CVE-2026-23225: Linux Kernel Use-After-Free Vulnerability

CVE-2026-23225 Overview

A memory corruption vulnerability exists in the Linux kernel's scheduler MMCID (Memory-Mapped Concurrency ID) management code. The vulnerability manifests as an out-of-bounds access that was initially reported as a KASAN (Kernel Address Sanitizer) use-after-free condition. The flaw occurs during mode switching in the CID management system when the kernel incorrectly assumes CPU ownership of a CID during task exit operations.

Critical Impact

Local attackers may exploit this out-of-bounds memory access vulnerability to cause system instability, kernel crashes, or potentially achieve privilege escalation on affected Linux kernel systems.

Affected Products

• Linux kernel (versions with vulnerable MMCID management code)
• Systems using per-CPU CID mode switching functionality
• Linux-based distributions with affected kernel versions

Discovery Timeline

• February 18, 2026 - CVE-2026-23225 published to NVD
• February 18, 2026 - Last updated in NVD database

Technical Details for CVE-2026-23225

Vulnerability Analysis

This vulnerability is classified as an Out-of-Bounds Access in the Linux kernel scheduler's MMCID management subsystem. The root issue involves improper validation of CID (Concurrency ID) ownership during mode transitions between per-CPU and per-task CID modes.

The vulnerability was discovered by Shinichiro, who identified that the kernel's sched_mm_cid_exit() function incorrectly assumes that a CID is CPU-owned based solely on the prior operational mode being per-CPU. This assumption fails in race condition scenarios where a task exits with the TRANSIT bit set but the ONCPU bit is not set.

When a task exits under these conditions, the mm_drop_cid_on_cpu() function is invoked, which attempts to clear the non-existent ONCPU bit. Subsequently, a clear_bit() operation is executed with an extremely large bit number (bit 29 from the TRANSIT flag), resulting in an out-of-bounds memory access.

Root Cause

The vulnerability stems from a race condition in the MMCID management code during concurrent task operations. When a parent task (T0) forks a new task and switches to per-CPU CID mode, it sets the MM_CID_TRANSIT bit on other tasks running in userspace. If one of these tasks (T1) exits before being scheduled again, it retains the TRANSIT bit set.

The sched_mm_cid_remove_user() function correctly clears the TRANSIT bit and drops the CID from the task structure but does not update the per-CPU storage. While this is functionally correct since CID ownership requires the ONCPU bit (which is mutually exclusive with TRANSIT), the subsequent sched_mm_cid_exit() function fails to validate actual CPU ownership before invoking cleanup operations.

Attack Vector

The attack vector for this vulnerability involves local access to the system. An attacker with the ability to execute code on the affected system could potentially trigger the race condition by manipulating task creation, scheduling, and exit timing. The exploitation scenario involves:

1. Creating multiple tasks that utilize the MMCID scheduling features
2. Triggering a mode switch from per-task to per-CPU CID mode via fork operations
3. Timing task exits to occur while the TRANSIT bit is set but before the task is rescheduled
4. Causing the out-of-bounds memory access when clear_bit() is called with bit 29

The vulnerability mechanism involves improper state validation in the kernel scheduler. During mode transitions, the kernel sets the MM_CID_TRANSIT flag on tasks, but fails to properly verify CID ownership state before performing cleanup operations. When mm_drop_cid_on_cpu() executes without validating the ONCPU bit status, passing the TRANSIT bit value (bit 29) to clear_bit() results in memory access far beyond the intended boundaries.

Detection Methods for CVE-2026-23225

Indicators of Compromise

• KASAN (Kernel Address Sanitizer) reports indicating use-after-free or out-of-bounds access in scheduler code
• Kernel panic or crash logs referencing sched_mm_cid_exit(), mm_drop_cid_on_cpu(), or related MMCID functions
• System instability during heavy multitasking workloads involving frequent process creation and termination

Detection Strategies

• Enable KASAN in kernel builds to detect out-of-bounds memory accesses during testing and development
• Monitor system logs for kernel oops or panic messages related to the scheduler subsystem
• Implement kernel crash dump analysis to identify exploitation attempts targeting MMCID code paths

Monitoring Recommendations

• Configure kernel crash dump collection (kdump) to capture detailed crash information for forensic analysis
• Set up automated alerting for kernel log entries containing mm_cid, sched_mm_cid, or related scheduler functions
• Monitor system stability metrics and investigate unexpected kernel crashes or reboots

How to Mitigate CVE-2026-23225

Immediate Actions Required

• Apply the official kernel patch that validates CID CPU ownership in mm_drop_cid_on_cpu() before performing cleanup operations
• Update to a patched kernel version that includes commit 81f29975631db8a78651b3140ecd0f88ffafc476
• Prioritize patching for systems running multi-threaded applications with heavy process creation patterns

Patch Information

The fix has been committed to the Linux kernel stable tree. The patch modifies mm_drop_cid_on_cpu() to properly validate that the CID is actually CPU-owned before attempting to clear bits, preventing the out-of-bounds access when the TRANSIT bit is set. Technical details and the complete patch are available in the Linux Kernel Commit.

Workarounds

• If immediate patching is not feasible, consider reducing multiprocessing workloads that involve frequent fork/exit patterns
• Monitor systems closely for signs of instability and prepare for emergency patching if exploitation is detected
• Enable KASAN in development and staging environments to identify potential exploitation attempts before production impact

# Verify current kernel version and check for vulnerable MMCID code
uname -r

# Check if the fix commit is present in your kernel source
git log --oneline | grep "81f29975631db8a78651b3140ecd0f88ffafc476"

# Apply kernel updates using your distribution's package manager
# For Debian/Ubuntu:
sudo apt update && sudo apt upgrade linux-image-$(uname -r)

# For RHEL/CentOS:
sudo yum update kernel