was dropped immediately via trace_event_put_ref(). If the target module was removed after the reference was released but before the RCU grace period expired, the trigger could still fire and dereference module-owned event structures that had already been freed.

This condition constitutes a use-after-free in kernel memory. The impact includes potential kernel information disclosure, memory corruption, and local privilege escalation depending on heap layout and the freed object's contents.

Root Cause

The root cause is a lifetime mismatch between two resources tied to the same trigger. The trigger's private data followed the delayed, RCU-safe free path, but the module refcount was decremented synchronously during trigger unregistration. This allowed the module to be unloaded while lingering trigger invocations still referenced its event objects.

Attack Vector

Exploitation requires local access with the ability to write to tracefs and configure an enable_event trigger that targets an event exported by a removable kernel module. The attacker registers the trigger, unregisters it, and unloads the module during the narrow window before the RCU grace period completes while another CPU is still executing the trigger. Winning this race yields access to freed memory referenced by the trigger.

No verified public exploit code is available. Technical details are documented in the upstream commits referenced in the Kernel Git Commit Log and the companion patch commit.

Detection Methods for CVE-2026-68177

Indicators of Compromise

Kernel oops or panic messages referencing event_trigger, enable_event, or trace_event_put_ref in dmesg or /var/log/kern.log
KASAN use-after-free reports involving tracing event structures on debug kernels
Unexpected module unload events (rmmod, delete_module) correlated with tracefs writes to events/*/trigger

Detection Strategies

Audit write syscalls to /sys/kernel/tracing/events/*/trigger and /sys/kernel/debug/tracing/events/*/trigger by non-root or non-administrative users
Correlate module load and unload activity with prior configuration of enable_event triggers targeting events in the same module
Monitor for repeated failed exploitation attempts producing kernel warnings or stack traces referencing the tracing subsystem

Monitoring Recommendations

Forward auditd records for init_module, finit_module, and delete_module syscalls into a centralized log platform
Alert on tracefs configuration changes from non-privileged sessions or containerized workloads that should not require tracing access
Retain kernel ring buffer output for post-incident analysis of use-after-free crash signatures

How to Mitigate CVE-2026-68177

Immediate Actions Required

Apply the upstream Linux kernel patches from commits 159fdc3e01dc and e091351b3881 or update to a distribution kernel that includes them
Restrict access to /sys/kernel/tracing and /sys/kernel/debug/tracing to trusted administrative users only
Inventory systems where local users, containers, or CI runners can load or unload kernel modules and configure trace triggers

Patch Information

The fix moves the trace_event_put_ref() call into the delayed private_data_free() callback. This ensures the module reference is held until the RCU grace period ends and all pending trigger invocations have completed. Patch details are available in the upstream commit and the stable tree backport.

Workarounds

Mount tracefs and debugfs with restrictive permissions (mode=0700, owned by root) until patches are deployed
Disable or unload kernel modules that expose triggerable trace events and are not required for production workloads
Remove CAP_SYS_ADMIN from unprivileged containers and workloads to prevent tracefs configuration

bash

# Configuration example: restrict tracefs access to root only
mount -o remount,mode=0700 /sys/kernel/tracing
chmod 700 /sys/kernel/debug/tracing

# Verify running kernel includes the fix commit
uname -r
grep -c "enable_event" /proc/kallsyms