CVE-2025-21898

5.5 MEDIUM

Denial of Service

📋 TL;DR

This CVE addresses a potential division by zero vulnerability in the Linux kernel's ftrace subsystem within the function_stat_show() function. The vulnerability could cause kernel panics or system crashes when accessing ftrace statistics. It affects Linux systems with ftrace enabled and requires local access to trigger.

💻 Affected Systems

Products:

Linux Kernel

Versions: Specific affected versions not explicitly stated in CVE, but patches exist for multiple stable branches

Operating Systems: Linux distributions using vulnerable kernel versions

Default Config Vulnerable: ✅ No

Notes: Requires ftrace to be enabled and debugfs mounted. Many distributions disable ftrace by default or restrict debugfs access.

📦 What is this software?

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Kernel panic leading to system crash and denial of service, potentially causing data loss or service disruption.

🟠

Likely Case

System crash or kernel panic when accessing ftrace statistics through debugfs, resulting in temporary denial of service.

🟢

If Mitigated

No impact if ftrace is disabled or proper access controls prevent unauthorized users from accessing debugfs.

🌐 Internet-Facing: LOW - Requires local access to debugfs interface, not directly exploitable over network.

🏢 Internal Only: MEDIUM - Local users or processes with debugfs access could cause system crashes.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: NO

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Requires local access to debugfs interface and specific conditions to trigger the division by zero. No known active exploitation.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Multiple stable kernel versions with commits: 3d738b53ed6cddb68e68c9874520a4bf846163b5, 5b3d32f607f0478b414b16516cf27f9170cf66c8, 746cc474a95473591853927b3a9792a2d671155b, 992775227843c9376773784b8b362add44592ad7, 9cdac46fa7e854e587eb5f393fe491b6d7a9bdf6

Vendor Advisory: https://git.kernel.org/stable/c/

Restart Required: Yes

Instructions:

1. Update Linux kernel to patched version from distribution vendor
2. Reboot system to load new kernel
3. Verify kernel version after reboot

🔧 Temporary Workarounds

Disable ftrace

linux

Disable ftrace functionality to prevent access to vulnerable code path

echo 0 > /sys/kernel/debug/tracing/tracing_on
umount /sys/kernel/debug

Restrict debugfs access

linux

Mount debugfs with restricted permissions or unmount it entirely

mount -o remount,noexec,nosuid,nodev -t debugfs debugfs /sys/kernel/debug
chmod 700 /sys/kernel/debug

🧯 If You Can't Patch

Disable ftrace functionality completely
Restrict access to debugfs to trusted users only using filesystem permissions

🔍 How to Verify

Check if Vulnerable:

Check if ftrace is enabled and accessible: ls -la /sys/kernel/debug/tracing/

Check Version:

uname -r

Verify Fix Applied:

Check kernel version against patched versions from distribution vendor

📡 Detection & Monitoring

Log Indicators:

Kernel panic messages in /var/log/messages or dmesg
Division by zero errors in kernel logs
System crashes when accessing ftrace statistics

Network Indicators:

None - local vulnerability only

SIEM Query:

source="kernel" AND ("division by zero" OR "panic" OR "Oops") AND "ftrace"

📊 Metadata

CVE ID: CVE-2025-21898

CVSS v3 Score: 5.5 (MEDIUM)

CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

CWE: CWE-369

EPSS Score: 0.11% exploit probability (29.1th percentile)

Published: April 1, 2025

Last Updated: November 3, 2025

🔗 References