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Restrict a Container's Syscalls with seccomp
Kubernetes v1.19 [stable]
Seccomp stands for secure computing mode and has been a feature of the Linux kernel since version 2.6.12. It can be used to sandbox the privileges of a process, restricting the calls it is able to make from userspace into the kernel. Kubernetes lets you automatically apply seccomp profiles loaded onto a node to your Pods and containers.
Identifying the privileges required for your workloads can be difficult. In this tutorial, you will go through how to load seccomp profiles into a local Kubernetes cluster, how to apply them to a Pod, and how you can begin to craft profiles that give only the necessary privileges to your container processes.
Objectives
- Learn how to load seccomp profiles on a node
- Learn how to apply a seccomp profile to a container
- Observe auditing of syscalls made by a container process
- Observe behavior when a missing profile is specified
- Observe a violation of a seccomp profile
- Learn how to create fine-grained seccomp profiles
- Learn how to apply a container runtime default seccomp profile
Before you begin
In order to complete all steps in this tutorial, you must install kind and kubectl.
The commands used in the tutorial assume that you are using
Docker as your container runtime. (The cluster that kind
creates may
use a different container runtime internally). You could also use
Podman but in that case, you would have to follow specific
instructions in order to complete the tasks
successfully.
This tutorial shows some examples that are still beta (since v1.25) and others that use only generally available seccomp functionality. You should make sure that your cluster is configured correctly for the version you are using.
The tutorial also uses the curl
tool for downloading examples to your computer.
You can adapt the steps to use a different tool if you prefer.
privileged: true
set in the container's securityContext
. Privileged containers always
run as Unconfined
.
Download example seccomp profiles
The contents of these profiles will be explored later on, but for now go ahead
and download them into a directory named profiles/
so that they can be loaded
into the cluster.
{
"defaultAction": "SCMP_ACT_LOG"
}
{
"defaultAction": "SCMP_ACT_ERRNO"
}
{
"defaultAction": "SCMP_ACT_ERRNO",
"architectures": [
"SCMP_ARCH_X86_64",
"SCMP_ARCH_X86",
"SCMP_ARCH_X32"
],
"syscalls": [
{
"names": [
"accept4",
"epoll_wait",
"pselect6",
"futex",
"madvise",
"epoll_ctl",
"getsockname",
"setsockopt",
"vfork",
"mmap",
"read",
"write",
"close",
"arch_prctl",
"sched_getaffinity",
"munmap",
"brk",
"rt_sigaction",
"rt_sigprocmask",
"sigaltstack",
"gettid",
"clone",
"bind",
"socket",
"openat",
"readlinkat",
"exit_group",
"epoll_create1",
"listen",
"rt_sigreturn",
"sched_yield",
"clock_gettime",
"connect",
"dup2",
"epoll_pwait",
"execve",
"exit",
"fcntl",
"getpid",
"getuid",
"ioctl",
"mprotect",
"nanosleep",
"open",
"poll",
"recvfrom",
"sendto",
"set_tid_address",
"setitimer",
"writev",
"fstatfs",
"getdents64",
"pipe2",
"getrlimit"
],
"action": "SCMP_ACT_ALLOW"
}
]
}
Run these commands:
mkdir ./profiles
curl -L -o profiles/audit.json https://k8s.io/examples/pods/security/seccomp/profiles/audit.json
curl -L -o profiles/violation.json https://k8s.io/examples/pods/security/seccomp/profiles/violation.json
curl -L -o profiles/fine-grained.json https://k8s.io/examples/pods/security/seccomp/profiles/fine-grained.json
ls profiles
You should see three profiles listed at the end of the final step:
audit.json fine-grained.json violation.json
Create a local Kubernetes cluster with kind
For simplicity, kind can be used to create a single node cluster with the seccomp profiles loaded. Kind runs Kubernetes in Docker, so each node of the cluster is a container. This allows for files to be mounted in the filesystem of each container similar to loading files onto a node.
