credentials/sts: PerRPCCreds Implementation (#3696)

credentials/sts/sts.go

@@ -0,0 +1,395 @@
+// +build go1.13
+
+/*
+ *
+ * Copyright 2020 gRPC 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 sts implements call credentials using STS (Security Token Service) as
+// defined in https://tools.ietf.org/html/rfc8693.
+//
+// Experimental
+//
+// Notice: All APIs in this package are experimental and may be changed or
+// removed in a later release.
+package sts
+
+import (
+	"bytes"
+	"context"
+	"crypto/tls"
+	"crypto/x509"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"sync"
+	"time"
+
+	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/grpclog"
+)
+
+const (
+	// HTTP request timeout set on the http.Client used to make STS requests.
+	stsRequestTimeout = 5 * time.Second
+	// If lifetime left in a cached token is lesser than this value, we fetch a
+	// new one instead of returning the current one.
+	minCachedTokenLifetime = 300 * time.Second
+
+	tokenExchangeGrantType    = "urn:ietf:params:oauth:grant-type:token-exchange"
+	defaultCloudPlatformScope = "https://www.googleapis.com/auth/cloud-platform"
+)
+
+// For overriding in tests.
+var (
+	loadSystemCertPool   = x509.SystemCertPool
+	makeHTTPDoer         = makeHTTPClient
+	readSubjectTokenFrom = ioutil.ReadFile
+	readActorTokenFrom   = ioutil.ReadFile
+)
+
+// Options configures the parameters used for an STS based token exchange.
+type Options struct {
+	// TokenExchangeServiceURI is the address of the server which implements STS
+	// token exchange functionality.
+	TokenExchangeServiceURI string // Required.
+
+	// Resource is a URI that indicates the target service or resource where the
+	// client intends to use the requested security token.
+	Resource string // Optional.
+
+	// Audience is the logical name of the target service where the client
+	// intends to use the requested security token
+	Audience string // Optional.
+
+	// Scope is a list of space-delimited, case-sensitive strings, that allow
+	// the client to specify the desired scope of the requested security token
+	// in the context of the service or resource where the token will be used.
+	// If this field is left unspecified, a default value of
+	// https://www.googleapis.com/auth/cloud-platform will be used.
+	Scope string // Optional.
+
+	// RequestedTokenType is an identifier, as described in
+	// https://tools.ietf.org/html/rfc8693#section-3, that indicates the type of
+	// the requested security token.
+	RequestedTokenType string // Optional.
+
+	// SubjectTokenPath is a filesystem path which contains the security token
+	// that represents the identity of the party on behalf of whom the request
+	// is being made.
+	SubjectTokenPath string // Required.
+
+	// SubjectTokenType is an identifier, as described in
+	// https://tools.ietf.org/html/rfc8693#section-3, that indicates the type of
+	// the security token in the "subject_token_path" parameter.
+	SubjectTokenType string // Required.
+
+	// ActorTokenPath is a  security token that represents the identity of the
+	// acting party.
+	ActorTokenPath string // Optional.
+
+	// ActorTokenType is an identifier, as described in
+	// https://tools.ietf.org/html/rfc8693#section-3, that indicates the type of
+	// the the security token in the "actor_token_path" parameter.
+	ActorTokenType string // Optional.
+}
+
+// NewCredentials returns a new PerRPCCredentials implementation, configured
+// using opts, which performs token exchange using STS.
+func NewCredentials(opts Options) (credentials.PerRPCCredentials, error) {
+	if err := validateOptions(opts); err != nil {
+		return nil, err
+	}
+
+	// Load the system roots to validate the certificate presented by the STS
+	// endpoint during the TLS handshake.
+	roots, err := loadSystemCertPool()
+	if err != nil {
+		return nil, err
+	}
+
+	return &callCreds{
+		opts:   opts,
+		client: makeHTTPDoer(roots),
+	}, nil
+}
+
+// callCreds provides the implementation of call credentials based on an STS
+// token exchange.
+type callCreds struct {
+	opts   Options
+	client httpDoer
+
+	// Cached accessToken to avoid an STS token exchange for every call to
+	// GetRequestMetadata.
+	mu            sync.Mutex
+	tokenMetadata map[string]string
+	tokenExpiry   time.Time
+}
+
+// GetRequestMetadata returns the cached accessToken, if available and valid, or
+// fetches a new one by performing an STS token exchange.
+func (c *callCreds) GetRequestMetadata(ctx context.Context, _ ...string) (map[string]string, error) {
+	if err := credentials.CheckSecurityLevel(ctx, credentials.PrivacyAndIntegrity); err != nil {
+		return nil, fmt.Errorf("unable to transfer STS PerRPCCredentials: %v", err)
+	}
+
+	// Holding the lock for the whole duration of the STS request and response
+	// processing ensures that concurrent RPCs don't end up in multiple
+	// requests being made.
