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AppArmor

AppArmor is a Linux kernel enhancement that can reduce the potential attack surface of an application and provide greater defense in depth for Applications. Beta support for AppArmor was added in Kubernetes v1.4.

What is AppArmor

AppArmor is a Linux kernel security module that supplements the standard Linux user and group based permissions to confine programs to a limited set of resources. AppArmor can be configured for any application to reduce its potential attack surface and provide greater defense in depth. It is configured through profiles tuned to whitelist the access needed by a specific program or container, such as Linux capabilities, network access, file permissions, etc. Each profile can be run in either enforcing mode, which blocks access to disallowed resources, or complain mode, which only reports violations.

AppArmor can help you to run a more secure deployment by restricting what containers are allowed to do, and /or providing better auditing through system logs. However, it is important to keep in mind that AppArmor is not a silver bullet, and can only do so much to protect against exploits in your application code. It is important to provide good, restrictive profiles, and harden your applications and cluster from other angles as well.

AppArmor support in Kubernetes is currently in beta.

Prerequisites

  1. Kubernetes version is at least v1.4. Kubernetes support for AppArmor was added in v1.4. Kubernetes components older than v1.4 are not aware of the new AppArmor annotations, and will silently ignore any AppArmor settings that are provided. To ensure that your Pods are receiving the expected protections, it is important to verify the Kubelet version of your nodes:

     $ kubectl get nodes -o=jsonpath=$'{range .items[*]}{@.metadata.name}: {@.status.nodeInfo.kubeletVersion}\n{end}'
 gke-test-default-pool-239f5d02-gyn2: v1.4.0
 gke-test-default-pool-239f5d02-x1kf: v1.4.0
 gke-test-default-pool-239f5d02-xwux: v1.4.0

  2. AppArmor kernel module is enabled. For the Linux kernel to enforce an AppArmor profile, the AppArmor kernel module must be installed and enabled. Several distributions enable the module by default, such as Ubuntu and SUSE, and many others provide optional support. To check whether the module is enabled, check the /sys/module/apparmor/parameters/enabled file:

     $ cat /sys/module/apparmor/parameters/enabled
 Y

    If the Kubelet contains AppArmor support (>= v1.4), it will refuse to run a Pod with AppArmor options if the kernel module is not enabled.

    Note: Ubuntu carries many AppArmor patches that have not been merged into the upstream Linux kernel, including patches that add additional hooks and features. Kubernetes has only been tested with the upstream version, and does not promise support for other features.

  3. Container runtime is Docker. Currently the only Kubernetes-supported container runtime that also supports AppArmor is Docker. As more runtimes add AppArmor support, the options will be expanded. You can verify that your nodes are running docker with:

     $ kubectl get nodes -o=jsonpath=$'{range .items[*]}{@.metadata.name}: {@.status.nodeInfo.containerRuntimeVersion}\n{end}'
 gke-test-default-pool-239f5d02-gyn2: docker://1.11.2
 gke-test-default-pool-239f5d02-x1kf: docker://1.11.2
 gke-test-default-pool-239f5d02-xwux: docker://1.11.2

    If the Kubelet contains AppArmor support (>= v1.4), it will refuse to run a Pod with AppArmor options if the runtime is not Docker.

  4. Profile is loaded. AppArmor is applied to a Pod by specifying an AppArmor profile that each container should be run with. If any of the specified profiles is not already loaded in the kernel, the Kubelet (>= v1.4) will reject the Pod. You can view which profiles are loaded on a node by checking the /sys/kernel/security/apparmor/profiles file. For example:

     $ ssh gke-test-default-pool-239f5d02-gyn2 "sudo cat /sys/kernel/security/apparmor/profiles | sort"
 apparmor-test-deny-write (enforce)
 apparmor-test-audit-write (enforce)
 docker-default (enforce)
 k8s-nginx (enforce)

    For more details on loading profiles on nodes, see Setting up nodes with profiles.

