CVE-2026-23415: Linux Kernel Use-After-Free Vulnerability

CVE-2026-23415 Overview

CVE-2026-23415 is a Use-After-Free vulnerability in the Linux kernel's futex subsystem. The flaw exists in the interaction between futex_key_to_node_opt() and vma_replace_policy() functions, where a race condition allows a freed mempolicy pointer to be dereferenced, resulting in unauthorized memory access.

During futex_key_to_node_opt() execution, the vma->vm_policy is read under speculative mmap lock and RCU (Read-Copy-Update). Concurrently, the mbind() system call may invoke vma_replace_policy(), which frees the old mempolicy immediately via kmem_cache_free(). This creates a dangerous race where __futex_key_to_node() dereferences a freed mempolicy pointer, causing a use-after-free read of mpol->mode.

Critical Impact

A local attacker could potentially exploit this race condition to read freed memory, potentially leading to information disclosure or system instability. The vulnerability affects kernel memory safety in the futex synchronization primitive.

Affected Products

• Linux kernel (versions with vulnerable futex implementation)
• Systems using NUMA memory policies with futex operations
• Environments with concurrent mbind() and futex operations

Discovery Timeline

• 2026-04-02 - CVE CVE-2026-23415 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-23415

Vulnerability Analysis

This vulnerability is classified as a Use-After-Free (UAF) memory corruption issue occurring in the Linux kernel's futex (fast userspace mutex) implementation. The core of the vulnerability lies in inadequate synchronization between memory policy operations and futex key resolution.

When a process performs futex operations, the kernel calls futex_key_to_node_opt() to determine the NUMA node associated with the futex address. This function reads vma->vm_policy under speculative mmap lock protection and RCU. However, this protection is insufficient when another thread concurrently executes mbind() to change memory binding policies.

The vma_replace_policy() function, called during mbind(), replaces the VMA's memory policy and immediately frees the old policy using kmem_cache_free(). If the futex code path is in the middle of reading the old policy when this happens, it will access freed memory—specifically attempting to read the mpol->mode field from a deallocated structure.

The kernel KASAN (Kernel Address Sanitizer) error trace clearly demonstrates this issue:

BUG: KASAN: slab-use-after-free in __futex_key_to_node (kernel/futex/core.c:349)
Read of size 2 at addr ffff888001c49634 by task e/87

Call Trace:
 __asan_load2 (mm/kasan/generic.c:271)
 __futex_key_to_node (kernel/futex/core.c:349)
 get_futex_key (kernel/futex/core.c:374 kernel/futex/core.c:386 kernel/futex/core.c:593)

Root Cause

The root cause is a Time-of-Check Time-of-Use (TOCTOU) race condition combined with improper RCU lifecycle management for mempolicy objects. The __mpol_put() function did not properly use RCU to defer freeing of memory policy structures, allowing immediate deallocation while other kernel paths still held references under RCU read-side critical sections.

The fix adds proper RCU synchronization to __mpol_put(), ensuring that mempolicy structures are only freed after all RCU read-side critical sections that might reference them have completed.

Attack Vector

The vulnerability can be triggered locally by an attacker who can:

1. Create a shared memory region with a specific NUMA memory policy
2. Initiate futex operations on addresses within that region
3. Concurrently call mbind() to change the memory policy, triggering the race condition

The attack requires local access and the ability to perform multi-threaded operations with precise timing. While exploitation for code execution would be complex, the UAF read could potentially leak kernel memory contents or cause system instability.

Detection Methods for CVE-2026-23415

Indicators of Compromise

• KASAN reports showing slab-use-after-free errors in __futex_key_to_node or related futex functions
• Kernel oops or crashes with call traces involving futex_key_to_node_opt(), get_futex_key(), and memory policy functions
• Unexpected kernel panics during heavy multi-threaded workloads using futexes and NUMA memory policies

Detection Strategies

• Enable KASAN (Kernel Address Sanitizer) in development and testing kernels to detect use-after-free conditions
• Monitor kernel logs for KASAN reports referencing futex/core.c or mempolicy-related functions
• Deploy kernel live patching solutions that can detect and alert on exploitation attempts targeting known vulnerabilities

Monitoring Recommendations

• Configure centralized logging to capture kernel error messages including KASAN reports and kernel oops
• Implement real-time alerting on kernel panic events, especially those mentioning futex or mempolicy subsystems
• Use SentinelOne Singularity Platform to monitor for suspicious process behavior involving rapid futex and mbind system calls

How to Mitigate CVE-2026-23415

Immediate Actions Required

• Update affected Linux systems to kernel versions containing the fix commits
• Prioritize patching systems running multi-threaded applications with NUMA memory policies
• Consider temporarily disabling NUMA memory policies on critical systems if patching is delayed

Patch Information

The vulnerability has been resolved in the Linux kernel stable branches. The fix adds proper RCU synchronization to __mpol_put() to ensure memory policy structures are not freed while still potentially in use.

The following kernel commits contain the fix:

• Kernel Commit 190a8c4
• Kernel Commit 7e19619
• Kernel Commit 853f70c

Apply these patches by updating to a kernel version that includes them, or by backporting the commits to your current kernel version.

Workarounds

• Limit the use of mbind() system calls on systems where patching is not immediately possible
• Restrict access to memory policy system calls using seccomp filters for untrusted applications
• Monitor and audit processes making concurrent futex and mbind system calls as potential exploitation attempts

# Check current kernel version
uname -r

# Verify if your kernel includes the fix (check commit log)
# Update kernel using your distribution's package manager
# For Debian/Ubuntu:
sudo apt update && sudo apt upgrade linux-image-generic

# For RHEL/CentOS:
sudo yum update kernel

# Reboot to apply new kernel
sudo reboot