Secure supply chain
A secure supply chain infrastructure can verify the validity of its parts, or links. It lets users and developers show the chain of custody of its software components, or artifacts. It's an active approach to mitigate security issues.
The Sigstore project provides tools and infrastructure for this. It's for validating the integrity of the artifact supply chain.
Kubewarden uses cosign
together with the fulcio and rekor infrastructure offered by the Sigstore project.
Cluster operators can configure Kubewarden to only run policies signed by trusted entities. Policy developers can sign their policies and publish them in a registry.
Prerequisites
In the following sections, you need a few tools to be installed.
These are so users can sign and verify OCI artifacts signatures.
The examples show the use of cosign
and kwctl
utilities for signing and inspecting policies.
Users may also want to use GitHub to sign their policies. In which case, they need to install Github actions
Keyless signing uses the default fulcio and rekor instances provided by the Sigstore project. Check the Sigstore documentation for details on how to use your own infrastructure for this, if needed.
Signing policies
Kubewarden recommends using Sigstore's cosign utility to signing policies.
This section shows a key-based method of signing policies.
Users need to generate a private-public key-pair for this.
The generated keys help to verify if the signed artifacts came from the expected user.
To generate this key-pair use this cosign generate-key-pair
command:
cosign generate-key-pair
Resulting in a prompt to type and verify a password:
Enter password for private key: ●●●●●●●●
Enter password for private key again: ●●●●●●●●
Private key written to cosign.key
Public key written to cosign.pub
Now you can use this key to sign policies.
The private key file, cosign.key
, shouldn't be shared.
This is a secret file only for use by the key owner for signing policies.
To sign a policy you can use cosign sign
passing the --key
command line argument with your private key file:
cosign sign --key cosign.key ghcr.io/kubewarden/policies/user-group-psp:latest
Resulting in a prompt for the password, for the specified private key:
an error occurred: no provider found for that key reference, will try to load key from disk...
Enter password for private key: ●●●●●●●●
Pushing signature to: ghcr.io/kubewarden/policies/user-group-psp
This command signs the policy by creating a new signature object. The signature object is then uploaded into the registry, with the policy. Now the policy is ready to use in a Kubewarden installation using signature verification.
The same policy can be signed multiple times, by the same user or different ones. These signatures are added to the signature object along with the original signature.
For more information about how the signing process works, check out the Sigstore project documentation.
Keyless signing
Often policies are automatically built in CI/CD pipelines. This complicates the key generation process. This Sigstore keyless workflow is for these situations. Instead of using long-lived singing keys, the keyless workflow uses certificate authorities (CAs) and certificate chains.
A short-lived certificate key is generated, and linked into a chain of trust. It's done by an identity challenge to confirm the signer's identity. The life of the certificate key is long enough for the signing to occur. The identity challenge is done by authenticating against an OpenID Connect (OIDC) provider. Sigstore's Fulcio public infrastructure is used for the chain of trust.
Signing uses Sigstore's cosign utility.
$ cosign sign ghcr.io/kubewarden/policies/user-group-psp:latest
cosign
output
Generating ephemeral keys...
Retrieving signed certificate...
Your browser will now be opened to:
https://oauth2.sigstore.dev/auth/auth?access_type=online&client_id=sigstore&code_challenge=<REDACTED>&code_challenge_method=S256&nonce=<REDACTED>&redirect_uri=http%3A%2F%2Flocalhost%3A34021%2Fauth%2Fcallback&response_type=code&scope=openid+email&state=<REDACTED>
client.go:196: root pinning is not supported in Spec 1.0.19
Successfully verified SCT...
tlog entry created with index: 1819248
Pushing signature to: ghcr.io/kubewarden/policies/user-group-psp
This signs the policy and pushes it to the repository. There are no keys generated as a byproduct.
How to sign artifacts in GitHub workflows
When using keyless signing, in a GitHub action,
cosign
doesn't need the user to log in to an OIDC provider.
A GitHub token is available during the execution of the GitHub workflow.
It's used to authenticate the user and generate the ephemeral keys.
The signing process is the same used in the keyless mode.