As long as the Kubelet version includes AppArmor support (>= v1.4), the Kubelet will reject a Pod with AppArmor options if any of the prerequisites are not met. You can also verify AppArmor support on nodes by checking the node ready condition message (though this is likely to be removed in a later release):

$ kubectl get nodes -o=jsonpath=$'{range .items[*]}{@.metadata.name}: {.status.conditions[?(@.reason=="KubeletReady")].message}\n{end}'
gke-test-default-pool-239f5d02-gyn2: kubelet is posting ready status. AppArmor enabled
gke-test-default-pool-239f5d02-x1kf: kubelet is posting ready status. AppArmor enabled
gke-test-default-pool-239f5d02-xwux: kubelet is posting ready status. AppArmor enabled

Securing a Pod

Note: AppArmor is currently in beta, so options are specified as annotations. Once support graduates to general availability, the annotations will be replaced with first-class fields (more details in Upgrade path to GA).

AppArmor profiles are specified per-container. To specify the AppArmor profile to run a Pod container with, add an annotation to the Pod’s metadata:

container.apparmor.security.beta.kubernetes.io/<container_name>: <profile_ref>

Where <container_name> is the name of the container to apply the profile to, and <profile_ref> specifies the profile to apply. The profile_ref can be one of:

See the API Reference for the full details on the annotation and profile name formats.

The Kubernetes AppArmor enforcement works by first checking that all the prerequisites have been met, and then forwarding the profile selection to the container runtime for enforcement. If the prerequisites have not been met, the Pod will be rejected, and will not run.

To verify that the profile was applied, you can expect to see the AppArmor security option listed in the container created event:

$ kubectl get events | grep Created
22s        22s         1         hello-apparmor     Pod       spec.containers{hello}   Normal    Created     {kubelet e2e-test-stclair-minion-group-31nt}   Created container with docker id 269a53b202d3; Security:[seccomp=unconfined apparmor=k8s-apparmor-example-deny-write]

You can also verify directly that the container’s root process is running with the correct profile by checking its proc attr:

$ kubectl exec <pod_name> cat /proc/1/attr/current
k8s-apparmor-example-deny-write (enforce)

Example

In this example you’ll see:

This example assumes you have already set up a cluster with AppArmor support.

First, we need to load the profile we want to use onto our nodes. The profile we’ll use simply denies all file writes:

deny-write.profile 
#include <tunables/global>

profile k8s-apparmor-example-deny-write flags=(attach_disconnected) {
  #include <abstractions/base>

  file,

  # Deny all file writes.
  deny /** w,
}

Since we don’t know where the Pod will be scheduled, we’ll need to load the profile on all our nodes. For this example we’ll just use SSH to install the profiles, but other approaches are discussed in Setting up nodes with profiles.

$ NODES=(
    # The SSH-accessible domain names of your nodes
    gke-test-default-pool-239f5d02-gyn2.us-central1-a.my-k8s
    gke-test-default-pool-239f5d02-x1kf.us-central1-a.my-k8s
    gke-test-default-pool-239f5d02-xwux.us-central1-a.my-k8s)
$ for NODE in ${NODES[*]}; do ssh $NODE 'sudo apparmor_parser -q <<EOF
#include <tunables/global>

profile k8s-apparmor-example-deny-write flags=(attach_disconnected) {
  #include <abstractions/base>

  file,

  # Deny all file writes.
  deny /** w,
}
EOF'
done

Next, we’ll run a simple “Hello AppArmor” pod with the deny-write profile:

hello-apparmor-pod.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: hello-apparmor
  annotations:
    # Tell Kubernetes to apply the AppArmor profile "k8s-apparmor-example-deny-write".
    # Note that this is ignored if the Kubernetes node is not running version 1.4 or greater.
    container.apparmor.security.beta.kubernetes.io/hello: localhost/k8s-apparmor-example-deny-write
spec:
  containers:
  - name: hello
    image: busybox
    command: [ "sh", "-c", "echo 'Hello AppArmor!' && sleep 1h" ]

