aws.go

// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package externalaccount

import (
	"bytes"
	"context"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
	"net/http"
	"net/url"
	"os"
	"path"
	"sort"
	"strings"
	"time"

	"golang.org/x/oauth2"
)

type awsSecurityCredentials struct {
	AccessKeyID     string `json:"AccessKeyID"`
	SecretAccessKey string `json:"SecretAccessKey"`
	SecurityToken   string `json:"Token"`
}

// awsRequestSigner is a utility class to sign http requests using a AWS V4 signature.
type awsRequestSigner struct {
	RegionName             string
	AwsSecurityCredentials awsSecurityCredentials
}

// getenv aliases os.Getenv for testing
var getenv = os.Getenv

const (
	// AWS Signature Version 4 signing algorithm identifier.
	awsAlgorithm = "AWS4-HMAC-SHA256"

	// The termination string for the AWS credential scope value as defined in
	// https://docs.aws.amazon.com/general/latest/gr/sigv4-create-string-to-sign.html
	awsRequestType = "aws4_request"

	// 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"

	// Supported AWS configuration environment variables.
	awsAccessKeyId     = "AWS_ACCESS_KEY_ID"
	awsDefaultRegion   = "AWS_DEFAULT_REGION"
	awsRegion          = "AWS_REGION"
	awsSecretAccessKey = "AWS_SECRET_ACCESS_KEY"
	awsSessionToken    = "AWS_SESSION_TOKEN"

	awsTimeFormatLong  = "20060102T150405Z"
	awsTimeFormatShort = "20060102"
)

func getSha256(input []byte) (string, error) {
	hash := sha256.New()
	if _, err := hash.Write(input); err != nil {
		return "", err
	}
	return hex.EncodeToString(hash.Sum(nil)), nil
}

func getHmacSha256(key, input []byte) ([]byte, error) {
	hash := hmac.New(sha256.New, key)
	if _, err := hash.Write(input); err != nil {
		return nil, err
	}
	return hash.Sum(nil), nil
}

func canonicalPath(req *http.Request) string {
	result := req.URL.EscapedPath()
	if result == "" {
		return "/"
	}
	return path.Clean(result)
}

func canonicalQuery(req *http.Request) string {
	queryValues := req.URL.Query()
	for queryKey := range queryValues {
		sort.Strings(queryValues[queryKey])
	}
	return queryValues.Encode()
}

func canonicalHeaders(req *http.Request) (string, string) {
	// Header keys need to be sorted alphabetically.
	var headers []string
	lowerCaseHeaders := make(http.Header)
	for k, v := range req.Header {
		k := strings.ToLower(k)
		if _, ok := lowerCaseHeaders[k]; ok {
			// include additional values
			lowerCaseHeaders[k] = append(lowerCaseHeaders[k], v...)
		} else {
			headers = append(headers, k)
			lowerCaseHeaders[k] = v
		}
	}
	sort.Strings(headers)

	var fullHeaders bytes.Buffer
	for _, header := range headers {
		headerValue := strings.Join(lowerCaseHeaders[header], ",")
		fullHeaders.WriteString(header)
		fullHeaders.WriteRune(':')
		fullHeaders.WriteString(headerValue)
		fullHeaders.WriteRune('\n')
	}

	return strings.Join(headers, ";"), fullHeaders.String()
}

func requestDataHash(req *http.Request) (string, error) {
	var requestData []byte
	if req.Body != nil {
		requestBody, err := req.GetBody()
		if err != nil {
			return "", err
		}
		defer requestBody.Close()

		requestData, err = ioutil.ReadAll(io.LimitReader(requestBody, 1<<20))
		if err != nil {
			return "", err
		}
	}

	return getSha256(requestData)
}

func requestHost(req *http.Request) string {
	if req.Host != "" {
		return req.Host
	}
	return req.URL.Host
}

func canonicalRequest(req *http.Request, canonicalHeaderColumns, canonicalHeaderData string) (string, error) {
	dataHash, err := requestDataHash(req)
	if err != nil {
		return "", err
	}

	return fmt.Sprintf("%s\n%s\n%s\n%s\n%s\n%s", req.Method, canonicalPath(req), canonicalQuery(req), canonicalHeaderData, canonicalHeaderColumns, dataHash), nil
}

