OpenVZ Host Compromise via Kernel Exploit

TL;DR

Yes, a kernel exploit can compromise an OpenVZ host, but it’s more complex than exploiting a full virtual machine. OpenVZ shares the host kernel, so a successful exploit gives access to the entire system, not just one container. However, security features like SELinux and resource limits can mitigate the risk.

Understanding the Risk

OpenVZ is a containerisation technology that relies on shared kernel functionality. Unlike full virtual machines (like those created with VMware or VirtualBox), containers don’t have their own dedicated kernels. They use the host operating system’s kernel, but are isolated through namespaces and other mechanisms.

How a Kernel Exploit Works

1. Exploit Development/Discovery: A vulnerability is found in the host kernel (e.g., a buffer overflow, race condition).
2. Exploit Delivery: The exploit needs to be executed within a container. This could happen through various means:
   • Malicious software uploaded into a container.
   • A vulnerability in an application running inside the container that allows code execution.
   • Compromised system libraries used by containers.
3. Kernel Compromise: If successful, the exploit gains control of the host kernel. Because OpenVZ shares the kernel, this means root access to the entire host system, not just the container.

Steps to Mitigate the Risk

1. Keep the Host Kernel Updated: This is the most important step. Regularly apply security patches released by your operating system vendor.

   sudo apt update && sudo apt upgrade

   (Example for Debian/Ubuntu)

2. Use SELinux or AppArmor: These Mandatory Access Control (MAC) systems provide an extra layer of security. They restrict what processes can do, even if they have root privileges.
   • SELinux: Configure SELinux in enforcing mode and create specific policies for OpenVZ containers to limit their access.
   • AppArmor: Similar to SELinux, AppArmor profiles define the capabilities of each container.
3. Resource Limits (vzctl): Use vzctl to set strict resource limits for containers.
   • CPU Usage: Limit the amount of CPU time a container can use.
   • Memory Usage: Prevent containers from consuming all available memory.
   • Disk I/O: Restrict disk read and write operations.
   • Network Bandwidth: Control network traffic.

   vzctl set 100 --cpulimit 50% --memlimit 2G

   (Example: Limit container ID 100 to 50% CPU and 2GB of memory)

4. Disable Unnecessary Kernel Modules: Reduce the attack surface by disabling kernel modules that aren’t required.

   sudo modprobe -r 

   (Example: Remove a specific module. Be careful when doing this!)

5. Regular Security Audits: Scan your host system and containers for vulnerabilities.
   • Use vulnerability scanners like Nessus or OpenVAS.
   • Review container configurations for potential security weaknesses.
6. Container Isolation: While OpenVZ provides isolation, it’s not perfect. Avoid running untrusted code in containers.
7. Monitor System Logs: Regularly check system logs (e.g., /var/log/syslog, /var/log/kern.log) for suspicious activity.

What Happens After a Successful Exploit?

If an attacker successfully exploits the kernel through an OpenVZ container, they typically gain root access to the host system. This allows them to:

• Install malware.
• Steal sensitive data.
• Modify system files.
• Compromise other containers on the same host.