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.
This tutorial shows some examples that are still alpha (since v1.22) 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"
],
"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 v1.23.
As an alpha 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.
Enable the use of RuntimeDefault as the default seccomp profile for all workloads
Kubernetes v1.22 [alpha]
SeccompDefault is an optional kubelet
feature gate as
well as corresponding --seccomp-default
command line flag.
Both have to be enabled simultaneously to use the feature.
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.
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
SeccompDefaultfeature 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.
More detailed information about a possible upgrade and downgrade strategy can be found in the related Kubernetes Enhancement Proposal (KEP).
Since the feature is in alpha state it is disabled per default. To enable it,
pass the flags --feature-gates=SeccompDefault=true --seccomp-default to the
kubelet CLI or enable it via 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
featureGates:
SeccompDefault: true
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:0.2.3
args:
- "-text=just made some syscalls!"
securityContext:
allowPrivilegeEscalation: falseseccomp.security.alpha.kubernetes.io/pod (for the whole pod) and
container.seccomp.security.alpha.kubernetes.io/[name] (for a single container)
is going to be removed with the release of Kubernetes v1.25. Please always use
the native API fields in favor of the annotations.
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 Services 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 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. Open up a new terminal window and tail the output for
calls from http-echo:
tail -f /var/log/syslog | grep 'http-echo'
You should already see some logs of syscalls made by http-echo, and if you
curl the endpoint in the control plane container you will see more written.
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.
Clean up that Pod and Service before moving to the next section:
kubectl delete service audit-pod --wait
kubectl delete pod audit-pod --wait --now
Create Pod with 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:0.2.3
args:
- "-text=just made some syscalls!"
securityContext:
allowPrivilegeEscalation: falseAttempt 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.
Clean up that Pod and Service before moving to the next section:
kubectl delete service violation-pod --wait
kubectl delete pod violation-pod --wait --now
Create Pod with seccomp profile that only allows necessary syscalls
If you take a look at the fine-pod.json, you will notice some of the syscalls
seen in 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:0.2.3
args:
- "-text=just made some syscalls!"
securityContext:
allowPrivilegeEscalation: falseCreate 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 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.
Clean up that Pod and Service before moving to the next section:
kubectl delete service fine-pod --wait
kubectl delete pod fine-pod --wait --now
Create 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 feature gate 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:0.2.3
args:
- "-text=just made some more syscalls!"
securityContext:
allowPrivilegeEscalation: falseCreate 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
Finally, now that you saw that work OK, clean up:
kubectl delete pod default-pod --wait --now
What's next
You can learn more about Linux seccomp: