Support reading secrets from a separate config file

Users can specify a new overlay config file. This file can contain
secrets. The file is specified in two ways:

- VOUCH_SECRETS_FILE env var: path of the overlay config file

- CREDENTIALS_DIRECTORY env var that contains a file called
  VOUCH_SECRETS_FILE. This can be used with systemd LoadCredential.

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 Coming soon! Please document any work in progress here as part of your PR. It will be moved to the next tag when released.
 
+- [Support reading secrets from a separate file](https://github.com/vouch/vouch-proxy/pull/487)
+
 ## v0.37.0
 
 - [allow configurable Write, Read and Idle timeouts for the http server](https://github.com/vouch/vouch-proxy/pull/468)

config/testing/secret_overlay.yml

@@ -0,0 +1,2 @@
+oauth:
+  client_secret: my client secret from overlay

pkg/cfg/cfg.go

@@ -229,6 +229,7 @@ func configureFromEnv() bool {
 	if err != nil {
 		log.Fatal(err.Error())
 	}
+
 	// did anything change?
 	if !reflect.DeepEqual(preEnvConfig, *Cfg) ||
 		!reflect.DeepEqual(preEnvGenOAuth, *GenOAuth) {
@@ -270,8 +271,31 @@ func setRootDir() {
 
 // parseConfig parse the config file
 func parseConfigFile() error {
-	configEnv := os.Getenv(Branding.UCName + "_CONFIG")
+	secretsFileVar := Branding.UCName + "_SECRETS_FILE"
+	overlayConfigPath := os.Getenv(secretsFileVar)
+	if overlayConfigPath == "" {
+		// see if systemd LoadCredential was used to pass in the overlay config
+		if credDir := os.Getenv("CREDENTIALS_DIRECTORY"); credDir != "" {
+			overlayConfigPath = filepath.Join(credDir, secretsFileVar)
+			if _, err := os.Stat(overlayConfigPath); os.IsNotExist(err) {
+				log.Warnf("%s not found in CREDENTIALS_DIRECTORY %s", secretsFileVar, credDir)
+				overlayConfigPath = ""
+			} else {
+				log.Infof("reading secrets file from CREDENTIALS_DIRECTORY: %s", overlayConfigPath)
+			}
+		}
+	} else {
+		log.Infof("reading secrets file from %s env var: %s", secretsFileVar, overlayConfigPath)
+	}
+	if overlayConfigPath != "" {
+		viper.SetConfigFile(overlayConfigPath)
+		if err := viper.ReadInConfig(); err != nil {
+			return err
+		}
+		viper.SetConfigFile("")
+	}
 
+	configEnv := os.Getenv(Branding.UCName + "_CONFIG")
 	if configEnv != "" {
 		log.Warnf("config file loaded from environmental variable %s: %s", Branding.UCName+"_CONFIG", configEnv)
 		configFile, _ := filepath.Abs(configEnv)
@@ -287,8 +311,8 @@ func parseConfigFile() error {
 		viper.SetConfigType("yaml")
 		viper.AddConfigPath(filepath.Join(RootDir, "config"))
 	}
-	err := viper.ReadInConfig() // Find and read the config file
-	if err != nil {             // Handle errors reading the config file
+	err := viper.MergeInConfig() // Find and read the config file
+	if err != nil {              // Handle errors reading the config file
 
 		return fmt.Errorf("%w: %s", errConfigNotFound, err)
 	}

pkg/cfg/oauth_test.go

@@ -11,7 +11,10 @@ OR CONDITIONS OF ANY KIND, either express or implied.
 package cfg
 
 import (
+	"io/ioutil"
 	"net/url"
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -43,3 +46,48 @@ func Test_configureOAuthWithClaims(t *testing.T) {
 	assert.Nil(t, err)
 	assert.Equal(t, authCodeURL.Query().Get("claims"), `{"userinfo":{"email":{"essential":true},"email_verified":{"essential":true},"given_name":{"essential":true},"http://example.info/claims/groups":null,"nickname":null,"picture":null},"id_token":{"acr":{"values":["urn:mace:incommon:iap:silver"]},"auth_time":{"essential":true}}}`)
 }
+
+func Test_readOverlayConfig_fileVar(t *testing.T) {
+	defer cleanupEnv()
+	rootDir := os.Getenv(Branding.UCName + "_ROOT")
+	assert.NotEmpty(t, rootDir)
+	assert.NoError(t, os.Setenv(Branding.UCName+"_SECRETS_FILE", filepath.Join(rootDir, "config/testing/secret_overlay.yml")))
+	setUp("config/testing/handler_login_url.yml")
+	assert.Equal(t, "my client secret from overlay", OAuthClient.ClientSecret)
+}
+
+func Test_readOverlayConfig_credentialsDir(t *testing.T) {
+	defer cleanupEnv()
+	rootDir := os.Getenv(Branding.UCName + "_ROOT")
+	assert.NotEmpty(t, rootDir)
+	tempDir, err := ioutil.TempDir("", "")
+	assert.NoError(t, err)
+	defer func() {
+		_ = os.RemoveAll(tempDir)
+	}()
+	destFileName := Branding.UCName + "_SECRETS_FILE"
+	srcFileName := filepath.Join(rootDir, "config/testing/secret_overlay.yml")
+	assert.NoError(t, os.Symlink(srcFileName, filepath.Join(tempDir, destFileName)))
+	assert.NoError(t, os.Setenv("CREDENTIALS_DIRECTORY", tempDir))
+	setUp("config/testing/handler_login_url.yml")
+	assert.Equal(t, "my client secret from overlay", OAuthClient.ClientSecret)
+}
+
+func Test_readOverlayConfig_emptyCredentialsDir(t *testing.T) {
+	defer cleanupEnv()
+	tempDir, err := ioutil.TempDir("", "")
+	assert.NoError(t, err)
+	defer func() {
+		_ = os.RemoveAll(tempDir)
+	}()
+	assert.NoError(t, os.Setenv("CREDENTIALS_DIRECTORY", tempDir))
+	setUp("config/testing/handler_login_url.yml")
+	assert.Equal(t, "", OAuthClient.ClientSecret)
+}
+
+func Test_readOverlayConfig_missingCredentialsDir(t *testing.T) {
+	defer cleanupEnv()
+	assert.NoError(t, os.Setenv("CREDENTIALS_DIRECTORY", "/this/doesnt/exist"))
+	setUp("config/testing/handler_login_url.yml")
+	assert.Equal(t, "", OAuthClient.ClientSecret)
+}