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if md := c.cachedMetadata(); md != nil {
+		return md, nil
+	}
+	req, err := constructRequest(ctx, c.opts)
+	if err != nil {
+		return nil, err
+	}
+	respBody, err := sendRequest(c.client, req)
+	if err != nil {
+		return nil, err
+	}
+	ti, err := tokenInfoFromResponse(respBody)
+	if err != nil {
+		return nil, err
+	}
+	c.tokenMetadata = map[string]string{"Authorization": fmt.Sprintf("%s %s", ti.tokenType, ti.token)}
+	c.tokenExpiry = ti.expiryTime
+	return c.tokenMetadata, nil
+}
+
+// RequireTransportSecurity indicates whether the credentials requires
+// transport security.
+func (c *callCreds) RequireTransportSecurity() bool {
+	return true
+}
+
+// httpDoer wraps the single method on the http.Client type that we use. This
+// helps with overriding in unittests.
+type httpDoer interface {
+	Do(req *http.Request) (*http.Response, error)
+}
+
+func makeHTTPClient(roots *x509.CertPool) httpDoer {
+	return &http.Client{
+		Timeout: stsRequestTimeout,
+		Transport: &http.Transport{
+			TLSClientConfig: &tls.Config{
+				RootCAs: roots,
+			},
+		},
+	}
+}
+
+// validateOptions performs the following validation checks on opts:
+// - tokenExchangeServiceURI is not empty
+// - tokenExchangeServiceURI is a valid URI with a http(s) scheme
+// - subjectTokenPath and subjectTokenType are not empty.
+func validateOptions(opts Options) error {
+	if opts.TokenExchangeServiceURI == "" {
+		return errors.New("empty token_exchange_service_uri in options")
+	}
+	u, err := url.Parse(opts.TokenExchangeServiceURI)
+	if err != nil {
+		return err
+	}
+	if u.Scheme != "http" && u.Scheme != "https" {
+		return fmt.Errorf("scheme is not supported: %s. Only http(s) is supported", u.Scheme)
+	}
+
+	if opts.SubjectTokenPath == "" {
+		return errors.New("required field SubjectTokenPath is not specified")
+	}
+	if opts.SubjectTokenType == "" {
+		return errors.New("required field SubjectTokenType is not specified")
+	}
+	return nil
+}
+
+// cachedMetadata returns the cached metadata provided it is not going to
+// expire anytime soon.
+//
+// Caller must hold c.mu.
+func (c *callCreds) cachedMetadata() map[string]string {
+	now := time.Now()
+	// If the cached token has not expired and the lifetime remaining on that
+	// token is greater than the minimum value we are willing to accept, go
+	// ahead and use it.
+	if c.tokenExpiry.After(now) && c.tokenExpiry.Sub(now) > minCachedTokenLifetime {
+		return c.tokenMetadata
+	}
+	return nil
+}
+
+// constructRequest creates the STS request body in JSON based on the provided
+// options.
+// - Contents of the subjectToken are read from the file specified in
+//   options. If we encounter an error here, we bail out.
+// - Contents of the actorToken are read from the file specified in options.
+//   If we encounter an error here, we ignore this field because this is
+//   optional.
+// - Most of the other fields in the request come directly from options.
+//
+// A new HTTP request is created by calling http.NewRequestWithContext() and
+// passing the provided context, thereby enforcing any timeouts specified in
+// the latter.
+func constructRequest(ctx context.Context, opts Options) (*http.Request, error) {
+	subToken, err := readSubjectTokenFrom(opts.SubjectTokenPath)
+	if err != nil {
+		return nil, err
+	}
+	reqScope := opts.Scope
+	if reqScope == "" {
+		reqScope = defaultCloudPlatformScope
+	}
+	reqParams := &requestParameters{
+		GrantType:          tokenExchangeGrantType,
+		Resource:           opts.Resource,
+		Audience:           opts.Audience,
+		Scope:              reqScope,
+		RequestedTokenType: opts.RequestedTokenType,
+		SubjectToken:       string(subToken),
+		SubjectTokenType:   opts.SubjectTokenType,
+	}
+	if opts.ActorTokenPath != "" {
+		actorToken, err := readActorTokenFrom(opts.ActorTokenPath)
+		if err != nil {
+			return nil, err
+		}
+		reqParams.ActorToken = string(actorToken)
+		reqParams.ActorTokenType = opts.ActorTokenType
+	}
+	jsonBody, err := json.Marshal(reqParams)
+	if err != nil {
+		return nil, err
+	}
+	req, err := http.NewRequestWithContext(ctx, "POST", opts.TokenExchangeServiceURI, bytes.NewBuffer(jsonBody))
+	if err != nil {
+		return nil, fmt.Errorf("failed to create http request: %v", err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+	return req, nil
+}
+
+func sendRequest(client httpDoer, req *http.Request) ([]byte, error) {
+	// http.Client returns a non-nil error only if it encounters an error
+	// caused by client policy (such as CheckRedirect), or failure to speak
+	// HTTP (such as a network connectivity problem). A non-2xx status code
+	// doesn't cause an error.
