How to Harden the Linux Kernel on Arch Linux

The Linux kernel is the core of your Arch Linux system. It manages hardware, schedules processes, and enforces security boundaries. By default, Linux provides a balanced configuration between usability and performance. However, if you’re running a sensitive system—such as a server, workstation with confidential data, or a minimal container host—kernel hardening is a critical step toward minimizing vulnerabilities and increasing security.

This guide explains how to harden the Linux kernel on Arch Linux using well-established practices, tools, and patches. We’ll walk through tuning kernel parameters, leveraging security modules, applying grsecurity-style configurations, and more. This is aimed at technically inclined users, sysadmins, and security enthusiasts.

1. Understanding the Scope of Kernel Hardening

Kernel hardening is about minimizing the attack surface and increasing protections against memory corruption, privilege escalation, and other kernel-level exploits. On Arch Linux, this can be achieved through:

• Configuring sysctl parameters (runtime kernel settings)
• Using Linux Security Modules (LSMs) like AppArmor and SELinux
• Applying hardened kernel patches (like linux-hardened)
• Enabling kernel compile-time options
• Removing unnecessary kernel modules
• Enforcing stricter permissions and capabilities

2. Use the linux-hardened Kernel

The Arch Linux community provides a custom kernel called linux-hardened, which includes patches and configuration options aimed at hardening the system.

Installing linux-hardened

sudo pacman -S linux-hardened linux-hardened-headers

Update your bootloader configuration after installation. For example, with GRUB:

sudo grub-mkconfig -o /boot/grub/grub.cfg

Reboot and select the hardened kernel from your boot menu.

Benefits of linux-hardened

• Restrictive dmesg access
• Hardened user namespaces
• Hardened syscall filtering
• Additional control over module loading
• Improved protection against information leaks

3. Lock Down Sysctl Settings

Sysctl is a runtime interface to modify kernel behavior. Settings can be made persistent in /etc/sysctl.d/.

Create a hardening file:

sudo nano /etc/sysctl.d/99-kernel-hardening.conf

Add the following secure defaults:

# Disable magic SysRq keys
kernel.sysrq = 0

# Disable kexec
kernel.kexec_load_disabled = 1

# Prevent kernel pointer exposure
kernel.kptr_restrict = 2

# Hide kernel info from unprivileged users
kernel.dmesg_restrict = 1

# Enable Exec Shield (DEP/NX)
vm.mmap_min_addr = 65536

# Enable address space layout randomization
kernel.randomize_va_space = 2

# Restrict access to BPF
kernel.unprivileged_bpf_disabled = 1

# Harden network stack
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
net.ipv4.icmp_echo_ignore_broadcasts = 1
net.ipv4.icmp_ignore_bogus_error_responses = 1
net.ipv4.tcp_syncookies = 1

Apply changes:

sudo sysctl --system

4. Restrict Kernel Module Loading

Dynamically loaded kernel modules can introduce attack vectors. Limit or disable this behavior on secure systems.

Option 1: Disable module loading at runtime

Add the following to your kernel boot parameters:

module.sig_enforce=1 modules_disabled=1

This fully disables loading of additional modules after boot.

Option 2: Blacklist unused modules

For more granular control, blacklist unnecessary or risky modules:

sudo nano /etc/modprobe.d/blacklist.conf

Example:

blacklist usb_storage
blacklist firewire-core
blacklist thunderbolt

Afterward, regenerate initramfs:

sudo mkinitcpio -P

5. Enable and Configure AppArmor or SELinux

Arch supports both AppArmor and SELinux, although AppArmor is easier to configure for desktops and general servers.

Install and Enable AppArmor

sudo pacman -S apparmor
sudo systemctl enable apparmor --now

Ensure your kernel is booted with AppArmor:

cat /sys/module/apparmor/parameters/enabled

You should see Y. Add to your GRUB kernel line if missing:

apparmor=1 security=apparmor

Then update GRUB and reboot:

sudo grub-mkconfig -o /boot/grub/grub.cfg

Set Profiles

AppArmor profiles control what applications can access. To load and audit them:

sudo apparmor_parser -r /etc/apparmor.d/*

For tight security, place custom profiles for services like nginx, mysql, or firefox.

6. Compile a Custom Kernel (Optional)

For maximum control, you can compile your own kernel with specific security options enabled.

Get the PKGBUILD

Clone Arch’s kernel source:

git clone https://gitlab.archlinux.org/archlinux/packaging/packages/linux-hardened.git
cd linux-hardened

Edit the Configuration

Modify .config to include or tighten options:

Examples:

CONFIG_STRICT_KERNEL_RWX=y
CONFIG_STRICT_MODULE_RWX=y
CONFIG_SECURITY=y
CONFIG_SECURITY_LOCKDOWN_LSM=y
CONFIG_SECURITY_YAMA=y
CONFIG_SECURITY_APPARMOR=y
CONFIG_SECURITY_DMESG_RESTRICT=y

Then build the package:

makepkg -si

Note: Kernel compilation is advanced and may take significant time.

7. Use Stack Protector Features

Make sure the kernel includes stack protection features to reduce buffer overflow attacks:

• CONFIG_CC_STACKPROTECTOR_STRONG=y
• CONFIG_STACKPROTECTOR=y
• CONFIG_RANDOMIZE_BASE=y

These options are enabled in linux-hardened and can be verified with:

zcat /proc/config.gz | grep STACKPROTECTOR

8. Audit Your System with lynis

lynis is a great auditing tool for system hardening:

sudo pacman -S lynis
sudo lynis audit system

It provides suggestions, including kernel security checks, and helps guide your hardening efforts.

9. Remove Setuid Binaries

Setuid binaries can be exploited to escalate privileges. Find them with:

find / -perm -4000 -type f 2>/dev/null

Remove or restrict as needed. For example:

chmod u-s /usr/bin/ping

Be careful: Some are necessary for system functionality.

10. Additional Tips for Kernel Hardening

• Enable lockdown mode: Use the lockdown LSM in “confidentiality” mode for unprivileged access control.
• Use a read-only /boot: Prevent unauthorized modifications of boot configuration.
• Use full disk encryption: Protects against physical access attacks.
• Enable Secure Boot (if supported): Prevents unsigned kernels from booting.
• Regularly update your kernel: Security patches are critical; avoid long outdated kernels.

Conclusion

Kernel hardening is a powerful way to reinforce the security of your Arch Linux system. While Arch provides flexibility, it also places responsibility on the user to implement best practices. By combining the linux-hardened kernel, strict sysctl settings, module restrictions, and kernel-level security modules like AppArmor or SELinux, you significantly reduce your system’s exposure to common and advanced threats.

Security is an ongoing process—what’s hardened today may not be secure tomorrow. Stay updated with kernel security advisories, review changes in Arch’s linux-hardened package, and periodically audit your configuration.

Ultimately, kernel hardening should be part of a larger defense-in-depth strategy that includes secure applications, user privilege separation, regular updates, and firewall configuration.