apiVersion: kind.x-k8s.io/v1alpha4
kind: Cluster
nodes:
- role: control-plane
extraMounts:
- hostPath: "./profiles"
containerPath: "/var/lib/kubelet/seccomp/profiles"
Download that example kind configuration, and save it to a file named kind.yaml
:
curl -L -O https://k8s.io/examples/pods/security/seccomp/kind.yaml
You can set a specific Kubernetes version by setting the node's container image. See Nodes within the kind documentation about configuration for more details on this. This tutorial assumes you are using Kubernetes v1.29.
As a beta feature, you can configure Kubernetes to use the profile that the
container runtime
prefers by default, rather than falling back to Unconfined
.
If you want to try that, see
enable the use of RuntimeDefault
as the default seccomp profile for all workloads
before you continue.
Once you have a kind configuration in place, create the kind cluster with that configuration:
kind create cluster --config=kind.yaml
After the new Kubernetes cluster is ready, identify the Docker container running as the single node cluster:
docker ps
You should see output indicating that a container is running with name
kind-control-plane
. The output is similar to:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
6a96207fed4b kindest/node:v1.18.2 "/usr/local/bin/entr…" 27 seconds ago Up 24 seconds 127.0.0.1:42223->6443/tcp kind-control-plane
If observing the filesystem of that container, you should see that the
profiles/
directory has been successfully loaded into the default seccomp path
of the kubelet. Use docker exec
to run a command in the Pod:
# Change 6a96207fed4b to the container ID you saw from "docker ps"
docker exec -it 6a96207fed4b ls /var/lib/kubelet/seccomp/profiles
audit.json fine-grained.json violation.json
You have verified that these seccomp profiles are available to the kubelet running within kind.
Create a Pod that uses the container runtime default seccomp profile
Most container runtimes provide a sane set of default syscalls that are allowed
or not. You can adopt these defaults for your workload by setting the seccomp
type in the security context of a pod or container to RuntimeDefault
.
seccompDefault
configuration
enabled, then Pods use the RuntimeDefault
seccomp profile whenever
no other seccomp profile is specified. Otherwise, the default is Unconfined
.
Here's a manifest for a Pod that requests the RuntimeDefault
seccomp profile
for all its containers:
apiVersion: v1
kind: Pod
metadata:
name: default-pod
labels:
app: default-pod
spec:
securityContext:
seccompProfile:
type: RuntimeDefault
containers:
- name: test-container
image: hashicorp/http-echo:1.0
args:
- "-text=just made some more syscalls!"
securityContext:
allowPrivilegeEscalation: false
Create that Pod:
kubectl apply -f https://k8s.io/examples/pods/security/seccomp/ga/default-pod.yaml
kubectl get pod default-pod
The Pod should be showing as having started successfully:
NAME READY STATUS RESTARTS AGE
default-pod 1/1 Running 0 20s
Delete the Pod before moving to the next section:
kubectl delete pod default-pod --wait --now
Create a Pod with a seccomp profile for syscall auditing
To start off, apply the audit.json
profile, which will log all syscalls of the
process, to a new Pod.
Here's a manifest for that Pod:
apiVersion: v1
kind: Pod
metadata:
name: audit-pod
labels:
app: audit-pod
spec:
securityContext:
seccompProfile:
type: Localhost
localhostProfile: profiles/audit.json
containers:
- name: test-container
image: hashicorp/http-echo:1.0
args:
- "-text=just made some syscalls!"
securityContext:
allowPrivilegeEscalation: false
.spec.securityContext
to configure seccomp, and this tutorial explains that
approach.