// SignRequest adds the appropriate headers to an http.Request
// or returns an error if something prevented this.
func (rs *awsRequestSigner) SignRequest(req *http.Request) error {
	signedRequest := req.Clone(req.Context())
	timestamp := now()

	signedRequest.Header.Add("host", requestHost(req))

	if rs.AwsSecurityCredentials.SecurityToken != "" {
		signedRequest.Header.Add(awsSecurityTokenHeader, rs.AwsSecurityCredentials.SecurityToken)
	}

	if signedRequest.Header.Get("date") == "" {
		signedRequest.Header.Add(awsDateHeader, timestamp.Format(awsTimeFormatLong))
	}

	authorizationCode, err := rs.generateAuthentication(signedRequest, timestamp)
	if err != nil {
		return err
	}
	signedRequest.Header.Set("Authorization", authorizationCode)

	req.Header = signedRequest.Header
	return nil
}

func (rs *awsRequestSigner) generateAuthentication(req *http.Request, timestamp time.Time) (string, error) {
	canonicalHeaderColumns, canonicalHeaderData := canonicalHeaders(req)

	dateStamp := timestamp.Format(awsTimeFormatShort)
	serviceName := ""
	if splitHost := strings.Split(requestHost(req), "."); len(splitHost) > 0 {
		serviceName = splitHost[0]
	}

	credentialScope := fmt.Sprintf("%s/%s/%s/%s", dateStamp, rs.RegionName, serviceName, awsRequestType)

	requestString, err := canonicalRequest(req, canonicalHeaderColumns, canonicalHeaderData)
	if err != nil {
		return "", err
	}
	requestHash, err := getSha256([]byte(requestString))
	if err != nil {
		return "", err
	}

	stringToSign := fmt.Sprintf("%s\n%s\n%s\n%s", awsAlgorithm, timestamp.Format(awsTimeFormatLong), credentialScope, requestHash)

	signingKey := []byte("AWS4" + rs.AwsSecurityCredentials.SecretAccessKey)
	for _, signingInput := range []string{
		dateStamp, rs.RegionName, serviceName, awsRequestType, stringToSign,
	} {
		signingKey, err = getHmacSha256(signingKey, []byte(signingInput))
		if err != nil {
			return "", err
		}
	}

	return fmt.Sprintf("%s Credential=%s/%s, SignedHeaders=%s, Signature=%s", awsAlgorithm, rs.AwsSecurityCredentials.AccessKeyID, credentialScope, canonicalHeaderColumns, hex.EncodeToString(signingKey)), nil
}

type awsCredentialSource struct {
	EnvironmentID               string
	RegionURL                   string
	RegionalCredVerificationURL string
	CredVerificationURL         string
	IMDSv2SessionTokenURL       string
	TargetResource              string
	requestSigner               *awsRequestSigner
	region                      string
	ctx                         context.Context
	client                      *http.Client
}

type awsRequestHeader struct {
	Key   string `json:"key"`
	Value string `json:"value"`
}

type awsRequest struct {
	URL     string             `json:"url"`
	Method  string             `json:"method"`
	Headers []awsRequestHeader `json:"headers"`
}

func (cs awsCredentialSource) validateMetadataServers() error {
	if err := cs.validateMetadataServer(cs.RegionURL, "region_url"); err != nil {
		return err
	}
	if err := cs.validateMetadataServer(cs.CredVerificationURL, "url"); err != nil {
		return err
	}
	return cs.validateMetadataServer(cs.IMDSv2SessionTokenURL, "imdsv2_session_token_url")
}

var validHostnames []string = []string{"169.254.169.254", "fd00:ec2::254"}

func (cs awsCredentialSource) isValidMetadataServer(metadataUrl string) bool {
	if metadataUrl == "" {
		// Zero value means use default, which is valid.
		return true
	}

	u, err := url.Parse(metadataUrl)
	if err != nil {
		// Unparseable URL means invalid
		return false
	}

	for _, validHostname := range validHostnames {
		if u.Hostname() == validHostname {
			// If it's one of the valid hostnames, everything is good
			return true
		}
	}

	// hostname not found in our allowlist, so not valid
	return false
}

func (cs awsCredentialSource) validateMetadataServer(metadataUrl, urlName string) error {
	if !cs.isValidMetadataServer(metadataUrl) {
		return fmt.Errorf("oauth2/google: invalid hostname %s for %s", metadataUrl, urlName)
	}

	return nil
}

func (cs awsCredentialSource) doRequest(req *http.Request) (*http.Response, error) {
	if cs.client == nil {
		cs.client = oauth2.NewClient(cs.ctx, nil)
	}
	return cs.client.Do(req.WithContext(cs.ctx))
}

func canRetrieveRegionFromEnvironment() bool {
	// The AWS region can be provided through AWS_REGION or AWS_DEFAULT_REGION. Only one is
	// required.
	return getenv(awsRegion) != "" || getenv(awsDefaultRegion) != ""
}

func canRetrieveSecurityCredentialFromEnvironment() bool {
	// Check if both AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY are available.
	return getenv(awsAccessKeyId) != "" && getenv(awsSecretAccessKey) != ""
}

func shouldUseMetadataServer() bool {
	return !canRetrieveRegionFromEnvironment() || !canRetrieveSecurityCredentialFromEnvironment()
}

func (cs awsCredentialSource) subjectToken() (string, error) {
	if cs.requestSigner == nil {
		headers := make(map[string]string)
		if shouldUseMetadataServer() {
			awsSessionToken, err := cs.getAWSSessionToken()
			if err != nil {
				return "", err
			}

			if awsSessionToken != "" {
				headers[awsIMDSv2SessionTokenHeader] = awsSessionToken
			}
		}

		awsSecurityCredentials, err := cs.getSecurityCredentials(headers)
		if err != nil {
			return "", err
		}

		if cs.region, err = cs.getRegion(headers); err != nil {
			return "", err
		}

		cs.requestSigner = &awsRequestSigner{
			RegionName:             cs.region,
			AwsSecurityCredentials: awsSecurityCredentials,
		}
	}