This is an example of how the Kubewarden project signs its policies:
YAML describing Kubewarden policy signing
# ... beginning of the workflow file ...
jobs:
build:
name: Build container image
runs-on: ubuntu-latest
steps:
# ... other steps building the container image ...
-
name: Login to GitHub Container Registry
uses: docker/login-action@v1
with:
registry: ghcr.io
username: ${{ github.repository_owner }}
password: ${{ inputs.GITHUB_TOKEN }}
-
name: Publish Wasm policy artifact to OCI registry with the 'latest' tag
shell: bash
if: ${{ startsWith(github.ref, 'refs/heads/') }}
env:
COSIGN_EXPERIMENTAL: 1
run: |
set -ex
echo Pushing policy to OCI container registry
IMMUTABLE_REF=$(kwctl push -o json ${{ PATH_TO_BUILT_WASM_FILE }} ghcr.io/myorg/policies/my-great-policy:latest | jq -r .immutable_ref)
echo Keyless signing of policy using cosign
cosign sign ${IMMUTABLE_REF}
# ... other build steps ...
# ... remainder of the workflow file ...
Policy developers can use the Kubewarden policy templates. They have GitHub actions to build, test, sign and publish policies.
Listing policy signatures
You can check signature in a published policy with kwctl inspect
.
This shows the information about the policy and its signatures as shown below:
kwctl inspect registry://ghcr.io/kubewarden/policies/us....
$ kwctl inspect registry://ghcr.io/kubewarden/policies/user-group-psp:v0.2.0
Details
title: psp-user-group
description: Short description
author: José Guilherme Vanz <jguilhermevanz@suse.com>
url: https://github.com/kubewarden/user-group-psp-policy
source: https://github.com/kubewarden/user-group-psp-policy
license: Apache-2.0
mutating: true
context aware: false
execution mode: kubewarden-wapc
protocol version: 1
Annotations
io.kubewarden.kwctl 0.2.5-rc2
Rules
────────────────────
---
- apiGroups:
- ""
apiVersions:
- v1
resources:
- pods
operations:
- CREATE
────────────────────
Usage
This policy enforce the user and group used in the container.
Sigstore signatures
Digest: sha256:026af67682a85d424e7d95db460171635f5c3957d67b53499bece912cc0413cc
Media type: application/vnd.dev.cosign.simplesigning.v1+json
Size: 258
Annotations
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-----END CERTIFICATE-----
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dev.sigstore.cosign/chain -----BEGIN CERTIFICATE-----
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-----END CERTIFICATE-----
dev.cosignproject.cosign/signature MEYCIQCW5fQgPTQ7ZNSnFHslrNNQkKgSIQi8cR159PA1sK8TiQIhAJwL9bOqBJmWn7YKdoSzo4slOgk8JBBjqXdsrt3ryQtB
Verifying policies
You can check if a policy is correctly signed with cosign
or kwctl
.
They have similar command line options for checking policy signatures.
To check if the binary is signed with a key, use kwctl
like this:
$ kwctl verify -k cosign.pub ghcr.io/kubewarden/policies/user-group-psp:latest
2022-03-29T14:49:31.878180Z INFO kwctl::verify: Policy successfully verified
Or cosign
:
$ cosign verify --key cosign.pub ghcr.io/kubewarden/policies/user-group-psp:latest
Verification for ghcr.io/kubewarden/policies/user-group-psp:latest --
The following checks were performed on each of these signatures:
- The cosign claims were validated
- The signatures were verified against the specified public key
- Any certificates were verified against the Fulcio roots.
[{"critical":{"identity":{"docker-reference":"ghcr.io/kubewarden/policies/user-group-psp"},"image":{"docker-manifest-digest":"sha256:af520a8ccee03811d426c48634b7007f1220c121cc23e14962bb64510585ce97"},"type":"cosign container image signature"},"optional":null}]
Configuring the policy server to check policy signatures
You can configure Kubewarden with a ConfigMap
to only run trusted policies.
The ConfigMap
structure described in Signature Config Reference.
It's used to verify a policy using kwctl
.
The ConfigMap
should define allowable configurations under the verification-config
field.
For example, you want to run policies signed by the Kubewarden GitHub organization. Then a sample ConfigMap
for this scenario would be:
$ cat kubewarden_signatures.yaml
$ cat kubewarden_signatures.yaml
apiVersion: v1
allOf:
- kind: githubAction
owner: kubewarden
# note that the data is stored under verification-config field
$ kubectl create configmap my-signatures-configuration --from-file=verification-config=kubewarden_signatures.yaml
$ kubectl get configmap -o yaml my-signatures-configuration
apiVersion: v1
data:
verification-config: |
apiVersion: v1
allOf:
- kind: githubAction
owner: kubewarden
kind: ConfigMap
metadata:
creationTimestamp: "2022-03-29T18:27:20Z"
name: my-signatures-configuration
namespace: default
resourceVersion: "10279"
uid: d53e1c56-1fee-45de-92f5-9bd73b8cead4
You can use kwctl scaffold verification-config
to generate a default verification configuration file for the ConfigMap
:
$ kwctl scaffold verification-config > verification_config.yaml
$ cat verification_config.yaml
$ kwctl scaffold verification-config > verification_config.yaml
$ cat verification_config.yaml
# Default Kubewarden verification config
#
# With this config, the only valid policies are those signed by Kubewarden
# infrastructure.
#
# This config can be saved to its default location (for this OS) with:
# kwctl scaffold verification-config > /home/kubewarden/.config/kubewarden/verification-config.yml
#
# Providing a config in the default location enables Sigstore verification.
# See https://docs.kubewarden.io for more Sigstore verification options.
---
apiVersion: v1
allOf:
- kind: githubAction
owner: kubewarden
repo: ~
annotations: ~
anyOf: ~
You can use this verification_config.yml
to create the ConfigMap
.
$ kubectl create configmap my-signatures-configuration --from-file==verification_config.yaml
configmap/my-signatures-configuration created
Then we can inspect with get configmap
.
kubectl get configmap
$ kubectl get configmap -o yaml my-signatures-configuration
apiVersion: v1
data:
verification-config: |+
# Default Kubewarden verification config
#
# With this config, the only valid policies are those signed by Kubewarden
# infrastructure.
#
# This config can be saved to its default location (for this OS) with:
# kwctl scaffold verification-config > /home/kubewarden/.config/kubewarden/verification-config.yml
#
# Providing a config in the default location enables Sigstore verification.
# See https://docs.kubewarden.io for more Sigstore verification options.
---
apiVersion: v1
allOf:
- kind: githubAction
owner: kubewarden
repo: ~
annotations: ~
anyOf: ~
kind: ConfigMap
metadata:
creationTimestamp: "2022-04-07T11:54:27Z"
name: my-signatures-configuration
namespace: default
resourceVersion: "1317"
uid: 74dec846-7fcd-4b4b-8184-700c816f685a
After creating the ConfigMap
to store the signature requirements, you can configure a Policy Server.
To start validating policy signatures by setting the ConfigMap
name in the highlighted field verificationConfig
.
apiVersion: policies.kubewarden.io/v1alpha2
kind: PolicyServer
metadata:
name: default
finalizers:
- kubewarden
spec:
image: ghcr.io/kubewarden/policy-server:v0.2.7
serviceAccountName: policy-server
replicas: 1
verificationConfig: your_configmap #name of the confimap with the signatures requirements
env:
- name: KUBEWARDEN_ENABLE_METRICS
value: "1"
- name: KUBEWARDEN_LOG_FMT
value: otlp
- name: "KUBEWARDEN_LOG_LEVEL"
value: "info"
If you deploy the default Policy Server using the kubewarden-defaults
Helm chart then you configure this field by setting the ConfigMap
name in the
policyServer.verificationConfig
value.
Now, the PolicyServer rejects untrusted AdmissionPolicies and ClusterAdmissionPolicies by refusing to start. You need to remove the untrusted policy, or change the signatures requirement, for a running PolicyServer.
Signature configuration reference
You can validate signature requirements contained in a file. Here is an example:
A file of signature requirements
apiVersion: v1
allOf:
- kind: githubAction
owner: kubewarden # mandatory
annotations:
env: prod
anyOf: # at least `anyOf.minimumMatches` are required to match
minimumMatches: 2 # default is 1
signatures:
- kind: pubKey
owner: flavio # optional
key: .... # mandatory
annotations: # optional
env: prod
foo: bar
- kind: pubKey
owner: victor # optional
key: .... # mandatory
- kind: genericIssuer
issuer: https://github.com/login/oauth
subject:
equal: alice@example.com
- kind: genericIssuer
issuer: https://token.actions.githubusercontent.com
subject:
equal: https://github.com/flavio/policy-secure-pod-images/.github/workflows/release.yml@refs/heads/main
- kind: genericIssuer
issuer: https://token.actions.githubusercontent.com
subject:
urlPrefix: https://github.com/flavio/
- kind: genericIssuer
issuer: https://token.actions.githubusercontent.com
subject:
urlPrefix: https://github.com/kubewarden # <- it will be post-fixed with `/` for security reasons
- kind: githubAction
owner: flavio # mandatory
repo: policy1 # optional
- kind: pubKey
owner: alice # optional
key: .... # mandatory
Signature validation
The configuration above contains the two highlighted sections, allOf
and anyOf
:
-
allOf
: The policy is trusted only if all signature requirements here are valid. -
anyOf
: The policy is trusted if theminimumMatches
criterion is met. Above, theminimumMatches
field is 2. So, at least two of the signature requirements must be met. The default value forminimumMatches
field is1
.
