Add logic to detect and use ambient OIDC from exec envs. (#644)

This is based on some work I have been doing here: https://github.com/mattmoor/oidc-magic

At present, it is fairly tedious to use the "keyless" flow inside of environments that have a form of ambient OIDC (e.g. GKE workload identity).  For example, in the context of Tekton, one needs to overlay `cosign` on an image like `docker.io/google/cloud-sdk:slim`, and then during execution have the step do something like:

```yaml
command: ["/bin/sh"]
args:
- "-c"
- |
  # Generate an identity token.
  IDENTITY_TOKEN=$(gcloud auth print-identity-token --audiences=sigstore)

  # Use the identity token to sign the image.
  cosign sign \
    -identity-token $IDENTITY_TOKEN \
    my.registry/the-image@sha256:deadbeef
```

This change adds support for detecting when `cosign` is executing within an environment with this kind of ambient authentication, and automatically producing one when `-identity-token` is not specified (and `COSIGN_EXPERIMENTAL=true`).

This means the same signing can now be done with:

```yaml
args: ["sign", "my.registry/the-image@sha256:deadbeef"]
```

This is much simpler, but also the image will be both smaller (distroless) and more portable (not just GCP, but any provider we link).

Signed-off-by: Matt Moore <mattomata@gmail.com>

KEYLESS.md

@@ -65,6 +65,9 @@ In automated environments, cosign also supports directly using OIDC Identity Tok
 These can be supplied on the command line with the `--identity-token` flag.
 The `audiences` field must contain `sigstore`.
 
+`cosign` also has support for detecting some of these automated environments
+and producing an identity token.  Currently this supports Google and Github.
+
 #### On GCP
 
 From a GCE VM, you can use the VM's service account identity to sign an image:

cmd/cosign/cli/sign.go

@@ -44,9 +44,14 @@ import (
 	"github.com/sigstore/cosign/pkg/cosign"
 	"github.com/sigstore/cosign/pkg/cosign/pivkey"
 	cremote "github.com/sigstore/cosign/pkg/cosign/remote"
+	"github.com/sigstore/cosign/pkg/providers"
 	fulcioClient "github.com/sigstore/fulcio/pkg/client"
 	"github.com/sigstore/rekor/pkg/generated/models"
 
+	// These are the ambient OIDC providers to link in.
+	_ "github.com/sigstore/cosign/pkg/providers/github"
+	_ "github.com/sigstore/cosign/pkg/providers/google"
+
 	rekorClient "github.com/sigstore/rekor/pkg/client"
 	"github.com/sigstore/sigstore/pkg/cryptoutils"
 	"github.com/sigstore/sigstore/pkg/signature"
@@ -511,7 +516,14 @@ func signerFromKeyOpts(ctx context.Context, certPath string, ko KeyOpts) (*certS
 		return nil, errors.Wrap(err, "parsing Fulcio URL")
 	}
 	fClient := fulcioClient.New(fulcioServer)
-	k, err := fulcio.NewSigner(ctx, ko.IDToken, ko.OIDCIssuer, ko.OIDCClientID, fClient)
+	tok := ko.IDToken
+	if providers.Enabled(ctx) {
+		tok, err = providers.Provide(ctx, "sigstore")
+		if err != nil {
+			return nil, errors.Wrap(err, "fetching ambient OIDC credentials")
+		}
+	}
+	k, err := fulcio.NewSigner(ctx, tok, ko.OIDCIssuer, ko.OIDCClientID, fClient)
 	if err != nil {
 		return nil, errors.Wrap(err, "getting key from Fulcio")
 	}

pkg/providers/doc.go

@@ -0,0 +1,18 @@
+//
+// Copyright 2021 The Sigstore Authors.
+//
+// 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 providers defines the APIs for providers to detect their relevance
+// and register themselves to furnish OIDC tokens within a given environment.
+package providers

pkg/providers/github/doc.go

@@ -0,0 +1,17 @@
+//
+// Copyright 2021 The Sigstore Authors.
+//
+// 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 github defines a github implementation of the providers.Interface.
+package github

