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191 lines
9.2 KiB
Markdown
191 lines
9.2 KiB
Markdown
# 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.md)):
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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](https://github.com/GoogleCloudPlatform/kubernetes/issues/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](https://github.com/GoogleCloudPlatform/kubernetes/pull/2297).
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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. It has a 1:1 correspondence to a
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[service account](https://github.com/GoogleCloudPlatform/kubernetes/pull/2297). 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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```go
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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.BoundPod, container *api.Container, config *docker.Config) error
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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.BoundPod, container *api.Container, hostConfig *docker.HostConfig)
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}
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```
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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 has a 1:1 correspondence to a service account and it can be included as
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part of the service account resource. Following is an example of an initial implementation:
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```go
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// SecurityContext specifies the security constraints associated with a service account
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type SecurityContext struct {
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// user is the uid to use when running the container
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User int
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// AllowPrivileged indicates whether this context allows privileged mode containers
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AllowPrivileged bool
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// AllowedVolumeTypes lists the types of volumes that a container can bind
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AllowedVolumeTypes []string
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// AddCapabilities is the list of Linux kernel capabilities to add
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AddCapabilities []string
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// RemoveCapabilities is the list of Linux kernel capabilities to remove
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RemoveCapabilities []string
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// Isolation specifies the type of isolation required for containers
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// in this security context
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Isolation ContainerIsolationSpec
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}
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// ContainerIsolationSpec indicates intent for container isolation
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type ContainerIsolationSpec struct {
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// Type is the container isolation type (None, Private)
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Type ContainerIsolationType
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// FUTURE: IDMapping specifies how users and groups from the host will be mapped
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IDMapping *IDMapping
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}
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// ContainerIsolationType is the type of container isolation for a security context
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type ContainerIsolationType string
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const (
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// ContainerIsolationNone means that no additional consraints are added to
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// containers to isolate them from their host
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ContainerIsolationNone ContainerIsolationType = "None"
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// ContainerIsolationPrivate means that containers are isolated in process
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// and storage from their host and other containers.
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ContainerIsolationPrivate ContainerIsolationType = "Private"
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)
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// IDMapping specifies the requested user and group mappings for containers
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// associated with a specific security context
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type IDMapping struct {
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// SharedUsers is the set of user ranges that must be unique to the entire cluster
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SharedUsers []IDMappingRange
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// SharedGroups is the set of group ranges that must be unique to the entire cluster
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SharedGroups []IDMappingRange
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// PrivateUsers are mapped to users on the host node, but are not necessarily
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// unique to the entire cluster
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PrivateUsers []IDMappingRange
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// PrivateGroups are mapped to groups on the host node, but are not necessarily
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// unique to the entire cluster
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PrivateGroups []IDMappingRange
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}
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// IDMappingRange specifies a mapping between container IDs and node IDs
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type IDMappingRange struct {
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// ContainerID is the starting container UID or GID
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ContainerID int
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// HostID is the starting host UID or GID
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HostID int
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// Length is the length of the UID/GID range
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Length int
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}
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```
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#### Security Context Lifecycle
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The lifecycle of a security context will be tied to that of a service account. It is expected that a service account with a default security context will be created for every Kubernetes namespace (without administrator intervention). If resources need to be allocated when creating a security context (for example, assign a range of host uids/gids), a pattern such as [finalizers](https://github.com/GoogleCloudPlatform/kubernetes/issues/3585) can be used before declaring the security context / service account / namespace ready for use.
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