	// Generate the signed request to AWS STS GetCallerIdentity API.
	// Use the required regional endpoint. Otherwise, the request will fail.
	req, err := http.NewRequest("POST", strings.Replace(cs.RegionalCredVerificationURL, "{region}", cs.region, 1), nil)
	if err != nil {
		return "", err
	}
	// The full, canonical resource name of the workload identity pool
	// provider, with or without the HTTPS prefix.
	// Including this header as part of the signature is recommended to
	// ensure data integrity.
	if cs.TargetResource != "" {
		req.Header.Add("x-goog-cloud-target-resource", cs.TargetResource)
	}
	cs.requestSigner.SignRequest(req)

	/*
	   The GCP STS endpoint expects the headers to be formatted as:
	   # [
	   #   {key: 'x-amz-date', value: '...'},
	   #   {key: 'Authorization', value: '...'},
	   #   ...
	   # ]
	   # And then serialized as:
	   # quote(json.dumps({
	   #   url: '...',
	   #   method: 'POST',
	   #   headers: [{key: 'x-amz-date', value: '...'}, ...]
	   # }))
	*/

	awsSignedReq := awsRequest{
		URL:    req.URL.String(),
		Method: "POST",
	}
	for headerKey, headerList := range req.Header {
		for _, headerValue := range headerList {
			awsSignedReq.Headers = append(awsSignedReq.Headers, awsRequestHeader{
				Key:   headerKey,
				Value: headerValue,
			})
		}
	}
	sort.Slice(awsSignedReq.Headers, func(i, j int) bool {
		headerCompare := strings.Compare(awsSignedReq.Headers[i].Key, awsSignedReq.Headers[j].Key)
		if headerCompare == 0 {
			return strings.Compare(awsSignedReq.Headers[i].Value, awsSignedReq.Headers[j].Value) < 0
		}
		return headerCompare < 0
	})

	result, err := json.Marshal(awsSignedReq)
	if err != nil {
		return "", err
	}
	return url.QueryEscape(string(result)), nil
}

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 canRetrieveRegionFromEnvironment() {
		if envAwsRegion := getenv(awsRegion); envAwsRegion != "" {
			return envAwsRegion, nil
		}
		return getenv("AWS_DEFAULT_REGION"), nil
	}

	if cs.RegionURL == "" {
		return "", errors.New("oauth2/google: unable to determine AWS region")
	}

	req, err := http.NewRequest("GET", cs.RegionURL, nil)
	if err != nil {
		return "", err
	}

	for name, value := range headers {
		req.Header.Add(name, value)
	}

	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 region - %s", string(respBody))
	}

	// This endpoint will return the region in format: us-east-2b.
	// Only the us-east-2 part should be used.
	respBodyEnd := 0
	if len(respBody) > 1 {
		respBodyEnd = len(respBody) - 1
	}
	return string(respBody[:respBodyEnd]), nil
}

func (cs *awsCredentialSource) getSecurityCredentials(headers map[string]string) (result awsSecurityCredentials, err error) {
	if canRetrieveSecurityCredentialFromEnvironment() {
		return awsSecurityCredentials{
			AccessKeyID:     getenv(awsAccessKeyId),
			SecretAccessKey: getenv(awsSecretAccessKey),
			SecurityToken:   getenv(awsSessionToken),
		}, nil
	}

	roleName, err := cs.getMetadataRoleName(headers)
	if err != nil {
		return
	}

	credentials, err := cs.getMetadataSecurityCredentials(roleName, headers)
	if err != nil {
		return
	}

	if credentials.AccessKeyID == "" {
		return result, errors.New("oauth2/google: missing AccessKeyId credential")
	}

	if credentials.SecretAccessKey == "" {
		return result, errors.New("oauth2/google: missing SecretAccessKey credential")
	}

	return credentials, nil
}

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)
	if err != nil {
		return result, err
	}
	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
	}
	defer resp.Body.Close()

	respBody, err := ioutil.ReadAll(io.LimitReader(resp.Body, 1<<20))
	if err != nil {
		return result, err
	}

	if resp.StatusCode != 200 {
		return result, fmt.Errorf("oauth2/google: unable to retrieve AWS security credentials - %s", string(respBody))
	}

	err = json.Unmarshal(respBody, &result)
	return result, err
}

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")
	}

	req, err := http.NewRequest("GET", cs.CredVerificationURL, nil)
	if err != nil {
		return "", err
	}

	for name, value := range headers {
		req.Header.Add(name, value)
	}

	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 role name - %s", string(respBody))
	}

	return string(respBody), nil
}