mirror of https://github.com/k3s-io/k3s
[Proposal] Security Contexts
parent
919b2d20e0
commit
8b72dd9000
|
@ -0,0 +1,158 @@
|
||||||
|
# Security Contexts
|
||||||
|
## Abstract
|
||||||
|
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)):
|
||||||
|
|
||||||
|
1. Ensure a clear isolation between container and the underlying host it runs on
|
||||||
|
2. Limit the ability of the container to negatively impact the infrastructure or other containers
|
||||||
|
|
||||||
|
## Background
|
||||||
|
|
||||||
|
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.
|
||||||
|
|
||||||
|
## Motivation
|
||||||
|
|
||||||
|
### Container isolation
|
||||||
|
|
||||||
|
In order to improve container isolation from host and other containers running on the host, containers should only be
|
||||||
|
granted the access they need to perform their work. To this end it should be possible to take advantage of Docker
|
||||||
|
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)
|
||||||
|
to the container process.
|
||||||
|
|
||||||
|
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.
|
||||||
|
|
||||||
|
### External integration with shared storage
|
||||||
|
In order to support external integration with shared storage, processes running in a Kubernetes cluster
|
||||||
|
should be able to be uniquely identified by their Unix UID, such that a chain of ownership can be established.
|
||||||
|
Processes in pods will need to have consistent UID/GID/SELinux category labels in order to access shared disks.
|
||||||
|
|
||||||
|
## Constraints and Assumptions
|
||||||
|
* It is out of the scope of this document to prescribe a specific set
|
||||||
|
of constraints to isolate containers from their host. Different use cases need different
|
||||||
|
settings.
|
||||||
|
* The concept of a security context should not be tied to a particular security mechanism or platform
|
||||||
|
(ie. SELinux, AppArmor)
|
||||||
|
* Applying a different security context to a scope (namespace or pod) requires a solution such as the one proposed for
|
||||||
|
[service accounts](https://github.com/GoogleCloudPlatform/kubernetes/pull/2297).
|
||||||
|
|
||||||
|
## Use Cases
|
||||||
|
|
||||||
|
In order of increasing complexity, following are example use cases that would
|
||||||
|
be addressed with security contexts:
|
||||||
|
|
||||||
|
1. Kubernetes is used to run a single cloud application. In order to protect
|
||||||
|
nodes from containers:
|
||||||
|
* All containers run as a single non-root user
|
||||||
|
* Privileged containers are disabled
|
||||||
|
* All containers run with a particular MCS label
|
||||||
|
* Kernel capabilities like CHOWN and MKNOD are removed from containers
|
||||||
|
|
||||||
|
2. Just like case #1, except that I have more than one application running on
|
||||||
|
the Kubernetes cluster.
|
||||||
|
* Each application is run in its own namespace to avoid name collisions
|
||||||
|
* For each application a different uid and MCS label is used
|
||||||
|
|
||||||
|
3. Kubernetes is used as the base for a PAAS with
|
||||||
|
multiple projects, each project represented by a namespace.
|
||||||
|
* Each namespace is associated with a range of uids/gids on the node that
|
||||||
|
are mapped to uids/gids on containers using linux user namespaces.
|
||||||
|
* Certain pods in each namespace have special privileges to perform system
|
||||||
|
actions such as talking back to the server for deployment, run docker
|
||||||
|
builds, etc.
|
||||||
|
* External NFS storage is assigned to each namespace and permissions set
|
||||||
|
using the range of uids/gids assigned to that namespace.
|
||||||
|
|
||||||
|
## Proposed Design
|
||||||
|
|
||||||
|
### Overview
|
||||||
|
A *security context* consists of a set of constraints that determine how a container
|
||||||
|
is secured before getting created and run. It has a 1:1 correspondence to a
|
||||||
|
[service account](https://github.com/GoogleCloudPlatform/kubernetes/pull/2297). A *security context provider* is passed to the Kubelet so it can have a chance
|
||||||
|
to mutate Docker API calls in order to apply the security context.
|
||||||
|
|
||||||
|
It is recommended that this design be implemented in two phases:
|
||||||
|
|
||||||
|
1. Implement the security context provider extension point in the Kubelet
|
||||||
|
so that a default security context can be applied on container run and creation.