All the signatures requirements from allOf
and the minimum number from anyOf
must be met.
Public key validation
To check a policy is signed with the correct public key, you specify the key data and the owner of the key.
In this example, kind
is set to pubKey
and the key
has the public key.
The owner field is optional, but can be useful to clarify who owns the key.
- kind: pubKey
owner: bob # optional
key: |
-----BEGIN PUBLIC KEY-----
MBFKHFDGHKIJH0CAQYIKoZIzj0DAQcDQgAEX0HFTtCfTtPmkx5p1RbDE6HJSGAVD
BVDF6SKFSF87AASUspkQsN3FO4iyWodCy5j3o0CdIJD/KJHDJFHDFIu6sA==
-----END PUBLIC KEY-----
Keyless signature validation
A policy signed in keyless mode doesn't have a public key we can verify.
You can still verify the policy with the OIDC data used during the signing process.
For that, it's necessary to define the signature validation as genericIssuer
.
It's possible to verify information from the signature:
issuer
(mandatory): this matches theIssuer
attribute in the certificate generated by Fulcio. This shows the OIDC used to sign the policy.subject
: field used to match theSubject
attribute in Fulcio's certificate. TheSubject
(Fulcio) field contains the user used to authenticate against the OIDC provider. The verification field,subject
, can have one of two sub fields:equal
: theSubject
(Fulcio) from the certificate must be equal to the value in the signature validation;urlPrefix
: the certificate'sSubject
(Fulcio) field value must be prefixed by the value defined in the signature validation.
Both the cosign verify
and the kwctl inspect
can show information about keyless signatures.
For example, this configuration means the policy must have a keyless signature from Alice using the GitHub OIDC:
- kind: genericIssuer
issuer: https://github.com/login/oauth
subject:
equal: alice@example.com
This configuration needs the policy to be signed in GitHub actions,
from a repository owned by the GitHub user flavio
:
- kind: genericIssuer
issuer: https://token.actions.githubusercontent.com
subject:
urlPrefix: https://github.com/flavio
GitHub actions signature verification
The "kind", githubAction
is to validate policies signed in GitHub Actions.
You can do this with the genericIssuer
kind as well.
To simplify the signature requirement process, use two extra fields for githubAction
:
owner
(mandatory): GitHub ID of the user or organization to trustrepo
: the name of the repository to trust
For example, the last snippet, using genericIssuer
, could be rewritten as:
- kind: githubAction
owner: flavio
Signature annotations validation
All signature types can have other optional validation fields, annotations
.
These fields are key/value data added by during the signing process.
With Kubewarden, you can ensure policies are signed by trusted users and have specific annotations.
The next validation checks 2 conditions for the policy:
- that it's signed with a specific key
- it has a production environment annotation.
- kind: pubKey
key: |
-----BEGIN PUBLIC KEY-----
MBFKHFDGHKIJH0CAQYIKoZIzj0DAQcDQgAEX0HFTtCfTtPmkx5p1RbDE6HJSGAVD
BVDF6SKFSF87AASUspkQsN3FO4iyWodCy5j3o0CdIJD/KJHDJFHDFIu6sA==
-----END PUBLIC KEY-----
annotations:
environment: production
Using a signature verification configuration file to check a policy OCI artifact
You can test if a policy passes verification using the verification config file.
Use the --verification-config-path
flag of the kwctl verify
command
$ cat signatures_requirements.yaml
apiVersion: v1
allOf:
- kind: pubKey
key: |
-----BEGIN PUBLIC KEY-----
MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE5Q+cN1Jj2S7N05J4AXnqwP2DyzSg
Mc+raYce2Wthrd30MSgFtoh5ADAkCd/nML2Nx8UD9KBuASRb0gG5jXqgMQ==
-----END PUBLIC KEY-----
$ kwctl verify --verification-config-path signatures_requirements.yaml ghcr.io/kubewarden/policies/user-group-psp:latest
2022-03-29T17:34:37.847169Z INFO kwctl::verify: Policy successfully verified
This last example tests if a given policy came from the Kubewarden organization:
$ cat kubewarden_signatures.yaml
apiVersion: v1
allOf:
- kind: githubAction
owner: kubewarden
$ kwctl verify --verification-config-path kubewarden_signatures.yaml ghcr.io/kubewarden/policies/user-group-psp:latest
2022-03-29T18:07:39.062292Z INFO kwctl::verify: Policy successfully verified