Containers enable you to simplify multi-tenancy deployments by deploying multiple applications on a single host, using the kernel and the docker runtime to spin up each container.
You must have an operating system (OS) that can secure the host kernel and secure containers from each other. In Linux, containers are just a special type of process, so securing containers is the same as securing any running process. Containers should run as a non-root user. Dropping the privilege level or creating containers with the least amount of privileges possible is recommended.
Because OpenShift Container Platform runs on Red Hat Enterprise Linux (RHEL) and RHEL Atomic Host, the following concepts apply by default to any deployed OpenShift Container Platform cluster and are at the core of what make containers secure on the platform.
Linux namespaces enable creating an abstraction of a particular global system resource to make it appear as a separate instance to processes within a namespace. Consequently, several containers can use the same resource simultaneously without creating a conflict. See Overview of Containers in Red Hat Systems for details on the types of namespaces (e.g., mount, PID, and network).
SELinux provides an additional layer of security to keep containers isolated from each other and from the host. SELinux allows administrators to enforce mandatory access controls (MAC) for every user, application, process, and file.
CGroups (control groups) limit, account for, and isolate the resource usage (CPU, memory, disk I/O, network, etc.) of a collection of processes. CGroups are used to ensure that containers on the same host are not impacted by each other.
Secure computing mode (seccomp) profiles can be associated with a container to restrict available system calls.
Deploying containers using RHEL Atomic Host reduces the attack surface by minimizing the host environment and tuning it for containers.
Linux man page: namespaces(7)
Red Hat Enterprise Linux Atomic Host Overview of Containers in Red Hat Systems: Secure Containers with SELinux
Red Hat Enterprise Linux Resource Management Guide: Introduction to Control Groups (CGroups)
Red Hat Enterprise Linux Atomic Host Container Security Guide: Linux Capabilities and seccomp
Kernel documentation: seccomp
Traditional virtualization also enables multi-tenancy, but in a very different way from containers. Virtualization relies on a hypervisor spinning up guest virtual machines (VMs), each of which has its own operating system (OS), as well as the running application and its dependencies.
With VMs, the hypervisor isolates the guests from each other and from the host kernel. Fewer individuals and processes have access to the hypervisor, reducing the attack surface on the physical server. That said, security must still be monitored: one guest VM may be able to use hypervisor bugs to gain access to another VM or the host kernel. And, when the OS needs patching, it must be patched on all guest VMs using that OS.
Containers can be run inside guest VMs, and there may be use cases where this is desirable. For example, you may be deploying a traditional application in a container, perhaps in order to lift-and-shift an application to the cloud. However, container multi-tenancy on a single host provides a more lightweight, flexible, and easier-to-scale deployment solution. This deployment model is particularly appropriate for cloud-native applications.
Red Hat Enterprise Linux Atomic Host Overview of Containers in Red Hat Systems: Linux Containers Compared to KVM Virtualization