$ kubectl create -f /dev/stdin <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: hello-apparmor
  annotations:
    container.apparmor.security.beta.kubernetes.io/hello: localhost/k8s-apparmor-example-deny-write
spec:
  containers:
  - name: hello
    image: busybox
    command: [ "sh", "-c", "echo 'Hello AppArmor!' && sleep 1h" ]
EOF
pod "hello-apparmor" created

If we look at the pod events, we can see that the Pod container was created with the AppArmor profile “k8s-apparmor-example-deny-write”:

$ kubectl get events | grep hello-apparmor
14s        14s         1         hello-apparmor   Pod                                Normal    Scheduled   {default-scheduler }                           Successfully assigned hello-apparmor to gke-test-default-pool-239f5d02-gyn2
14s        14s         1         hello-apparmor   Pod       spec.containers{hello}   Normal    Pulling     {kubelet gke-test-default-pool-239f5d02-gyn2}   pulling image "busybox"
13s        13s         1         hello-apparmor   Pod       spec.containers{hello}   Normal    Pulled      {kubelet gke-test-default-pool-239f5d02-gyn2}   Successfully pulled image "busybox"
13s        13s         1         hello-apparmor   Pod       spec.containers{hello}   Normal    Created     {kubelet gke-test-default-pool-239f5d02-gyn2}   Created container with docker id 06b6cd1c0989; Security:[seccomp=unconfined apparmor=k8s-apparmor-example-deny-write]
13s        13s         1         hello-apparmor   Pod       spec.containers{hello}   Normal    Started     {kubelet gke-test-default-pool-239f5d02-gyn2}   Started container with docker id 06b6cd1c0989

We can verify that the container is actually running with that profile by checking its proc attr:

$ kubectl exec hello-apparmor cat /proc/1/attr/current
k8s-apparmor-example-deny-write (enforce)

Finally, we can see what happens if we try to violate the profile by writing to a file:

$ kubectl exec hello-apparmor touch /tmp/test
touch: /tmp/test: Permission denied
error: error executing remote command: command terminated with non-zero exit code: Error executing in Docker Container: 1

To wrap up, let’s look at what happens if we try to specify a profile that hasn’t been loaded:

$ kubectl create -f /dev/stdin <<EOF
apiVersion: v1
kind: Pod
metadata:
  name: hello-apparmor-2
  annotations:
    container.apparmor.security.beta.kubernetes.io/hello: localhost/k8s-apparmor-example-allow-write
spec:
  containers:
  - name: hello
    image: busybox
    command: [ "sh", "-c", "echo 'Hello AppArmor!' && sleep 1h" ]
EOF
pod "hello-apparmor-2" created

$ kubectl describe pod hello-apparmor-2
Name:          hello-apparmor-2
Namespace:     default
Node:          gke-test-default-pool-239f5d02-x1kf/
Start Time:    Tue, 30 Aug 2016 17:58:56 -0700
Labels:        <none>
Status:        Failed
Reason:        AppArmor
Message:       Pod Cannot enforce AppArmor: profile "k8s-apparmor-example-allow-write" is not loaded
IP:
Controllers:   <none>
Containers:
  hello:
    Image:     busybox
    Port:
    Command:
      sh
      -c
      echo 'Hello AppArmor!' && sleep 1h
    Requests:
      cpu:            100m
    Environment Variables:    <none>
Volumes:
  default-token-dnz7v:
    Type:    Secret (a volume populated by a Secret)
    SecretName:    default-token-dnz7v
QoS Tier:    Burstable
Events:
  FirstSeen    LastSeen    Count    From                        SubobjectPath    Type        Reason        Message
  ---------    --------    -----    ----                        -------------    --------    ------        -------
  23s          23s         1        {default-scheduler }                         Normal      Scheduled     Successfully assigned hello-apparmor-2 to e2e-test-stclair-minion-group-t1f5
  23s          23s         1        {kubelet e2e-test-stclair-minion-group-t1f5}             Warning        AppArmor    Cannot enforce AppArmor: profile "k8s-apparmor-example-allow-write" is not loaded

Note the pod status is Failed, with a helpful error message: Pod Cannot enforce AppArmor: profile "k8s-apparmor-example-allow-write" is not loaded. An event was also recorded with the same message.

