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Chapter 6. Docker SELinux Security Policy

The Docker SELinux security policy is similar to the libvirt security policy and is based on the libvirt security policy.

The libvirt security policy is a series of SELinux policies that defines two ways of isolating virtual machines. Generally, virtual machines are prevented from accessing parts of the network. Specifically, individual virtual machines are denied access to one another’s resources. Red Hat extends the libvirt-SELinux model to Docker. The Docker SELinux role and Docker SELinux types are based on libvirt. For example, by default, Docker has access to /usr/var/ and some other locations, but it has complete access to things that are labeled with svirt_sandbox_file_t. - this explains the entire Docker SELinux policy. It is not in layman’s terms, but it is complete.


^      ^           ^          ^     ^--- unique category
|      |           |          |----  secret-level 0
|      |           |--- a shared type
|      |---SELinux role
|------ SELinux user

If a file is labeled svirt_sandbox_file_t, then by default all containers can read it. But if the containers write into a directory that has svirt_sandbox_file_t ownership, they write using their own category (which in this case is "c186,c641). If you start the same container twice, it will get a new category the second time ( a different category than it had the first time). The category system isolates containers from one another.

Types can be applied to processes and to files.

6.1. MCS - Multi-Category Security

MCS - Multi-Category Security - this is similar to Multi-Level Authentication. Each container is given a unique ID at startup, and each file that a container writes carries that unique ID. Although this is an opt-in system, failure to make use of it means that you will have no isolation between containers. If you do not make use of MCS, you will have isolation between containers and the host, but you will not have isolation of containers from one another. That means that one container could access another container’s files. - this will be used later to build the MCS example that we will include in the MCS.

6.2. Leveraging the Docker SELinux Security Model

Properly Labeling Content - By default, docker gets access to everything in /usr and most things in /etc. To give docker access to more than that, relabel content on the host. To restrict access to things in /usr or things in /etc, relabel them. If you want to restrict access to only one or two containers, then you’ll need to use the opt-in MCS system.

Important Booleans and Other Restrictions - "privileged" under docker is not really privileged. Even privileged docker processes cannot access arbitrary socket files. An SElinux Boolean, docker_connect_any, makes it possible for privileged docker processes to access arbitrary socket files. Even if run privileged, docker is restricted by the Booleans that are in effect.

restricting kernel capabilities - docker supports two commands as part of "docker run": (1) "--cap-add=" and (2) "--cap-drop=". these allow us to add and drop kernel capabilites to and from the containers. root powers have been broken up into a number of groups of capabilities (for instance "cap-chown", which lets you change the ownership of files). by default, docker has a very restricted list of capabilites.

This provides more information about capabilites. Capabilites constitute the heart of the isolation of containers. If you have used capabilites in the manner described in this guide, an attacker who does not have a kernel exploit will be able to do nothing even if they have root on your system.

restricting kernel calls with seccomp - This is a kernel call that renounces capabilities. A seccomp call that has no root capabilities will make the call to the kernel. A capable process creates a restricted process that makes the kernel call. "seccomp" is an abbreviation of "secure computing mode". - seccomp is even more fine-grained than capabilites. This feature restricts the kernel calls that containers can make. This is useful for general security reasons, because (for instance) you can prevent a container from calling "cd". Almost all kernel exploits rely on making kernel calls (usually to rarely used parts of the kernel). With seccomp you can drop lots of kernel calls, and dropped kernel calls can’t be exploited as attack vectors.

docker network security and routing - By default, docker creates a virtual ethernet card for each container. Each container has its own routing tables and iptables. When specific ports are forwarded, docker creates certain host iptables rules. The docker daemon itself does some of the proxying. If you map applications to containers, you provide flexibility to yourself by limiting network access on a per-application basis. Because containers have their own routing tables, they can be used to limit incoming and outgoing traffic: use the ip route command in the same way you would use it on a host.

scenario: using a containerized firewall to segregate a particular kind of internet traffic from other kinds of internet traffic. This is an easy and potentially diverting exercise for the reader, and might involve concocting scenarios in which certain kinds of traffic are to be kept separate from an official network (one that is constrained, for instance, by the surveillance of a spouse or an employer).

cgroups - "control groups". cgroups provides the core functionality that permits docker to work. In its original implementation, cgroups controlled access only to resources like the CPU. You could put a process in a cgroup, and then instruct the kernel to give that cgroup only up to 10 percent of the cpu. This functions as a kind of a way of providing SLA or quota.

By default, docker creates a unique cgroup for each container. If you have existing cgroup policy on the docker daemon host, you can make use of that existing cgroup policy to control the resource consumption of the specified container.

freezing and unfreezing a container - You can completely stall a container in the state that it is in a any given moment, and then restart it at that point later. This is done by giving the container zero percent CPU. cgroups is the protection that docker provides against DDoS attacks. We could host a service on a machine and give it a cgroup priority so that the service can never get less than ten percent of the CPU: then if other services became compromised, they would be unable to stall out the service, because the service is guaranteed to get a minimum of ten percent of the CPU. This makes it possible to ensure that essential processes need never relinquish control of a part of the CPU, no matter how strongly they are attacked.