Create the Pod in the cluster:
kubectl apply -f https://k8s.io/examples/pods/security/seccomp/ga/audit-pod.yaml
This profile does not restrict any syscalls, so the Pod should start successfully.
kubectl get pod audit-pod
NAME READY STATUS RESTARTS AGE
audit-pod 1/1 Running 0 30s
In order to be able to interact with this endpoint exposed by this container, create a NodePort Service that allows access to the endpoint from inside the kind control plane container.
kubectl expose pod audit-pod --type NodePort --port 5678
Check what port the Service has been assigned on the node.
kubectl get service audit-pod
The output is similar to:
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
audit-pod NodePort 10.111.36.142 <none> 5678:32373/TCP 72s
Now you can use curl
to access that endpoint from inside the kind control plane container,
at the port exposed by this Service. Use docker exec
to run the curl
command within the
container belonging to that control plane container:
# Change 6a96207fed4b to the control plane container ID and 32373 to the port number you saw from "docker ps"
docker exec -it 6a96207fed4b curl localhost:32373
just made some syscalls!
You can see that the process is running, but what syscalls did it actually make?
Because this Pod is running in a local cluster, you should be able to see those
in /var/log/syslog
on your local system. Open up a new terminal window and tail
the output for
calls from http-echo
:
# The log path on your computer might be different from "/var/log/syslog"
tail -f /var/log/syslog | grep 'http-echo'
You should already see some logs of syscalls made by http-echo
, and if you run curl
again inside
the control plane container you will see more output written to the log.
For example:
Jul 6 15:37:40 my-machine kernel: [369128.669452] audit: type=1326 audit(1594067860.484:14536): auid=4294967295 uid=0 gid=0 ses=4294967295 pid=29064 comm="http-echo" exe="/http-echo" sig=0 arch=c000003e syscall=51 compat=0 ip=0x46fe1f code=0x7ffc0000
Jul 6 15:37:40 my-machine kernel: [369128.669453] audit: type=1326 audit(1594067860.484:14537): auid=4294967295 uid=0 gid=0 ses=4294967295 pid=29064 comm="http-echo" exe="/http-echo" sig=0 arch=c000003e syscall=54 compat=0 ip=0x46fdba code=0x7ffc0000
Jul 6 15:37:40 my-machine kernel: [369128.669455] audit: type=1326 audit(1594067860.484:14538): auid=4294967295 uid=0 gid=0 ses=4294967295 pid=29064 comm="http-echo" exe="/http-echo" sig=0 arch=c000003e syscall=202 compat=0 ip=0x455e53 code=0x7ffc0000
Jul 6 15:37:40 my-machine kernel: [369128.669456] audit: type=1326 audit(1594067860.484:14539): auid=4294967295 uid=0 gid=0 ses=4294967295 pid=29064 comm="http-echo" exe="/http-echo" sig=0 arch=c000003e syscall=288 compat=0 ip=0x46fdba code=0x7ffc0000
Jul 6 15:37:40 my-machine kernel: [369128.669517] audit: type=1326 audit(1594067860.484:14540): auid=4294967295 uid=0 gid=0 ses=4294967295 pid=29064 comm="http-echo" exe="/http-echo" sig=0 arch=c000003e syscall=0 compat=0 ip=0x46fd44 code=0x7ffc0000
Jul 6 15:37:40 my-machine kernel: [369128.669519] audit: type=1326 audit(1594067860.484:14541): auid=4294967295 uid=0 gid=0 ses=4294967295 pid=29064 comm="http-echo" exe="/http-echo" sig=0 arch=c000003e syscall=270 compat=0 ip=0x4559b1 code=0x7ffc0000
Jul 6 15:38:40 my-machine kernel: [369188.671648] audit: type=1326 audit(1594067920.488:14559): auid=4294967295 uid=0 gid=0 ses=4294967295 pid=29064 comm="http-echo" exe="/http-echo" sig=0 arch=c000003e syscall=270 compat=0 ip=0x4559b1 code=0x7ffc0000
Jul 6 15:38:40 my-machine kernel: [369188.671726] audit: type=1326 audit(1594067920.488:14560): auid=4294967295 uid=0 gid=0 ses=4294967295 pid=29064 comm="http-echo" exe="/http-echo" sig=0 arch=c000003e syscall=202 compat=0 ip=0x455e53 code=0x7ffc0000
You can begin to understand the syscalls required by the http-echo
process by
looking at the syscall=
entry on each line. While these are unlikely to
encompass all syscalls it uses, it can serve as a basis for a seccomp profile
for this container.