Administration

Setting up nodes with profiles

Kubernetes does not currently provide any native mechanisms for loading AppArmor profiles onto nodes. There are lots of ways to setup the profiles though, such as:

The scheduler is not aware of which profiles are loaded onto which node, so the full set of profiles must be loaded onto every node. An alternative approach is to add a node label for each profile (or class of profiles) on the node, and use a node selector to ensure the Pod is run on a node with the required profile.

Restricting profiles with the PodSecurityPolicy

If the PodSecurityPolicy extension is enabled, cluster-wide AppArmor restrictions can be applied. To enable the PodSecurityPolicy, two flags must be set on the apiserver:

--admission-control=PodSecurityPolicy[,others...]
--runtime-config=extensions/v1beta1/podsecuritypolicy[,others...]

With the extension enabled, the AppArmor options can be specified as annotations on the PodSecurityPolicy:

apparmor.security.beta.kubernetes.io/defaultProfileName: <profile_ref>
apparmor.security.beta.kubernetes.io/allowedProfileNames: <profile_ref>[,others...]

The default profile name option specifies the profile to apply to containers by default when none is specified. The allowed profile names option specifies a list of profiles that Pod containers are allowed to be run with. If both options are provided, the default must be allowed. The profiles are specified in the same format as on containers. See the API Reference for the full specification.

Disabling AppArmor

If you do not want AppArmor to be available on your cluster, it can be disabled by a command-line flag:

--feature-gates=AppArmor=false

When disabled, any Pod that includes an AppArmor profile will fail validation with a “Forbidden” error. Note that by default docker always enables the “docker-default” profile on non-privileged pods (if the AppArmor kernel module is enabled), and will continue to do so even if the feature-gate is disabled. The option to disable AppArmor will be removed when AppArmor graduates to general availability (GA).

Upgrading to Kubernetes v1.4 with AppArmor

No action is required with respect to AppArmor to upgrade your cluster to v1.4. However, if any existing pods had an AppArmor annotation, they will not go through validation (or PodSecurityPolicy admission). If permissive profiles are loaded on the nodes, a malicious user could pre-apply a permissive profile to escalate the pod privileges above the docker-default. If this is a concern, it is recommended to scrub the cluster of any pods containing an annotation with apparmor.security.beta.kubernetes.io.

Upgrade path to General Availability

When AppArmor is ready to be graduated to general availability (GA), the options currently specified through annotations will be converted to fields. Supporting all the upgrade and downgrade paths through the transition is very nuanced, and will be explained in detail when the transition occurs. We will commit to supporting both fields and annotations for at least 2 releases, and will explicitly reject the annotations for at least 2 releases after that.

Authoring Profiles

Getting AppArmor profiles specified correctly can be a tricky business. Fortunately there are some tools to help with that:

It is recommended to run your application through Docker on a development workstation to generate the profiles, but there is nothing preventing running the tools on the Kubernetes node where your Pod is running.

To debug problems with AppArmor, you can check the system logs to see what, specifically, was denied. AppArmor logs verbose messages to dmesg, and errors can usually be found in the system logs or through journalctl. More information is provided in AppArmor failures.

Additional resources:

API Reference

Pod Annotation:

Specifying the profile a container will run with:

Profile Reference:

Any other profile reference format is invalid.

PodSecurityPolicy Annotations

Specifying the default profile to apply to containers when none is provided:

Specifying the list of profiles Pod containers is allowed to specify:

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