Add Sigstore on workload attestor for SPIRE

conf/agent/agent_full.conf

@@ -359,6 +359,30 @@ plugins {
             # node_name: The name of the node. Overrides the value obtained by
             # the environment variable specified by node_name_env.
             # node_name = ""
+
+            # experimental: Experimental features.
+            experimental {
+                # sigstore: sigstore options. Enables signature checking.
+                # sigstore {
+                    # rekor_url: The URL for the rekor STL Server to use with cosign. Required.
+                    # rekor_url = "https://rekor.sigstore.dev"
+
+                    # skip_signature_verification_image_list: List of images that should
+                    # not be verified by cosign. They will receive a default
+                    # sigstore-validation:passed selector, but no other sigstore related selectors.
+                    # skip_signature_verification_image_list = ["sha:image1hash","sha:image2hash"]
+
+                    # allowed_subjects_list: Map of subjects that image signatures
+                    # will be checked against, keyed by OIDC Provider URI.
+                    # Signatures from subjects outside this list will be ignored. These should be email addresses.
+                    # allowed_subjects_list {
+                    #    "https://accounts.google.com" = ["[email protected]","[email protected]"]
+                    # }
+
+                    # enforce_sct: to be set as false in case of a private deployment not using the public CT
+                    # enforce_sct = true
+                # }
+            }
         }
     }
 

doc/plugin_agent_workloadattestor_k8s.md

@@ -59,6 +59,43 @@ since [hostprocess](https://kubernetes.io/docs/tasks/configure-pod-container/cre
 | `use_anonymous_authentication` | If true, use anonymous authentication for kubelet communication                                                                                                                                                                         |
 | `node_name_env`                | The environment variable used to obtain the node name. Defaults to `MY_NODE_NAME`.                                                                                                                                                      |
 | `node_name`                    | The name of the node. Overrides the value obtained by the environment variable specified by `node_name_env`.                                                                                                                            |
+| `experimental`                 | The experimental options that are subject to change or removal.                                                                                                                                                                         |
+
+| Experimental options | Description                                                                                                                  |
+|----------------------|----------------------------------------------------------------------------------------------------------------------------- |
+| `sigstore`           | Sigstore options. Options described below. See [Sigstore workload attestor for SPIRE](#sigstore-workload-attestor-for-spire) |
+
+| Sigstore options                         | Description                                                                                                                                                                                                                                                                                                                                                         |
+|------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `skip_signature_verification_image_list` | The list of images, described as digest hashes, that should be skipped in signature verification. Defaults to empty list.                                                                                                                                                                                                                                           |
+| `allowed_subjects_list`                  | A map of allowed subject strings, keyed by the OIDC Provider URI, that are trusted and are allowed to sign container images artifacts. Defaults to empty. If empty, no workload will pass signature validation, unless listed on `skip_signature_verification_image_list`. (eg. `"https://accounts.google.com" = ["[email protected]","[email protected]"]`). |
+| `rekor_url`                              | The rekor URL to use with cosign. Required. See notes below.                                                                                                                                                                                                                                                                                                        |
+| `enforce_sct`                            | A boolean to be set to false in case of a private deployment, not using public CT                                                                                                                                                                                                                                                                                   |
+
+> **Note** Cosign discourages the use of image tags for referencing docker images, and this plugin does not support attestation of sigstore selectors for workloads running on containers using tag-referenced images, which will then fail attestation for both sigstore and k8s selectors. In cases where this is necessary, add the digest string for the image in the `skip_signature_verification_image_list` setting (eg. `"sha256:abcdef0123456789abcdef0123456789abcdef0123456789abcdef0123456789"`). Note that sigstore signature attestation will still not be performed, but this will allow k8s selectors to be returned, along with the `"k8s:sigstore-validation:passed"` selector.
+
+<!-- different notes -->
+
+> **Note** Since the SPIRE Agent can also go through workload attestation, it will also need to be included in the skip list if either its image is not signed or has a digest reference string.
+
+<!-- different notes -->
+
+> **Note** The sigstore project contains a transparency log called Rekor that provides an immutable, tamper-resistant ledger to record signed metadata to an immutable record. While it is possible to run your own instance, a public instance of rekor is available at `https://rekor.sigstore.dev/`.
+
+## Sigstore workload attestor for SPIRE
+
+### Platform support
+
+This capability is only supported on Unix systems.
+
+The k8s workload attestor plugin also has capabilities to validate container images signatures through [sigstore](https://www.sigstore.dev/)
+
+Cosign supports container signing, verification, and storage in an OCI registry. Cosign aims to make signatures invisible infrastructure. For this, we’ve chosen the Sigstore ecosystem and artifacts. Digging deeper, we are using: Rekor (signature transparency log), Fulcio (signing certificate issuer and certificate transparency log) and Cosign (container image signing tool) to guarantee the authenticity of the running workload.
+
+> **Note** you can provide your own CA roots signed through TUF via the cosign initialize command.
+This effectively securely pins the CA roots. We allow you to also specify trusted roots via the `SIGSTORE_ROOT_FILE` flag
+
+### K8s selectors
 
 | Selector                 | Value                                                                                                                                                                                                                                                                                                                                                                                                                  |
 |--------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
@@ -77,6 +114,15 @@ since [hostprocess](https://kubernetes.io/docs/tasks/configure-pod-container/cre
 | k8s:pod-init-image       | An Image OR ImageID of any init container in the workload's pod, [as reported by K8S](https://pkg.go.dev/k8s.io/api/core/v1#ContainerStatus). Selector value may be an image tag, such as: `docker.io/envoyproxy/envoy-alpine:v1.16.0`, or a resolved SHA256 image digest, such as `docker.io/envoyproxy/envoy-alpine@sha256:bf862e5f5eca0a73e7e538224578c5cf867ce2be91b5eaed22afc153c00363eb`                         |
 | k8s:pod-init-image-count | The number of init container images in workload's pod                                                                                                                                                                                                                                                                                                                                                                  |
 
+Sigstore enabled selectors (available when configured to use sigstore)
+
+| Selector                                           | Value                                                                                                                                                                                                                                                      |
+|----------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| k8s:${containerID}:image-signature-content         | A containerID is an unique alphanumeric number for each container. The value of the signature itself in a hash (eg. "k8s:000000:image-signature-content:MEUCIQCyem8Gcr0sPFMP7fTXazCN57NcN5+MjxJw9Oo0x2eM+AIgdgBP96BO1Te/NdbjHbUeb0BUye6deRgVtQEv5No5smA=") |
+| k8s:${containerID}:image-signature-subject         | OIDC principal that signed it​ (eg. "k8s:000000:image-signature-subject:[email protected]")                                                                                                                                                               |
+| k8s:${containerID}:image-signature-logid           | A unique LogID for the Rekor transparency log​ (eg. "k8s:000000:image-signature-logid:samplelogID")                                                                                                                                                        |
+| k8s:${containerID}:image-signature-integrated-time | The time (in Unix timestamp format) when the image signature was integrated into the signature transparency log​ (eg. "k8s:000000:image-signature-integrated-time:12345")                                                                                  |
+| k8s:sigstore-validation                            | The confirmation if the signature is valid, has value of "passed" (eg. "k8s:sigstore-validation:passed")                                                                                                                                                   |
 > **Note** `container-image` will ONLY match against the specific container in the pod that is contacting SPIRE on behalf of
 > the pod, whereas `pod-image` and `pod-init-image` will match against ANY container or init container in the Pod,
 > respectively.