|
||||||
|
2. Implement a security context structure that is part of a service account. The
|
||||||
|
default context provider can then be used to apply a security context based
|
||||||
|
on the service account associated with the pod.
|
||||||
|
|
||||||
|
### Security Context Provider
|
||||||
|
|
||||||
|
The Kubelet will have an interface that points to a `SecurityContextProvider`. The `SecurityContextProvider` is invoked before creating and running a given container:
|
||||||
|
|
||||||
|
```go
|
||||||
|
type SecurityContextProvider interface {
|
||||||
|
// ModifyContainerConfig is called before the Docker createContainer call.
|
||||||
|
// The security context provider can make changes to the Config with which
|
||||||
|
// the container is created.
|
||||||
|
// An error is returned if it's not possible to secure the container as
|
||||||
|
// requested with a security context.
|
||||||
|
ModifyContainerConfig(pod *api.BoundPod, container *api.Container, config *docker.Config) error
|
||||||
|
|
||||||
|
// ModifyHostConfig is called before the Docker runContainer call.
|
||||||
|
// The security context provider can make changes to the HostConfig, affecting
|
||||||
|
// security options, whether the container is privileged, volume binds, etc.
|
||||||
|
// An error is returned if it's not possible to secure the container as requested
|
||||||
|
// with a security context.
|
||||||
|
ModifyHostConfig(pod *api.BoundPod, container *api.Container, hostConfig *docker.HostConfig)
|
||||||
|
}
|
||||||
|
```
|
||||||
|
If the value of the SecurityContextProvider field on the Kubelet is nil, the kubelet will create and run the container as it does today.
|
||||||
|
|
||||||
|
### Security Context
|
||||||
|
|
||||||
|
A security context has a 1:1 correspondence to a service account and it can be included as
|
||||||
|
part of the service account resource. Following is an example of an initial implementation:
|
||||||
|
|
||||||
|
```go
|
||||||
|
type SecurityContext struct {
|
||||||
|
// user is the uid to use when running the container
|
||||||
|
User int
|
||||||
|
|
||||||
|
// allowPrivileged indicates whether this context allows privileged mode containers
|
||||||
|
AllowPrivileged bool
|
||||||
|
|
||||||
|
// allowedVolumeTypes lists the types of volumes that a container can bind
|
||||||
|
AllowedVolumeTypes []string
|
||||||
|
|
||||||
|
// addCapabilities is the list of Linux kernel capabilities to add
|
||||||
|
AddCapabilities []string
|
||||||
|
|
||||||
|
// removeCapabilities is the list of Linux kernel capabilities to remove
|
||||||
|
RemoveCapabilities []string
|
||||||
|
|
||||||
|
// SELinux specific settings (optional)
|
||||||
|
SELinux *SELinuxContext
|
||||||
|
|
||||||
|
// AppArmor specific settings (optional)
|
||||||
|
AppArmor *AppArmorContext
|
||||||
|
|
||||||
|
// FUTURE:
|
||||||
|
// With Linux user namespace support, it should be possible to map
|
||||||
|
// a range of container uids/gids to arbitrary host uids/gids
|
||||||
|
// UserMappings []IDMapping
|
||||||
|
// GroupMappings []IDMapping
|
||||||
|
}
|
||||||
|
|
||||||
|
type SELinuxContext struct {
|
||||||
|
// MCS label/SELinux level to run the container under
|
||||||
|
Level string
|
||||||
|
|
||||||
|
// SELinux type label for container processes
|
||||||
|
Type string
|
||||||
|
|
||||||
|
// FUTURE:
|
||||||
|
// LabelVolumeMountsExclusive []Volume
|
||||||
|
// LabelVolumeMountsShared []Volume
|
||||||
|
}
|
||||||
|
|
||||||
|
type AppArmorContext struct {
|
||||||
|
// AppArmor profile
|
||||||
|
Profile string
|
||||||
|
}
|
||||||
|
```
|
||||||
|
|
||||||
|
#### Security Context Lifecycle
|
||||||
|
|
||||||
|
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.
|
Loading…
Reference in New Issue