Container image signatures - Container security | Security and compliance

Enabling signature verification for Red Hat Container Registries
Verifying the signature verification configuration
Understanding the verification of container images lacking verifiable signatures
- Automated verification during updates
- Using skopeo to verify signatures of Red Hat container images
Additional resources

Red Hat delivers signatures for the images in the Red Hat Container Registries. Those signatures can be automatically verified when being pulled to OKD 4 clusters by using the Machine Config Operator (MCO).

Quay.io serves most of the images that make up OKD, and only the release image is signed. Release images refer to the approved OKD images, offering a degree of protection against supply chain attacks. However, some extensions to OKD, such as logging, monitoring, and service mesh, are shipped as Operators from the Operator Lifecycle Manager (OLM). Those images ship from the Red Hat Ecosystem Catalog Container images registry.

To verify the integrity of those images between Red Hat registries and your infrastructure, enable signature verification.

Enabling signature verification for Red Hat Container Registries

Enabling container signature validation for Red Hat Container Registries requires writing a signature verification policy file specifying the keys to verify images from these registries. For RHEL8 nodes, the registries are already defined in /etc/containers/registries.d by default.

Procedure

Create a Butane config file, 51-worker-rh-registry-trust.bu, containing the necessary configuration for the worker nodes.

See "Creating machine configs with Butane" for information about Butane.

variant: openshift
version: 4.11.0
metadata:
  name: 51-worker-rh-registry-trust
  labels:
    machineconfiguration.openshift.io/role: worker
storage:
  files:
  - path: /etc/containers/policy.json
    mode: 0644
    overwrite: true
    contents:
      inline: |
        {
          "default": [
            {
              "type": "insecureAcceptAnything"
            }
          ],
          "transports": {
            "docker": {
              "registry.access.redhat.com": [
                {
                  "type": "signedBy",
                  "keyType": "GPGKeys",
                  "keyPath": "/etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release"
                }
              ],
              "registry.redhat.io": [
                {
                  "type": "signedBy",
                  "keyType": "GPGKeys",
                  "keyPath": "/etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release"
                }
              ]
            },
            "docker-daemon": {
              "": [
                {
                  "type": "insecureAcceptAnything"
                }
              ]
            }
          }
        }

Use Butane to generate a machine config YAML file, 51-worker-rh-registry-trust.yaml, containing the file to be written to disk on the worker nodes:
```
$ butane 51-worker-rh-registry-trust.bu -o 51-worker-rh-registry-trust.yaml
```

Apply the created machine config:

$ oc apply -f 51-worker-rh-registry-trust.yaml

Check that the worker machine config pool has rolled out with the new machine config:

Check that the new machine config was created:

$ oc get mc

Sample output

NAME                                               GENERATEDBYCONTROLLER                      IGNITIONVERSION   AGE
00-master                                          a2178ad522c49ee330b0033bb5cb5ea132060b0a   3.2.0             25m
00-worker                                          a2178ad522c49ee330b0033bb5cb5ea132060b0a   3.2.0             25m
01-master-container-runtime                        a2178ad522c49ee330b0033bb5cb5ea132060b0a   3.2.0             25m
01-master-kubelet                                  a2178ad522c49ee330b0033bb5cb5ea132060b0a   3.2.0             25m
01-worker-container-runtime                        a2178ad522c49ee330b0033bb5cb5ea132060b0a   3.2.0             25m
01-worker-kubelet                                  a2178ad522c49ee330b0033bb5cb5ea132060b0a   3.2.0             25m
51-master-rh-registry-trust                                                                   3.2.0             13s
51-worker-rh-registry-trust                                                                   3.2.0             53s (1)
99-master-generated-crio-seccomp-use-default                                                  3.2.0             25m
99-master-generated-registries                     a2178ad522c49ee330b0033bb5cb5ea132060b0a   3.2.0             25m
99-master-ssh                                                                                 3.2.0             28m
99-worker-generated-crio-seccomp-use-default                                                  3.2.0             25m
99-worker-generated-registries                     a2178ad522c49ee330b0033bb5cb5ea132060b0a   3.2.0             25m
99-worker-ssh                                                                                 3.2.0             28m
rendered-master-af1e7ff78da0a9c851bab4be2777773b   a2178ad522c49ee330b0033bb5cb5ea132060b0a   3.2.0             8s
rendered-master-cd51fd0c47e91812bfef2765c52ec7e6   a2178ad522c49ee330b0033bb5cb5ea132060b0a   3.2.0             24m
rendered-worker-2b52f75684fbc711bd1652dd86fd0b82   a2178ad522c49ee330b0033bb5cb5ea132060b0a   3.2.0             24m
rendered-worker-be3b3bce4f4aa52a62902304bac9da3c   a2178ad522c49ee330b0033bb5cb5ea132060b0a   3.2.0             48s (2)

1	New machine config
2	New rendered machine config

Check that the worker machine config pool is updating with the new machine config:

$ oc get mcp

Sample output

NAME     CONFIG                                             UPDATED   UPDATING   DEGRADED   MACHINECOUNT   READYMACHINECOUNT   UPDATEDMACHINECOUNT   DEGRADEDMACHINECOUNT   AGE
master   rendered-master-af1e7ff78da0a9c851bab4be2777773b   True      False      False      3              3                   3                     0                      30m
worker   rendered-worker-be3b3bce4f4aa52a62902304bac9da3c   False     True       False      3              0                   0                     0                      30m (1)

1	When the `UPDATING` field is `True`, the machine config pool is updating with the new machine config. When the field becomes `False`, the worker machine config pool has rolled out to the new machine config.

If your cluster uses any RHEL7 worker nodes, when the worker machine config pool is updated, create YAML files on those nodes in the /etc/containers/registries.d directory, which specify the location of the detached signatures for a given registry server. The following example works only for images hosted in registry.access.redhat.com and registry.redhat.io.
1. Start a debug session to each RHEL7 worker node:
  $ oc debug node/<node_name>
2. Change your root directory to /host:
  sh-4.2# chroot /host
3. Create a /etc/containers/registries.d/registry.redhat.io.yaml file that contains the following:
  docker: registry.redhat.io: sigstore: https://registry.redhat.io/containers/sigstore
4. Create a /etc/containers/registries.d/registry.access.redhat.com.yaml file that contains the following:
  docker: registry.access.redhat.com: sigstore: https://access.redhat.com/webassets/docker/content/sigstore
5. Exit the debug session.

Verifying the signature verification configuration

After you apply the machine configs to the cluster, the Machine Config Controller detects the new MachineConfig object and generates a new rendered-worker-<hash> version.

Prerequisites

You enabled signature verification by using a machine config file.

Procedure

On the command line, run the following command to display information about a desired worker:

$ oc describe machineconfigpool/worker

Example output of initial worker monitoring

Name:         worker
Namespace:
Labels:       machineconfiguration.openshift.io/mco-built-in=
Annotations:  <none>
API Version:  machineconfiguration.openshift.io/v1
Kind:         MachineConfigPool
Metadata:
  Creation Timestamp:  2019-12-19T02:02:12Z
  Generation:          3
  Resource Version:    16229
  Self Link:           /apis/machineconfiguration.openshift.io/v1/machineconfigpools/worker
  UID:                 92697796-2203-11ea-b48c-fa163e3940e5
Spec:
  Configuration:
    Name:  rendered-worker-f6819366eb455a401c42f8d96ab25c02
    Source:
      API Version:  machineconfiguration.openshift.io/v1
      Kind:         MachineConfig
      Name:         00-worker
      API Version:  machineconfiguration.openshift.io/v1
      Kind:         MachineConfig
      Name:         01-worker-container-runtime
      API Version:  machineconfiguration.openshift.io/v1
      Kind:         MachineConfig
      Name:         01-worker-kubelet
      API Version:  machineconfiguration.openshift.io/v1
      Kind:         MachineConfig
      Name:         51-worker-rh-registry-trust
      API Version:  machineconfiguration.openshift.io/v1
      Kind:         MachineConfig
      Name:         99-worker-92697796-2203-11ea-b48c-fa163e3940e5-registries
      API Version:  machineconfiguration.openshift.io/v1
      Kind:         MachineConfig
      Name:         99-worker-ssh
  Machine Config Selector:
    Match Labels:
      machineconfiguration.openshift.io/role:  worker
  Node Selector:
    Match Labels:
      node-role.kubernetes.io/worker:
  Paused:                              false
Status:
  Conditions:
    Last Transition Time:  2019-12-19T02:03:27Z
    Message:
    Reason:
    Status:                False
    Type:                  RenderDegraded
    Last Transition Time:  2019-12-19T02:03:43Z
    Message:
    Reason:
    Status:                False
    Type:                  NodeDegraded
    Last Transition Time:  2019-12-19T02:03:43Z
    Message:
    Reason:
    Status:                False
    Type:                  Degraded
    Last Transition Time:  2019-12-19T02:28:23Z
    Message:
    Reason:
    Status:                False
    Type:                  Updated
    Last Transition Time:  2019-12-19T02:28:23Z
    Message:               All nodes are updating to rendered-worker-f6819366eb455a401c42f8d96ab25c02
    Reason:
    Status:                True
    Type:                  Updating
  Configuration:
    Name:  rendered-worker-d9b3f4ffcfd65c30dcf591a0e8cf9b2e
    Source:
      API Version:            machineconfiguration.openshift.io/v1
      Kind:                   MachineConfig
      Name:                   00-worker
      API Version:            machineconfiguration.openshift.io/v1
      Kind:                   MachineConfig
      Name:                   01-worker-container-runtime
      API Version:            machineconfiguration.openshift.io/v1
      Kind:                   MachineConfig
      Name:                   01-worker-kubelet
      API Version:            machineconfiguration.openshift.io/v1
      Kind:                   MachineConfig
      Name:                   99-worker-92697796-2203-11ea-b48c-fa163e3940e5-registries
      API Version:            machineconfiguration.openshift.io/v1
      Kind:                   MachineConfig
      Name:                   99-worker-ssh
  Degraded Machine Count:     0
  Machine Count:              1
  Observed Generation:        3
  Ready Machine Count:        0
  Unavailable Machine Count:  1
  Updated Machine Count:      0
Events:                       <none>

Run the oc describe command again:

$ oc describe machineconfigpool/worker

Example output after the worker is updated

...
    Last Transition Time:  2019-12-19T04:53:09Z
    Message:               All nodes are updated with rendered-worker-f6819366eb455a401c42f8d96ab25c02
    Reason:
    Status:                True
    Type:                  Updated
    Last Transition Time:  2019-12-19T04:53:09Z
    Message:
    Reason:
    Status:                False
    Type:                  Updating
  Configuration:
    Name:  rendered-worker-f6819366eb455a401c42f8d96ab25c02
    Source:
      API Version:            machineconfiguration.openshift.io/v1
      Kind:                   MachineConfig
      Name:                   00-worker
      API Version:            machineconfiguration.openshift.io/v1
      Kind:                   MachineConfig
      Name:                   01-worker-container-runtime
      API Version:            machineconfiguration.openshift.io/v1
      Kind:                   MachineConfig
      Name:                   01-worker-kubelet
      API Version:            machineconfiguration.openshift.io/v1
      Kind:                   MachineConfig
      Name:                   51-worker-rh-registry-trust
      API Version:            machineconfiguration.openshift.io/v1
      Kind:                   MachineConfig
      Name:                   99-worker-92697796-2203-11ea-b48c-fa163e3940e5-registries
      API Version:            machineconfiguration.openshift.io/v1
      Kind:                   MachineConfig
      Name:                   99-worker-ssh
  Degraded Machine Count:     0
  Machine Count:              3
  Observed Generation:        4
  Ready Machine Count:        3
  Unavailable Machine Count:  0
  Updated Machine Count:      3
...

The Observed Generation parameter shows an increased count based on the generation of the controller-produced configuration. This controller updates this value even if it fails to process the specification and generate a revision. The Configuration Source value points to the 51-worker-rh-registry-trust configuration.

Confirm that the policy.json file exists with the following command:

$ oc debug node/<node> -- chroot /host cat /etc/containers/policy.json

Example output

Starting pod/<node>-debug ...
To use host binaries, run `chroot /host`
{
  "default": [
    {
      "type": "insecureAcceptAnything"
    }
  ],
  "transports": {
    "docker": {
      "registry.access.redhat.com": [
        {
          "type": "signedBy",
          "keyType": "GPGKeys",
          "keyPath": "/etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release"
        }
      ],
      "registry.redhat.io": [
        {
          "type": "signedBy",
          "keyType": "GPGKeys",
          "keyPath": "/etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release"
        }
      ]
    },
    "docker-daemon": {
      "": [
        {
          "type": "insecureAcceptAnything"
        }
      ]
    }
  }
}

Confirm that the registry.redhat.io.yaml file exists with the following command:

$ oc debug node/<node> -- chroot /host cat /etc/containers/registries.d/registry.redhat.io.yaml

Example output

Starting pod/<node>-debug ...
To use host binaries, run `chroot /host`
docker:
     registry.redhat.io:
         sigstore: https://registry.redhat.io/containers/sigstore

Confirm that the registry.access.redhat.com.yaml file exists with the following command:

$ oc debug node/<node> -- chroot /host cat /etc/containers/registries.d/registry.access.redhat.com.yaml

Example output

Starting pod/<node>-debug ...
To use host binaries, run `chroot /host`
docker:
     registry.access.redhat.com:
         sigstore: https://access.redhat.com/webassets/docker/content/sigstore

Understanding the verification of container images lacking verifiable signatures

Each OKD release image is immutable and signed with a Red Hat production key. During an OKD update or installation, a release image might deploy container images that do not have verifiable signatures. Each signed release image digest is immutable. Each reference in the release image is to the immutable digest of another image, so the contents can be trusted transitively. In other words, the signature on the release image validates all release contents.

For example, the image references lacking a verifiable signature are contained in the signed OKD release image:

Example release info output

$ oc adm release info  quay.io/openshift-release-dev/ ocp-release@sha256:2309578b68c5666dad62aed696f1f9d778ae1a089ee461060ba7b9514b7ca417 -o pullspec (1)
quay.io/openshift-release-dev/ocp-v4.0-art-dev@sha256:9aafb914d5d7d0dec4edd800d02f811d7383a7d49e500af548eab5d00c1bffdb (2)

1	Signed release image SHA.
2	Container image lacking a verifiable signature included in the release.

Automated verification during updates

Verification of signatures is automatic. The OpenShift Cluster Version Operator (CVO) verifies signatures on the release images during an OKD update. This is an internal process. An OKD installation or update fails if the automated verification fails.

Verification of signatures can also be done manually using the skopeo command-line utility.

Additional resources

Introduction to OpenShift Updates

Using skopeo to verify signatures of Red Hat container images

You can verify the signatures for container images included in an OKD release image by pulling those signatures from OCP release mirror site. Because the signatures on the mirror site are not in a format readily understood by Podman or CRI-O, you can use the skopeo standalone-verify command to verify that the your release images are signed by Red Hat.

Prerequisites

You have installed the skopeo command-line utility.

Procedure

Get the full SHA for your release by running the following command:

$ oc adm release info <release_version>  \ (1)

1	Substitute <release_version> with your release number, for example, `4.14.3`.

Example output snippet

---
Pull From: quay.io/openshift-release-dev/ocp-release@sha256:e73ab4b33a9c3ff00c9f800a38d69853ca0c4dfa5a88e3df331f66df8f18ec55
---

Pull down the Red Hat release key by running the following command:

$ curl -o pub.key https://access.redhat.com/security/data/fd431d51.txt

Get the signature file for the specific release that you want to verify by running the following command:

$ curl -o signature-1 https://mirror.openshift.com/pub/openshift-v4/signatures/openshift-release-dev/ocp-release/sha256%<sha_from_version>/signature-1 \ (1)

1 Replace <sha_from_version> with SHA value from the full link to the mirror site that matches the SHA of your release. For example, the link to the signature for the 4.12.23 release is https://mirror.openshift.com/pub/openshift-v4/signatures/openshift-release-dev/ocp-release/sha256%e73ab4b33a9c3ff00c9f800a38d69853ca0c4dfa5a88e3df331f66df8f18ec55/signature-1, and the SHA value is e73ab4b33a9c3ff00c9f800a38d69853ca0c4dfa5a88e3df331f66df8f18ec55.

Get the manifest for the release image by running the following command:

$ skopeo inspect --raw docker://<quay_link_to_release> > manifest.json \ (1)

1	Replace `<quay_link_to_release>` with the output of the `oc adm release info` command. For example, `quay.io/openshift-release-dev/ocp-release@sha256:e73ab4b33a9c3ff00c9f800a38d69853ca0c4dfa5a88e3df331f66df8f18ec55`.

Use skopeo to verify the signature:

$ skopeo standalone-verify manifest.json quay.io/openshift-release-dev/ocp-release:<release_number>-<arch> any signature-1 --public-key-file pub.key

where:

<release_number>

Specifies the release number, for example 4.14.3.

<arch>

Specifies the architecture, for example x86_64.

Example output

Signature verified using fingerprint 567E347AD0044ADE55BA8A5F199E2F91FD431D51, digest sha256:e73ab4b33a9c3ff00c9f800a38d69853ca0c4dfa5a88e3df331f66df8f18ec55

Additional resources

Machine Config Overview