+	resp, err := client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+
+	// When the http.Client returns a non-nil error, it is the
+	// responsibility of the caller to read the response body till an EOF is
+	// encountered and to close it.
+	body, err := ioutil.ReadAll(resp.Body)
+	resp.Body.Close()
+	if err != nil {
+		return nil, err
+	}
+
+	if resp.StatusCode == http.StatusOK {
+		return body, nil
+	}
+	grpclog.Warningf("http status %d, body: %s", resp.StatusCode, string(body))
+	return nil, fmt.Errorf("http status %d, body: %s", resp.StatusCode, string(body))
+}
+
+func tokenInfoFromResponse(respBody []byte) (*tokenInfo, error) {
+	respData := &responseParameters{}
+	if err := json.Unmarshal(respBody, respData); err != nil {
+		return nil, fmt.Errorf("json.Unmarshal(%v): %v", respBody, err)
+	}
+	if respData.AccessToken == "" {
+		return nil, fmt.Errorf("empty accessToken in response (%v)", string(respBody))
+	}
+	return &tokenInfo{
+		tokenType:  respData.TokenType,
+		token:      respData.AccessToken,
+		expiryTime: time.Now().Add(time.Duration(respData.ExpiresIn) * time.Second),
+	}, nil
+}
+
+// requestParameters stores all STS request attributes defined in
+// https://tools.ietf.org/html/rfc8693#section-2.1.
+type requestParameters struct {
+	// REQUIRED. The value "urn:ietf:params:oauth:grant-type:token-exchange"
+	// indicates that a token exchange is being performed.
+	GrantType string `json:"grant_type"`
+	// OPTIONAL. Indicates the location of the target service or resource where
+	// the client intends to use the requested security token.
+	Resource string `json:"resource,omitempty"`
+	// OPTIONAL. The logical name of the target service where the client intends
+	// to use the requested security token.
+	Audience string `json:"audience,omitempty"`
+	// OPTIONAL. A list of space-delimited, case-sensitive strings, that allow
+	// the client to specify the desired scope of the requested security token
+	// in the context of the service or Resource where the token will be used.
+	Scope string `json:"scope,omitempty"`
+	// OPTIONAL. An identifier, for the type of the requested security token.
+	RequestedTokenType string `json:"requested_token_type,omitempty"`
+	// REQUIRED. A security token that represents the identity of the party on
+	// behalf of whom the request is being made.
+	SubjectToken string `json:"subject_token"`
+	// REQUIRED. An identifier, that indicates the type of the security token in
+	// the "subject_token" parameter.
+	SubjectTokenType string `json:"subject_token_type"`
+	// OPTIONAL. A security token that represents the identity of the acting
+	// party.
+	ActorToken string `json:"actor_token,omitempty"`
+	// An identifier, that indicates the type of the security token in the
+	// "actor_token" parameter.
+	ActorTokenType string `json:"actor_token_type,omitempty"`
+}
+
+// nesponseParameters stores all attributes sent as JSON in a successful STS
+// response. These attributes are defined in
+// https://tools.ietf.org/html/rfc8693#section-2.2.1.
+type responseParameters struct {
+	// REQUIRED. The security token issued by the authorization server
+	// in response to the token exchange request.
+	AccessToken string `json:"access_token"`
+	// REQUIRED. An identifier, representation of the issued security token.
+	IssuedTokenType string `json:"issued_token_type"`
+	// REQUIRED. A case-insensitive value specifying the method of using the access
+	// token issued. It provides the client with information about how to utilize the
+	// access token to access protected resources.
+	TokenType string `json:"token_type"`
+	// RECOMMENDED. The validity lifetime, in seconds, of the token issued by the
+	// authorization server.
+	ExpiresIn int64 `json:"expires_in"`
+	// OPTIONAL, if the Scope of the issued security token is identical to the
+	// Scope requested by the client; otherwise, REQUIRED.
+	Scope string `json:"scope"`
+	// OPTIONAL. A refresh token will typically not be issued when the exchange is
+	// of one temporary credential (the subject_token) for a different temporary
+	// credential (the issued token) for use in some other context.
+	RefreshToken string `json:"refresh_token"`
+}
+
+// tokenInfo wraps the information received in a successful STS response.
+type tokenInfo struct {
+	tokenType  string
+	token      string
+	expiryTime time.Time
+}