Harden the Kernel with Grsecurity

Posted January 19Jan 19

You are reading Part 50 of the 57-part series: Harden and Secure Linux Servers. [Level 5]

This series covers progressive security measures, from fundamental hardening techniques to enterprise-grade defense strategies. Each article delves into a specific security practice, explaining its importance and providing step-by-step guidance for implementation.

To explore more security best practices, visit the main guide for a full breakdown of all levels and recommendations.

Grsecurity is a set of kernel patches that enhance Linux security by adding exploit mitigations, access controls, and protection against kernel vulnerabilities. It helps:

✅ Prevent privilege escalation attacks by hardening memory protections.
✅ Protect against zero-day exploits with advanced runtime security features.
✅ Restrict unauthorized system calls to prevent code execution vulnerabilities.
✅ Improve access control by enforcing stricter security policies.

🔹 Grsecurity is commonly used in security-sensitive environments like finance, government, and enterprise servers.

Note: Grsecurity is available for commercial use and requires a subscription for access to the latest patches.

How to Install and Configure Grsecurity on a Linux Kernel

1. Download the Grsecurity Patches and Kernel Source

Since Grsecurity is a patch to the Linux kernel, you need to download and apply it manually.

Step 1: Install Required Dependencies

Before patching the kernel, install the necessary tools:

sudo apt update && sudo apt install build-essential libncurses-dev bc flex bison -y

Step 2: Download the Kernel Source Code

Check your current kernel version:

uname -r

Download the corresponding Linux kernel source:

wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.15.85.tar.xz
tar -xvf linux-5.15.85.tar.xz
cd linux-5.15.85

Step 3: Download the Grsecurity Patch

Grsecurity is available for paid users via grsecurity.net.
After obtaining a subscription, download the patch:

wget https://grsecurity.net/test/grsecurity-5.15.85-1.patch

2. Apply the Grsecurity Patch to the Kernel

Navigate to the kernel source directory and apply the patch:

patch -p1 < ../grsecurity-5.15.85-1.patch

If successful, you will see output similar to:

patching file security/grsecurity/grsec.c
patching file include/linux/grsecurity.h

3. Configure and Compile the Hardened Kernel

Step 1: Configure Kernel with Grsecurity Settings

Run the kernel configuration tool:

make menuconfig

Navigate to:
🔹 Security Options → Enable Grsecurity
🔹 Harden Memory Protections → Enable PaX (Prevents buffer overflow and memory exploits)
🔹 Restrict Privileged Processes → Enable RBAC (Role-Based Access Control)

Save and exit.

Step 2: Compile and Install the Patched Kernel

Compile the kernel (this process may take some time):
```
make -j$(nproc)
```
Install kernel modules:
```
sudo make modules_install
```
Install the new kernel:
```
sudo make install
```
Update the bootloader (GRUB) to load the new kernel:
```
sudo update-grub
```
Reboot into the new hardened kernel:
```
sudo reboot
```

4. Verify That Grsecurity Is Active

After rebooting, check if Grsecurity is running:

dmesg | grep grsecurity

If correctly installed, you should see Grsecurity-related logs confirming the hardened kernel is loaded.

5. Configure Grsecurity Security Settings

A. Enable Role-Based Access Control (RBAC)

Enable RBAC in Grsecurity:

sudo nano /etc/grsec/policy

Add:

subject /bin/bash o {
    /bin/bash rxi
    /etc/shadow h
}

Apply RBAC changes:
```
gradm -E
```

B. Enforce Memory Protections (PaX)

To harden against memory-based exploits, enable PaX protections:

paxctl -c /bin/bash
paxctl -m /bin/bash

6. Regularly Update and Maintain Grsecurity

To ensure continued protection, keep Grsecurity updated:

cd linux-5.15.85
wget https://grsecurity.net/test/grsecurity-5.15.86-1.patch
patch -p1 < grsecurity-5.15.86-1.patch
make -j$(nproc)
sudo make modules_install
sudo make install
sudo update-grub
sudo reboot

Best Practices for Grsecurity Kernel Hardening

✅ Use Grsecurity with SELinux or AppArmor for multi-layered security.
✅ Regularly patch the kernel to keep up with new security updates.
✅ Monitor kernel logs for unauthorized access attempts.
✅ Test Grsecurity in a non-production environment first before deployment.
✅ Use Role-Based Access Control (RBAC) to restrict system privileges.

By applying Grsecurity patches, you significantly strengthen the Linux kernel against advanced threats, exploits, and privilege escalation attacks, making your system more resilient and secure.

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