pkg/providers/github/github.go

@@ -0,0 +1,76 @@
+//
+// Copyright 2021 The Sigstore Authors.
+//
+// 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 github
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+	"os"
+
+	"github.com/sigstore/cosign/pkg/providers"
+)
+
+func init() {
+	providers.Register("github-actions", &githubActions{})
+}
+
+type githubActions struct{}
+
+var _ providers.Interface = (*githubActions)(nil)
+
+const (
+	RequestTokenEnvKey = "ACTIONS_ID_TOKEN_REQUEST_TOKEN"
+	RequestURLEnvKey   = "ACTIONS_ID_TOKEN_REQUEST_URL"
+)
+
+// Enabled implements providers.Interface
+func (ga *githubActions) Enabled(ctx context.Context) bool {
+	if os.Getenv(RequestTokenEnvKey) == "" {
+		return false
+	}
+	if os.Getenv(RequestURLEnvKey) == "" {
+		return false
+	}
+	return true
+}
+
+// Provide implements providers.Interface
+func (ga *githubActions) Provide(ctx context.Context, audience string) (string, error) {
+	url := os.Getenv(RequestURLEnvKey) + "&audience=" + audience
+
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		return "", err
+	}
+
+	req.Header.Add("Authorization", "bearer "+os.Getenv(RequestTokenEnvKey))
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+
+	var payload struct {
+		Value string `json:"value"`
+	}
+
+	decoder := json.NewDecoder(resp.Body)
+	if err := decoder.Decode(&payload); err != nil {
+		return "", err
+	}
+	return payload.Value, nil
+}

pkg/providers/google/doc.go

@@ -0,0 +1,17 @@
+//
+// Copyright 2021 The Sigstore Authors.
+//
+// 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 google defines a google implementation of the providers.Interface.
+package google

pkg/providers/google/google.go

@@ -0,0 +1,68 @@
+//
+// Copyright 2021 The Sigstore Authors.
+//
+// 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 google
+
+import (
+	"context"
+	"io/ioutil"
+	"strings"
+
+	"google.golang.org/api/idtoken"
+
+	"github.com/sigstore/cosign/pkg/providers"
+)
+
+func init() {
+	providers.Register("google-workload-identity", &googleWorkloadIdentity{})
+}
+
+type googleWorkloadIdentity struct{}
+
+var _ providers.Interface = (*googleWorkloadIdentity)(nil)
+
+// gceProductNameFile is the product file path that contains the cloud service name.
+// This is a variable instead of a const to enable testing.
+var gceProductNameFile = "/sys/class/dmi/id/product_name"
+
+// Enabled implements providers.Interface
+// This is based on k8s.io/kubernetes/pkg/credentialprovider/gcp
+func (gwi *googleWorkloadIdentity) Enabled(ctx context.Context) bool {
+	data, err := ioutil.ReadFile(gceProductNameFile)
+	if err != nil {
+		return false
+	}
+	name := strings.TrimSpace(string(data))
+	if name == "Google" || name == "Google Compute Engine" {
+		// Just because we're on Google, does not mean workload identity is available.
+		// TODO(mattmoor): do something better than this.
+		_, err := gwi.Provide(ctx, "garbage")
+		return err == nil
+	}
+	return false
+}
+
+// Provide implements providers.Interface
+func (gwi *googleWorkloadIdentity) Provide(ctx context.Context, audience string) (string, error) {
+	ts, err := idtoken.NewTokenSource(ctx, audience)
+	if err != nil {
+		return "", err
+	}
+	tok, err := ts.Token()
+	if err != nil {
+		return "", err
+	}
+	return tok.AccessToken, nil
+}

pkg/providers/interface.go

@@ -0,0 +1,85 @@
+//
+// Copyright 2021 The Sigstore Authors.
+//
+// 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 providers
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"sync"
+)
+
+var (
+	m         sync.Mutex
+	providers = make(map[string]Interface)
+)
+
+// Interface is what providers need to implement to participate in furnishing OIDC tokens.
+type Interface interface {
+	// Enabled returns true if the provider is enabled.
+	Enabled(ctx context.Context) bool
+
+	// Provide returns an OIDC token scoped to the provided audience.
+	Provide(ctx context.Context, audience string) (string, error)
+}
+
+// Register is used by providers to participate in furnishing OIDC tokens.
+func Register(name string, p Interface) {
+	m.Lock()
+	defer m.Unlock()
+
+	if prev, ok := providers[name]; ok {
+		panic(fmt.Sprintf("duplicate provider for name %q, %T and %T", name, prev, p))
+	}
+	providers[name] = p
+}
+
+// Enabled checks whether any of the registered providers are enabled in this execution context.
+func Enabled(ctx context.Context) bool {
+	m.Lock()
+	defer m.Unlock()
+
+	for _, provider := range providers {
+		if provider.Enabled(ctx) {
+			return true
+		}
+	}
+	return false
+}
+
+// Provide fetches an OIDC token from one of the active providers.
+func Provide(ctx context.Context, audience string) (string, error) {
+	m.Lock()
+	defer m.Unlock()
+
+	var id string
+	var err error
+	for _, provider := range providers {
+		if !provider.Enabled(ctx) {
+			continue
+		}
+		id, err = provider.Provide(ctx, audience)
+		if err == nil {
+			return id, err
+		}
+	}
+	// return the last id/err combo, unless there wasn't an error in
+	// which case provider.Enabled() wasn't checked.
+	if err == nil {
+		err = errors.New("no providers are enabled, check providers.Enabled() before providers.Provide()")
+	}
+	return id, err
+}