CVE-2026-89750: Linux Kernel Use-After-Free Vulnerability

CVE-2026-89750 Overview

CVE-2026-89750 is a use-after-free (UAF) vulnerability in the Linux kernel's tracing/user_events subsystem. The flaw resides in fork duplication logic, where dup_task_struct() copies the user_event_mm pointer from parent to child without incrementing its reference count. When user_event_mm_alloc() fails inside user_event_mm_dup(), the copied pointer is left unmodified, producing a dangling reference. When the child later exits, user_event_mm_remove() frees memory the parent still references, resulting in a UAF detectable by KASAN.

Critical Impact
A local unprivileged user can trigger a kernel use-after-free through fork operations, enabling potential privilege escalation, kernel memory corruption, and denial of service on affected Linux systems.

Affected Products

Linux kernel versions containing the tracing/user_events subsystem prior to the fix
Distributions shipping the vulnerable user_event_mm_dup() code path
Systems with CONFIG_USER_EVENTS enabled

Discovery Timeline

2026-09-11 - CVE-2026-89750 published to NVD
2026-09-14 - Last updated in NVD database

Technical Details for CVE-2026-89750

Vulnerability Analysis

The vulnerability sits in the interaction between dup_task_struct() and user_event_mm_dup() during process fork. The parent's user_event_mm pointer is memcpy'd into the child task without acquiring a reference. user_event_mm_dup() is expected to overwrite this stale pointer with a fresh allocation, but the failure path of user_event_mm_alloc() returns without clearing the child's inherited pointer.

The child task now shares an unowned pointer with the parent. On child exit, user_event_mm_remove() decrements the reference count and frees the underlying user_event_mm structure. The parent retains a dangling pointer to freed slab memory. Any subsequent invocation of current_user_event_mm() from the parent — for example via user_events_ioctl() — dereferences freed memory.

KASAN reports the condition as slab-use-after-free in current_user_event_mm+0x51/0x1d0, with the write occurring during an ioctl syscall path reaching __x64_sys_ioctl. The freed allocation originated from user_event_mm_alloc() and was released via kfree() inside a workqueue context.

Root Cause

The root cause is missing state initialization after structure duplication. dup_task_struct() performs a shallow copy of task fields, including pointers to reference-counted objects. The user_event_mm field lacks a corresponding clear-or-take-reference operation in the fork path. When the allocation-based replacement fails, the shallow-copied pointer persists, yielding two owners for a single reference count.

Attack Vector

Exploitation requires local access and the ability to fork processes and invoke user_events ioctls. An attacker forks a child while inducing memory pressure to fail user_event_mm_alloc(), then allows the child to exit. The parent's next user_events_ioctl() call operates on freed slab memory. Reclaiming the freed slab with attacker-controlled data before the dereference converts the UAF into arbitrary kernel write primitives suitable for privilege escalation.

No verified public exploit code is available. The upstream fix and KASAN report can be reviewed in the Linux Kernel Commit 25a0758 and related backports.

Detection Methods for CVE-2026-89750

Indicators of Compromise

KASAN log entries reporting slab-use-after-free in current_user_event_mm
Kernel oops or panic traces referencing user_events_ioctl, user_event_mm_remove, or current_user_event_mm
Unexpected process crashes in workloads that fork children using the user_events tracing interface
Anomalous ioctl activity against /sys/kernel/tracing/user_events_data from unprivileged processes

Detection Strategies

Enable KASAN on test and staging kernels to surface the UAF at runtime
Audit kernel version and confirm whether backport commits 25a0758, 33ce55b, 390f6bd, 63b39e4, or b799f67 are applied
Correlate local process activity with ioctl syscalls targeting user_events file descriptors using eBPF or auditd
Monitor kernel ring buffer (dmesg) for slab corruption warnings following fork-heavy workloads

Monitoring Recommendations

Ingest kernel logs into a centralized logging pipeline and alert on KASAN or BUG: messages referencing user_events symbols
Track syscall telemetry for unprivileged users invoking ioctl on tracing file descriptors
Baseline fork rates and child exit patterns for workloads with access to CONFIG_USER_EVENTS

How to Mitigate CVE-2026-89750

Immediate Actions Required

Identify all Linux systems built with CONFIG_USER_EVENTS=y and prioritize them for patching
Apply the vendor kernel update containing the upstream fix that clears the copied user_event_mm pointer before any failure path
Restrict access to the user_events tracing interface to trusted administrators until patched
Reboot systems after kernel package upgrades to activate the fixed image

Patch Information

The upstream fix clears the child task's copied user_event_mm pointer before any allocation attempt in user_event_mm_dup(). If allocation fails, the child holds no reference and has nothing to free on exit, eliminating the double-owner condition. The fix is available across the stable trees in commits 25a0758, 33ce55b, 390f6bd, 63b39e4, and b799f67.

Workarounds

Build kernels without CONFIG_USER_EVENTS where the tracing interface is not required
Restrict permissions on /sys/kernel/tracing/user_events_data so only privileged users can open the interface
Deny the tracefs mount to untrusted containers and workloads through seccomp, LSM policy, or namespace restrictions

bash

# Configuration example: restrict user_events access
mount -o remount,mode=0700 /sys/kernel/tracing
chmod 0600 /sys/kernel/tracing/user_events_data

# Verify kernel does not expose user_events to unprivileged users
grep CONFIG_USER_EVENTS /boot/config-$(uname -r)