google/internal: Add AWS Session Token to Metadata Requests

AWS released a new instance metadata service (IMDSv2). IMDSv2 brought a requirement that a session token header is now required on every call to metadata endpoint.
Modify the AWS credential retrieval flow to fetch the session token and send it along with the calls to metadata endpoints

Change-Id: I539912ab38f5e591658b29a1e7a99d2b828a1128
GitHub-Last-Rev: 29e1f4a
GitHub-Pull-Request: #554
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/390794
Reviewed-by: Cody Oss <[email protected]>
Trust: Cody Oss <[email protected]>
Run-TryBot: Cody Oss <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Leo Siracusa <[email protected]>
Trust: Tyler Bui-Palsulich <[email protected]>

Author: sai-sunder-s

google/internal/externalaccount/aws.go

@@ -52,6 +52,13 @@ const (
 	// The AWS authorization header name for the security session token if available.
 	awsSecurityTokenHeader = "x-amz-security-token"
 
+	// The name of the header containing the session token for metadata endpoint calls
+	awsIMDSv2SessionTokenHeader = "X-aws-ec2-metadata-token"
+
+	awsIMDSv2SessionTtlHeader = "X-aws-ec2-metadata-token-ttl-seconds"
+
+	awsIMDSv2SessionTtl = "300"
+
 	// The AWS authorization header name for the auto-generated date.
 	awsDateHeader = "x-amz-date"
 
@@ -241,6 +248,7 @@ type awsCredentialSource struct {
 	RegionURL                   string
 	RegionalCredVerificationURL string
 	CredVerificationURL         string
+	IMDSv2SessionTokenURL       string
 	TargetResource              string
 	requestSigner               *awsRequestSigner
 	region                      string
@@ -268,12 +276,22 @@ func (cs awsCredentialSource) doRequest(req *http.Request) (*http.Response, erro
 
 func (cs awsCredentialSource) subjectToken() (string, error) {
 	if cs.requestSigner == nil {
-		awsSecurityCredentials, err := cs.getSecurityCredentials()
+		awsSessionToken, err := cs.getAWSSessionToken()
+		if err != nil {
+			return "", err
+		}
+
+		headers := make(map[string]string)
+		if awsSessionToken != "" {
+			headers[awsIMDSv2SessionTokenHeader] = awsSessionToken
+		}
+
+		awsSecurityCredentials, err := cs.getSecurityCredentials(headers)
 		if err != nil {
 			return "", err
 		}
 
-		if cs.region, err = cs.getRegion(); err != nil {
+		if cs.region, err = cs.getRegion(headers); err != nil {
 			return "", err
 		}
 
@@ -340,7 +358,37 @@ func (cs awsCredentialSource) subjectToken() (string, error) {
 	return url.QueryEscape(string(result)), nil
 }
 
-func (cs *awsCredentialSource) getRegion() (string, error) {
+func (cs *awsCredentialSource) getAWSSessionToken() (string, error) {
+	if cs.IMDSv2SessionTokenURL == "" {
+		return "", nil
+	}
+
+	req, err := http.NewRequest("PUT", cs.IMDSv2SessionTokenURL, nil)
+	if err != nil {
+		return "", err
+	}
+
+	req.Header.Add(awsIMDSv2SessionTtlHeader, awsIMDSv2SessionTtl)
+
+	resp, err := cs.doRequest(req)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+
+	respBody, err := ioutil.ReadAll(io.LimitReader(resp.Body, 1<<20))
+	if err != nil {
+		return "", err
+	}
+
+	if resp.StatusCode != 200 {
+		return "", fmt.Errorf("oauth2/google: unable to retrieve AWS session token - %s", string(respBody))
+	}
+
+	return string(respBody), nil
+}
+
+func (cs *awsCredentialSource) getRegion(headers map[string]string) (string, error) {
 	if envAwsRegion := getenv("AWS_REGION"); envAwsRegion != "" {
 		return envAwsRegion, nil
 	}
@@ -357,6 +405,10 @@ func (cs *awsCredentialSource) getRegion() (string, error) {
 		return "", err
 	}
 
+	for name, value := range headers {
+		req.Header.Add(name, value)
+	}
+
 	resp, err := cs.doRequest(req)
 	if err != nil {
 		return "", err
@@ -381,7 +433,7 @@ func (cs *awsCredentialSource) getRegion() (string, error) {
 	return string(respBody[:respBodyEnd]), nil
 }
 
-func (cs *awsCredentialSource) getSecurityCredentials() (result awsSecurityCredentials, err error) {
+func (cs *awsCredentialSource) getSecurityCredentials(headers map[string]string) (result awsSecurityCredentials, err error) {
 	if accessKeyID := getenv("AWS_ACCESS_KEY_ID"); accessKeyID != "" {
 		if secretAccessKey := getenv("AWS_SECRET_ACCESS_KEY"); secretAccessKey != "" {
 			return awsSecurityCredentials{
@@ -392,12 +444,12 @@ func (cs *awsCredentialSource) getSecurityCredentials() (result awsSecurityCrede
 		}
 	}
 
-	roleName, err := cs.getMetadataRoleName()
+	roleName, err := cs.getMetadataRoleName(headers)
 	if err != nil {
 		return
 	}
 
-	credentials, err := cs.getMetadataSecurityCredentials(roleName)
+	credentials, err := cs.getMetadataSecurityCredentials(roleName, headers)
 	if err != nil {
 		return
 	}
@@ -413,7 +465,7 @@ func (cs *awsCredentialSource) getSecurityCredentials() (result awsSecurityCrede
 	return credentials, nil
 }
 
-func (cs *awsCredentialSource) getMetadataSecurityCredentials(roleName string) (awsSecurityCredentials, error) {
+func (cs *awsCredentialSource) getMetadataSecurityCredentials(roleName string, headers map[string]string) (awsSecurityCredentials, error) {
 	var result awsSecurityCredentials
 
 	req, err := http.NewRequest("GET", fmt.Sprintf("%s/%s", cs.CredVerificationURL, roleName), nil)
@@ -422,6 +474,10 @@ func (cs *awsCredentialSource) getMetadataSecurityCredentials(roleName string) (
 	}
 	req.Header.Add("Content-Type", "application/json")
 
+	for name, value := range headers {
+		req.Header.Add(name, value)
+	}
+
 	resp, err := cs.doRequest(req)
 	if err != nil {
 		return result, err
@@ -441,7 +497,7 @@ func (cs *awsCredentialSource) getMetadataSecurityCredentials(roleName string) (
 	return result, err
 }
 
-func (cs *awsCredentialSource) getMetadataRoleName() (string, error) {
+func (cs *awsCredentialSource) getMetadataRoleName(headers map[string]string) (string, error) {
 	if cs.CredVerificationURL == "" {
 		return "", errors.New("oauth2/google: unable to determine the AWS metadata server security credentials endpoint")
 	}
@@ -451,6 +507,10 @@ func (cs *awsCredentialSource) getMetadataRoleName() (string, error) {
 		return "", err
 	}
 
+	for name, value := range headers {
+		req.Header.Add(name, value)
+	}
+
 	resp, err := cs.doRequest(req)
 	if err != nil {
 		return "", err