Delete the Service and the Pod before moving to the next section:
kubectl delete service audit-pod --wait
kubectl delete pod audit-pod --wait --now
Create a Pod with a seccomp profile that causes violation
For demonstration, apply a profile to the Pod that does not allow for any syscalls.
The manifest for this demonstration is:
apiVersion: v1
kind: Pod
metadata:
name: violation-pod
labels:
app: violation-pod
spec:
securityContext:
seccompProfile:
type: Localhost
localhostProfile: profiles/violation.json
containers:
- name: test-container
image: hashicorp/http-echo:1.0
args:
- "-text=just made some syscalls!"
securityContext:
allowPrivilegeEscalation: false
Attempt to create the Pod in the cluster:
kubectl apply -f https://k8s.io/examples/pods/security/seccomp/ga/violation-pod.yaml
The Pod creates, but there is an issue. If you check the status of the Pod, you should see that it failed to start.
kubectl get pod violation-pod
NAME READY STATUS RESTARTS AGE
violation-pod 0/1 CrashLoopBackOff 1 6s
As seen in the previous example, the http-echo
process requires quite a few
syscalls. Here seccomp has been instructed to error on any syscall by setting
"defaultAction": "SCMP_ACT_ERRNO"
. This is extremely secure, but removes the
ability to do anything meaningful. What you really want is to give workloads
only the privileges they need.
Delete the Pod before moving to the next section:
kubectl delete pod violation-pod --wait --now
Create a Pod with a seccomp profile that only allows necessary syscalls
If you take a look at the fine-grained.json
profile, you will notice some of the syscalls
seen in syslog of the first example where the profile set "defaultAction": "SCMP_ACT_LOG"
. Now the profile is setting "defaultAction": "SCMP_ACT_ERRNO"
,
but explicitly allowing a set of syscalls in the "action": "SCMP_ACT_ALLOW"
block. Ideally, the container will run successfully and you will see no messages
sent to syslog
.
The manifest for this example is:
apiVersion: v1
kind: Pod
metadata:
name: fine-pod
labels:
app: fine-pod
spec:
securityContext:
seccompProfile:
type: Localhost
localhostProfile: profiles/fine-grained.json
containers:
- name: test-container
image: hashicorp/http-echo:1.0
args:
- "-text=just made some syscalls!"
securityContext:
allowPrivilegeEscalation: false
Create the Pod in your cluster:
kubectl apply -f https://k8s.io/examples/pods/security/seccomp/ga/fine-pod.yaml
kubectl get pod fine-pod
The Pod should be showing as having started successfully:
NAME READY STATUS RESTARTS AGE
fine-pod 1/1 Running 0 30s
Open up a new terminal window and use tail
to monitor for log entries that
mention calls from http-echo
:
# The log path on your computer might be different from "/var/log/syslog"
tail -f /var/log/syslog | grep 'http-echo'
Next, expose the Pod with a NodePort Service:
kubectl expose pod fine-pod --type NodePort --port 5678
Check what port the Service has been assigned on the node:
kubectl get service fine-pod
The output is similar to:
NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE
fine-pod NodePort 10.111.36.142 <none> 5678:32373/TCP 72s
Use curl
to access that endpoint from inside the kind control plane container:
# Change 6a96207fed4b to the control plane container ID and 32373 to the port number you saw from "docker ps"
docker exec -it 6a96207fed4b curl localhost:32373
just made some syscalls!
