Initial checkin of v1.1 -- does not build
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John Mulhausen
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layout: docwithnav
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title: "Security Contexts"
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---
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# Security Contexts
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## Abstract
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A security context is a set of constraints that are applied to a container in order to achieve the following goals (from [security design](security.html)):
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1. Ensure a clear isolation between container and the underlying host it runs on
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2. Limit the ability of the container to negatively impact the infrastructure or other containers
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## Background
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The problem of securing containers in Kubernetes has come up [before](http://issue.k8s.io/398) and the potential problems with container security are [well known](http://opensource.com/business/14/7/docker-security-selinux). Although it is not possible to completely isolate Docker containers from their hosts, new features like [user namespaces](https://github.com/docker/libcontainer/pull/304) make it possible to greatly reduce the attack surface.
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## Motivation
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### Container isolation
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In order to improve container isolation from host and other containers running on the host, containers should only be
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granted the access they need to perform their work. To this end it should be possible to take advantage of Docker
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features such as the ability to [add or remove capabilities](https://docs.docker.com/reference/run/#runtime-privilege-linux-capabilities-and-lxc-configuration) and [assign MCS labels](https://docs.docker.com/reference/run/#security-configuration)
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to the container process.
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Support for user namespaces has recently been [merged](https://github.com/docker/libcontainer/pull/304) into Docker's libcontainer project and should soon surface in Docker itself. It will make it possible to assign a range of unprivileged uids and gids from the host to each container, improving the isolation between host and container and between containers.
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### External integration with shared storage
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In order to support external integration with shared storage, processes running in a Kubernetes cluster
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should be able to be uniquely identified by their Unix UID, such that a chain of ownership can be established.
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Processes in pods will need to have consistent UID/GID/SELinux category labels in order to access shared disks.
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## Constraints and Assumptions
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* It is out of the scope of this document to prescribe a specific set
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of constraints to isolate containers from their host. Different use cases need different
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settings.
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* The concept of a security context should not be tied to a particular security mechanism or platform
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(ie. SELinux, AppArmor)
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* Applying a different security context to a scope (namespace or pod) requires a solution such as the one proposed for
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[service accounts](service_accounts.html).
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## Use Cases
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In order of increasing complexity, following are example use cases that would
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be addressed with security contexts:
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1. Kubernetes is used to run a single cloud application. In order to protect
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nodes from containers:
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* All containers run as a single non-root user
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* Privileged containers are disabled
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* All containers run with a particular MCS label
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* Kernel capabilities like CHOWN and MKNOD are removed from containers
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2. Just like case #1, except that I have more than one application running on
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the Kubernetes cluster.
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* Each application is run in its own namespace to avoid name collisions
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* For each application a different uid and MCS label is used
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3. Kubernetes is used as the base for a PAAS with
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multiple projects, each project represented by a namespace.
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* Each namespace is associated with a range of uids/gids on the node that
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are mapped to uids/gids on containers using linux user namespaces.
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* Certain pods in each namespace have special privileges to perform system
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actions such as talking back to the server for deployment, run docker
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builds, etc.
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* External NFS storage is assigned to each namespace and permissions set
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using the range of uids/gids assigned to that namespace.
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## Proposed Design
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### Overview
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A *security context* consists of a set of constraints that determine how a container
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is secured before getting created and run. A security context resides on the container and represents the runtime parameters that will
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be used to create and run the container via container APIs. A *security context provider* is passed to the Kubelet so it can have a chance
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to mutate Docker API calls in order to apply the security context.
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It is recommended that this design be implemented in two phases:
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1. Implement the security context provider extension point in the Kubelet
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so that a default security context can be applied on container run and creation.
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2. Implement a security context structure that is part of a service account. The
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default context provider can then be used to apply a security context based
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on the service account associated with the pod.
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### Security Context Provider
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The Kubelet will have an interface that points to a `SecurityContextProvider`. The `SecurityContextProvider` is invoked before creating and running a given container:
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{% highlight go %}
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{% raw %}
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type SecurityContextProvider interface {
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// ModifyContainerConfig is called before the Docker createContainer call.
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// The security context provider can make changes to the Config with which
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// the container is created.
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// An error is returned if it's not possible to secure the container as
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// requested with a security context.
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ModifyContainerConfig(pod *api.Pod, container *api.Container, config *docker.Config)
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// ModifyHostConfig is called before the Docker runContainer call.
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// The security context provider can make changes to the HostConfig, affecting
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// security options, whether the container is privileged, volume binds, etc.
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// An error is returned if it's not possible to secure the container as requested
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// with a security context.
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ModifyHostConfig(pod *api.Pod, container *api.Container, hostConfig *docker.HostConfig)
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}
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{% endraw %}
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{% endhighlight %}
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If the value of the SecurityContextProvider field on the Kubelet is nil, the kubelet will create and run the container as it does today.
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### Security Context
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A security context resides on the container and represents the runtime parameters that will
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be used to create and run the container via container APIs. Following is an example of an initial implementation:
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{% highlight go %}
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{% raw %}
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type Container struct {
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... other fields omitted ...
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// Optional: SecurityContext defines the security options the pod should be run with
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SecurityContext *SecurityContext
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}
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// SecurityContext holds security configuration that will be applied to a container. SecurityContext
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// contains duplication of some existing fields from the Container resource. These duplicate fields
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// will be populated based on the Container configuration if they are not set. Defining them on
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// both the Container AND the SecurityContext will result in an error.
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type SecurityContext struct {
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// Capabilities are the capabilities to add/drop when running the container
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Capabilities *Capabilities
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// Run the container in privileged mode
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Privileged *bool
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// SELinuxOptions are the labels to be applied to the container
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// and volumes
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SELinuxOptions *SELinuxOptions
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// RunAsUser is the UID to run the entrypoint of the container process.
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RunAsUser *int64
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}
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// SELinuxOptions are the labels to be applied to the container.
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type SELinuxOptions struct {
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// SELinux user label
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User string
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// SELinux role label
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Role string
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// SELinux type label
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Type string
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// SELinux level label.
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Level string
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}
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{% endraw %}
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{% endhighlight %}
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### Admission
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It is up to an admission plugin to determine if the security context is acceptable or not. At the
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time of writing, the admission control plugin for security contexts will only allow a context that
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has defined capabilities or privileged. Contexts that attempt to define a UID or SELinux options
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will be denied by default. In the future the admission plugin will base this decision upon
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configurable policies that reside within the [service account](http://pr.k8s.io/2297).
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<!-- BEGIN MUNGE: IS_VERSIONED -->
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[]()
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<!-- END MUNGE: GENERATED_ANALYTICS -->
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