KVM Security: Risks & Mitigation

TL;DR

Yes, a malicious user with sufficient privileges can do significant harm using /dev/kvm. It allows direct access to the host machine’s virtualisation capabilities, potentially leading to privilege escalation, data theft, and denial of service. Properly configuring permissions and using security features like SELinux or AppArmor are crucial.

Understanding /dev/kvm

/dev/kvm is a character device in Linux that provides an interface for Kernel-based Virtual Machine (KVM) hypervisor. It’s the entry point for creating and managing virtual machines. Because it operates at such a low level, misuse or compromise can have severe consequences.

Potential Risks

1. Privilege Escalation: A user who can access /dev/kvm might be able to create a VM and use it as a stepping stone to gain root privileges on the host system.
2. Data Theft: Malicious VMs could be used to access sensitive data stored on the host or other virtual machines.
3. Denial of Service: A rogue VM can consume excessive resources (CPU, memory, disk I/O), leading to a denial-of-service condition for the host and other VMs.
4. Host System Compromise: Direct access to hardware through KVM could allow attackers to bypass security measures and compromise the entire host system.

Mitigation Steps

1. Restrict Access with Permissions: The most important step is to limit who can access /dev/kvm.
   • Check current permissions using:

     ls -l /dev/kvm

   • Typically, only the libvirt group (or a dedicated VM management group) should have read and write access. Ensure no other users or groups have unnecessary permissions.
   • Change permissions using:

     sudo chown root:libvirt /dev/kvm

     sudo chmod 660 /dev/kvm

2. Use SELinux or AppArmor: These Mandatory Access Control (MAC) systems provide an extra layer of security by defining strict rules about what processes can do.
   • Ensure that your system is running SELinux or AppArmor and properly configured.
   • Create policies specifically for VM management tools like libvirt to restrict their access to only the necessary resources. This prevents a compromised VM manager from abusing KVM.
3. Keep Software Updated: Regularly update your kernel and virtualisation software (e.g., QEMU, libvirt) to patch security vulnerabilities.
   • Use your distribution’s package manager:

     sudo apt update && sudo apt upgrade

     (Debian/Ubuntu)
     or

     sudo yum update

     (CentOS/RHEL)

4. Secure VM Configuration: Properly configure your virtual machines to minimise the attack surface.
   • Disable unnecessary devices and features.
   • Use strong passwords or SSH keys for access.
   • Keep guest operating systems updated.
5. Monitor System Logs: Regularly monitor system logs (e.g., /var/log/syslog, /var/log/audit/audit.log) for suspicious activity related to KVM.
   • Look for unexpected VM creation attempts or unusual resource usage.
6. Use a Hypervisor with Built-in Security Features: Some hypervisors offer advanced security features like integrity monitoring and secure boot.

Checking SELinux Status

To check if SELinux is enabled, use:

sestatus

If it’s disabled, consider enabling it according to your distribution’s documentation.