You should see no output in the syslog
. This is because the profile allowed all
necessary syscalls and specified that an error should occur if one outside of
the list is invoked. This is an ideal situation from a security perspective, but
required some effort in analyzing the program. It would be nice if there was a
simple way to get closer to this security without requiring as much effort.
Delete the Service and the Pod before moving to the next section:
kubectl delete service fine-pod --wait
kubectl delete pod fine-pod --wait --now
Enable the use of RuntimeDefault
as the default seccomp profile for all workloads
Kubernetes v1.27 [stable]
To use seccomp profile defaulting, you must run the kubelet with the
--seccomp-default
command line flag
enabled for each node where you want to use it.
If enabled, the kubelet will use the RuntimeDefault
seccomp profile by default, which is
defined by the container runtime, instead of using the Unconfined
(seccomp disabled) mode.
The default profiles aim to provide a strong set
of security defaults while preserving the functionality of the workload. It is
possible that the default profiles differ between container runtimes and their
release versions, for example when comparing those from CRI-O and containerd.
securityContext.seccompProfile
API field nor add the deprecated annotations of
the workload. This provides users the possibility to rollback anytime without
actually changing the workload configuration. Tools like
crictl inspect
can be used to
verify which seccomp profile is being used by a container.
Some workloads may require a lower amount of syscall restrictions than others.
This means that they can fail during runtime even with the RuntimeDefault
profile. To mitigate such a failure, you can:
- Run the workload explicitly as
Unconfined
. - Disable the
SeccompDefault
feature for the nodes. Also making sure that workloads get scheduled on nodes where the feature is disabled. - Create a custom seccomp profile for the workload.
If you were introducing this feature into production-like cluster, the Kubernetes project recommends that you enable this feature gate on a subset of your nodes and then test workload execution before rolling the change out cluster-wide.
You can find more detailed information about a possible upgrade and downgrade strategy in the related Kubernetes Enhancement Proposal (KEP): Enable seccomp by default.
Kubernetes 1.29 lets you configure the seccomp profile that applies when the spec for a Pod doesn't define a specific seccomp profile. However, you still need to enable this defaulting for each node where you would like to use it.
If you are running a Kubernetes 1.29 cluster and want to
enable the feature, either run the kubelet with the --seccomp-default
command
line flag, or enable it through the kubelet configuration
file. To enable the
feature gate in kind, ensure that kind
provides
the minimum required Kubernetes version and enables the SeccompDefault
feature
in the kind configuration:
kind: Cluster
apiVersion: kind.x-k8s.io/v1alpha4
nodes:
- role: control-plane
image: kindest/node:v1.28.0@sha256:9f3ff58f19dcf1a0611d11e8ac989fdb30a28f40f236f59f0bea31fb956ccf5c
kubeadmConfigPatches:
- |
kind: JoinConfiguration
nodeRegistration:
kubeletExtraArgs:
seccomp-default: "true"
- role: worker
image: kindest/node:v1.28.0@sha256:9f3ff58f19dcf1a0611d11e8ac989fdb30a28f40f236f59f0bea31fb956ccf5c
kubeadmConfigPatches:
- |
kind: JoinConfiguration
nodeRegistration:
kubeletExtraArgs:
seccomp-default: "true"
If the cluster is ready, then running a pod:
kubectl run --rm -it --restart=Never --image=alpine alpine -- sh
Should now have the default seccomp profile attached. This can be verified by
using docker exec
to run crictl inspect
for the container on the kind
worker:
docker exec -it kind-worker bash -c \
'crictl inspect $(crictl ps --name=alpine -q) | jq .info.runtimeSpec.linux.seccomp'
{
"defaultAction": "SCMP_ACT_ERRNO",
"architectures": ["SCMP_ARCH_X86_64", "SCMP_ARCH_X86", "SCMP_ARCH_X32"],
"syscalls": [
{
"names": ["..."]
}
]
}
What's next
You can learn more about Linux seccomp: