external-secrets/pkg/provider/gcp/secretmanager/workload_identity.go

/*
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

	http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package secretmanager

import (
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"net/http"
	"time"

	iam "cloud.google.com/go/iam/credentials/apiv1"
	"cloud.google.com/go/iam/credentials/apiv1/credentialspb"
	secretmanager "cloud.google.com/go/secretmanager/apiv1"
	"github.com/googleapis/gax-go/v2"
	"golang.org/x/oauth2"
	"google.golang.org/api/option"
	"google.golang.org/grpc"
	"google.golang.org/grpc/credentials"
	"grpc.go4.org/credentials/oauth"
	authenticationv1 "k8s.io/api/authentication/v1"
	v1 "k8s.io/api/core/v1"
	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/types"
	"k8s.io/client-go/kubernetes"
	clientcorev1 "k8s.io/client-go/kubernetes/typed/core/v1"
	kclient "sigs.k8s.io/controller-runtime/pkg/client"
	ctrlcfg "sigs.k8s.io/controller-runtime/pkg/client/config"

	esv1beta1 "github.com/external-secrets/external-secrets/apis/externalsecrets/v1beta1"
	"github.com/external-secrets/external-secrets/pkg/constants"
	"github.com/external-secrets/external-secrets/pkg/metrics"
)

const (
	gcpSAAnnotation = "iam.gke.io/gcp-service-account"

	errFetchPodToken  = "unable to fetch pod token: %w"
	errFetchIBToken   = "unable to fetch identitybindingtoken: %w"
	errGenAccessToken = "unable to generate gcp access token: %w"
	errNoProjectID    = "unable to find ProjectID in storeSpec"
)

// workloadIdentity holds all clients and generators needed
// to create a gcp oauth token.
type workloadIdentity struct {
	iamClient            IamClient
	idBindTokenGenerator idBindTokenGenerator
	saTokenGenerator     saTokenGenerator
	clusterProjectID     string
}

// interface to GCP IAM API.
type IamClient interface {
	GenerateAccessToken(ctx context.Context, req *credentialspb.GenerateAccessTokenRequest, opts ...gax.CallOption) (*credentialspb.GenerateAccessTokenResponse, error)
	Close() error
}

// interface to securetoken/identitybindingtoken API.
type idBindTokenGenerator interface {
	Generate(context.Context, *http.Client, string, string, string) (*oauth2.Token, error)
}

// interface to kubernetes serviceaccount token request API.
type saTokenGenerator interface {
	Generate(context.Context, []string, string, string) (*authenticationv1.TokenRequest, error)
}

func newWorkloadIdentity(ctx context.Context, projectID string) (*workloadIdentity, error) {
	satg, err := newSATokenGenerator()
	if err != nil {
		return nil, err
	}
	iamc, err := newIAMClient(ctx)
	if err != nil {
		return nil, err
	}
	return &workloadIdentity{
		iamClient:            iamc,
		idBindTokenGenerator: newIDBindTokenGenerator(),
		saTokenGenerator:     satg,
		clusterProjectID:     projectID,
	}, nil
}

func (w *workloadIdentity) TokenSource(ctx context.Context, auth esv1beta1.GCPSMAuth, isClusterKind bool, kube kclient.Client, namespace string) (oauth2.TokenSource, error) {
	wi := auth.WorkloadIdentity
	if wi == nil {
		return nil, nil
	}
	saKey := types.NamespacedName{
		Name:      wi.ServiceAccountRef.Name,
		Namespace: namespace,
	}

	// only ClusterStore is allowed to set namespace (and then it's required)
	if isClusterKind && wi.ServiceAccountRef.Namespace != nil {
		saKey.Namespace = *wi.ServiceAccountRef.Namespace
	}

	sa := &v1.ServiceAccount{}
	err := kube.Get(ctx, saKey, sa)
	if err != nil {
		return nil, err
	}

	idProvider := fmt.Sprintf("https://container.googleapis.com/v1/projects/%s/locations/%s/clusters/%s",
		w.clusterProjectID,
		wi.ClusterLocation,
		wi.ClusterName)
	idPool := fmt.Sprintf("%s.svc.id.goog", w.clusterProjectID)
	audiences := []string{idPool}
	if len(wi.ServiceAccountRef.Audiences) > 0 {
		audiences = append(audiences, wi.ServiceAccountRef.Audiences...)
	}
	gcpSA := sa.Annotations[gcpSAAnnotation]

	resp, err := w.saTokenGenerator.Generate(ctx, audiences, saKey.Name, saKey.Namespace)
	metrics.ObserveAPICall(constants.ProviderGCPSM, constants.CallGCPSMGenerateSAToken, err)
	if err != nil {
		return nil, fmt.Errorf(errFetchPodToken, err)
	}

	idBindToken, err := w.idBindTokenGenerator.Generate(ctx, http.DefaultClient, resp.Status.Token, idPool, idProvider)
	metrics.ObserveAPICall(constants.ProviderGCPSM, constants.CallGCPSMGenerateIDBindToken, err)
	if err != nil {
		return nil, fmt.Errorf(errFetchIBToken, err)
	}

	// If no `iam.gke.io/gcp-service-account` annotation is present the
	// identitybindingtoken will be used directly, allowing bindings on secrets
	// of the form "serviceAccount:<project>.svc.id.goog[<namespace>/<sa>]".
	if gcpSA == "" {
		return oauth2.StaticTokenSource(idBindToken), nil
	}
	gcpSAResp, err := w.iamClient.GenerateAccessToken(ctx, &credentialspb.GenerateAccessTokenRequest{
		Name:  fmt.Sprintf("projects/-/serviceAccounts/%s", gcpSA),
		Scope: secretmanager.DefaultAuthScopes(),
	}, gax.WithGRPCOptions(grpc.PerRPCCredentials(oauth.TokenSource{TokenSource: oauth2.StaticTokenSource(idBindToken)})))
	metrics.ObserveAPICall(constants.ProviderGCPSM, constants.CallGCPSMGenerateAccessToken, err)
	if err != nil {
		return nil, fmt.Errorf(errGenAccessToken, err)
	}
	return oauth2.StaticTokenSource(&oauth2.Token{
		AccessToken: gcpSAResp.GetAccessToken(),
	}), nil
}

func (w *workloadIdentity) Close() error {
	if w.iamClient != nil {
		return w.iamClient.Close()
	}
	return nil
}

func newIAMClient(ctx context.Context) (IamClient, error) {
	iamOpts := []option.ClientOption{
		option.WithUserAgent("external-secrets-operator"),
		// tell the secretmanager library to not add transport-level ADC since
		// we need to override on a per call basis
		option.WithoutAuthentication(),
		// grpc oauth TokenSource credentials require transport security, so
		// this must be set explicitly even though TLS is used
		option.WithGRPCDialOption(grpc.WithTransportCredentials(credentials.NewTLS(nil))),
		option.WithGRPCConnectionPool(5),
	}
	return iam.NewIamCredentialsClient(ctx, iamOpts...)
}

type k8sSATokenGenerator struct {
	corev1 clientcorev1.CoreV1Interface
}

func (g *k8sSATokenGenerator) Generate(ctx context.Context, audiences []string, name, namespace string) (*authenticationv1.TokenRequest, error) {
	// Request a serviceaccount token for the pod
	ttl := int64((15 * time.Minute).Seconds())
	return g.corev1.
		ServiceAccounts(namespace).
		CreateToken(ctx, name,
			&authenticationv1.TokenRequest{
				Spec: authenticationv1.TokenRequestSpec{
					ExpirationSeconds: &ttl,
					Audiences:         audiences,
				},
			},
			metav1.CreateOptions{},
		)
}

func newSATokenGenerator() (saTokenGenerator, error) {
	cfg, err := ctrlcfg.GetConfig()
	if err != nil {
		return nil, err
	}
	clientset, err := kubernetes.NewForConfig(cfg)
	if err != nil {
		return nil, err
	}
	return &k8sSATokenGenerator{
		corev1: clientset.CoreV1(),
	}, nil
}

// Trades the kubernetes token for an identitybindingtoken token.
type gcpIDBindTokenGenerator struct {
	targetURL string
}

func newIDBindTokenGenerator() idBindTokenGenerator {
	return &gcpIDBindTokenGenerator{
		targetURL: "https://securetoken.googleapis.com/v1/identitybindingtoken",
	}
}

func (g *gcpIDBindTokenGenerator) Generate(ctx context.Context, client *http.Client, k8sToken, idPool, idProvider string) (*oauth2.Token, error) {
	body, err := json.Marshal(map[string]string{
		"grant_type":           "urn:ietf:params:oauth:grant-type:token-exchange",
		"subject_token_type":   "urn:ietf:params:oauth:token-type:jwt",
		"requested_token_type": "urn:ietf:params:oauth:token-type:access_token",
		"subject_token":        k8sToken,
		"audience":             fmt.Sprintf("identitynamespace:%s:%s", idPool, idProvider),
		"scope":                "https://www.googleapis.com/auth/cloud-platform",
	})
	if err != nil {
		return nil, err
	}

	req, err := http.NewRequestWithContext(ctx, "POST", g.targetURL, bytes.NewBuffer(body))
	if err != nil {
		return nil, err
	}

	req.Header.Set("Content-Type", "application/json")
	resp, err := client.Do(req)
	if err != nil {
		return nil, err
	}
	if resp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("could not get idbindtoken token, status: %v", resp.StatusCode)
	}

	defer resp.Body.Close()
	respBody, err := io.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}

	idBindToken := &oauth2.Token{}
	if err := json.Unmarshal(respBody, idBindToken); err != nil {
		return nil, err
	}
	return idBindToken, nil
}