Introduce STS client grants API and OPA policy integration (#6168)

This PR introduces two new features

- AWS STS compatible STS API named AssumeRoleWithClientGrants

```
POST /?Action=AssumeRoleWithClientGrants&Token=<jwt>
```

This API endpoint returns temporary access credentials, access
tokens signature types supported by this API

  - RSA keys
  - ECDSA keys

Fetches the required public key from the JWKS endpoints, provides
them as rsa or ecdsa public keys.

- External policy engine support, in this case OPA policy engine

- Credentials are stored on disks

cmd/config-current.go

@@ -25,10 +25,11 @@ import (
 
 	"github.com/minio/minio/cmd/crypto"
 	"github.com/minio/minio/cmd/logger"
-
 	"github.com/minio/minio/pkg/auth"
 	"github.com/minio/minio/pkg/event"
 	"github.com/minio/minio/pkg/event/target"
+	"github.com/minio/minio/pkg/iam/policy"
+	"github.com/minio/minio/pkg/iam/validator"
 )
 
 // Steps to move from version N to version N+1
@@ -40,9 +41,9 @@ import (
 // 6. Make changes in config-current_test.go for any test change
 
 // Config version
-const serverConfigVersion = "30"
+const serverConfigVersion = "31"
 
-type serverConfig = serverConfigV30
+type serverConfig = serverConfigV31
 
 var (
 	// globalServerConfig server config.
@@ -173,55 +174,55 @@ func (s *serverConfig) Validate() error {
 	// Worm, Cache and StorageClass values are already validated during json unmarshal
 	for _, v := range s.Notify.AMQP {
 		if err := v.Validate(); err != nil {
-			return fmt.Errorf("amqp: %s", err.Error())
+			return fmt.Errorf("amqp: %s", err)
 		}
 	}
 
 	for _, v := range s.Notify.Elasticsearch {
 		if err := v.Validate(); err != nil {
-			return fmt.Errorf("elasticsearch: %s", err.Error())
+			return fmt.Errorf("elasticsearch: %s", err)
 		}
 	}
 
 	for _, v := range s.Notify.Kafka {
 		if err := v.Validate(); err != nil {
-			return fmt.Errorf("kafka: %s", err.Error())
+			return fmt.Errorf("kafka: %s", err)
 		}
 	}
 
 	for _, v := range s.Notify.MQTT {
 		if err := v.Validate(); err != nil {
-			return fmt.Errorf("mqtt: %s", err.Error())
+			return fmt.Errorf("mqtt: %s", err)
 		}
 	}
 
 	for _, v := range s.Notify.MySQL {
 		if err := v.Validate(); err != nil {
-			return fmt.Errorf("mysql: %s", err.Error())
+			return fmt.Errorf("mysql: %s", err)
 		}
 	}
 
 	for _, v := range s.Notify.NATS {
 		if err := v.Validate(); err != nil {
-			return fmt.Errorf("nats: %s", err.Error())
+			return fmt.Errorf("nats: %s", err)
 		}
 	}
 
 	for _, v := range s.Notify.PostgreSQL {
 		if err := v.Validate(); err != nil {
-			return fmt.Errorf("postgreSQL: %s", err.Error())
+			return fmt.Errorf("postgreSQL: %s", err)
 		}
 	}
 
 	for _, v := range s.Notify.Redis {
 		if err := v.Validate(); err != nil {
-			return fmt.Errorf("redis: %s", err.Error())
+			return fmt.Errorf("redis: %s", err)
 		}
 	}
 
 	for _, v := range s.Notify.Webhook {
 		if err := v.Validate(); err != nil {
-			return fmt.Errorf("webhook: %s", err.Error())
+			return fmt.Errorf("webhook: %s", err)
 		}
 	}
 
@@ -503,17 +504,31 @@ func (s *serverConfig) loadToCachedConfigs() {
 			globalKMSKeyID = globalKMSConfig.Vault.Key.Name
 		}
 	}
+
 	if !globalIsCompressionEnabled {
 		compressionConf := s.GetCompressionConfig()
 		globalCompressExtensions = compressionConf.Extensions
 		globalCompressMimeTypes = compressionConf.MimeTypes
 		globalIsCompressionEnabled = compressionConf.Enabled
 	}
+
+	if globalIAMValidators == nil {
+		globalIAMValidators = getAuthValidators(s)
+	}
+
+	if globalPolicyOPA == nil {
+		if s.Policy.OPA.URL != nil && s.Policy.OPA.URL.String() != "" {
+			globalPolicyOPA = iampolicy.NewOpa(iampolicy.OpaArgs{
+				URL:       s.Policy.OPA.URL,
+				AuthToken: s.Policy.OPA.AuthToken,
+			})
+		}
+	}
 }
 
-// newConfig - initialize a new server config, saves env parameters if
+// newSrvConfig - initialize a new server config, saves env parameters if
 // found, otherwise use default parameters
-func newConfig(objAPI ObjectLayer) error {
+func newSrvConfig(objAPI ObjectLayer) error {
 	// Initialize server config.
 	srvCfg := newServerConfig()
 
@@ -564,6 +579,20 @@ func loadConfig(objAPI ObjectLayer) error {
 	return nil
 }
 
+// getAuthValidators - returns ValidatorList which contains
+// enabled providers in server config.
+// A new authentication provider is added like below
+// * Add a new provider in pkg/iam/validator package.
+func getAuthValidators(config *serverConfig) *validator.Validators {
+	validators := validator.NewValidators()
+
+	if config.OpenID.JWKS.URL != nil {
+		validators.Add(validator.NewJWT(config.OpenID.JWKS))
+	}
+
+	return validators
+}
+
 // getNotificationTargets - returns TargetList which contains enabled targets in serverConfig.
 // A new notification target is added like below
 // * Add a new target in pkg/event/target package.