feat: support AWS assume role

.gitignore

@@ -1,3 +1,4 @@
+/.idea
 /restic
 /restic.exe
 /.vagrant

changelog/unreleased/issue-4472

@@ -0,0 +1,14 @@
+Enhancement: Allow AWS Assume Role to be used for S3 backend
+
+Previously only credentials discovered via the Minio Click discovery methods
+would be used to authenticate. However there are many circumstances where the
+discovered credentials have lower permissions and need to assume a specific role.
+
+New Environment Variables:
+
+- RESTIC_AWS_ASSUME_ROLE_ARN
+- RESTIC_AWS_ASSUME_ROLE_SESSION_NAME
+- RESTIC_AWS_ASSUME_ROLE_EXTERNAL_ID
+- RESTIC_AWS_ASSUME_ROLE_REGION (if need to override from us-east-1)
+- RESTIC_AWS_ASSUME_ROLE_POLICY
+- RESTIC_AWS_ASSUME_ROLE_STS_ENDPOINT

doc/040_backup.rst

@@ -628,6 +628,10 @@ environment variables. The following lists these environment variables:
     AWS_DEFAULT_REGION                  Amazon S3 default region
     AWS_PROFILE                         Amazon credentials profile (alternative to specifying key and region)
     AWS_SHARED_CREDENTIALS_FILE         Location of the AWS CLI shared credentials file (default: ~/.aws/credentials)
+    RESTIC_AWS_ASSUME_ROLE_ARN          Amazon IAM Role ARN to assume using discovered credentials
+    RESTIC_AWS_ASSUME_ROLE_SESSION_NAME Session Name to use with the role assumption
+    RESTIC_AWS_ASSUME_ROLE_EXTERNAL_ID  External ID to use with the role assumption
+    RESTIC_AWS_ASSUME_ROLE_REGION       Region to use for IAM calls for the role assumption
 
     AZURE_ACCOUNT_NAME                  Account name for Azure
     AZURE_ACCOUNT_KEY                   Account key for Azure

internal/backend/s3/s3.go

@@ -51,40 +51,9 @@ func open(ctx context.Context, cfg Config, rt http.RoundTripper) (*Backend, erro
 		minio.MaxRetry = int(cfg.MaxRetries)
 	}
 
-	// Chains all credential types, in the following order:
+	creds, err := getCredentials(cfg)
-	// 	- Static credentials provided by user
-	//	- AWS env vars (i.e. AWS_ACCESS_KEY_ID)
-	//  - Minio env vars (i.e. MINIO_ACCESS_KEY)
-	//  - AWS creds file (i.e. AWS_SHARED_CREDENTIALS_FILE or ~/.aws/credentials)
-	//  - Minio creds file (i.e. MINIO_SHARED_CREDENTIALS_FILE or ~/.mc/config.json)
-	//  - IAM profile based credentials. (performs an HTTP
-	//    call to a pre-defined endpoint, only valid inside
-	//    configured ec2 instances)
-	creds := credentials.NewChainCredentials([]credentials.Provider{
-		&credentials.EnvAWS{},
-		&credentials.Static{
-			Value: credentials.Value{
-				AccessKeyID:     cfg.KeyID,
-				SecretAccessKey: cfg.Secret.Unwrap(),
-			},
-		},
-		&credentials.EnvMinio{},
-		&credentials.FileAWSCredentials{},
-		&credentials.FileMinioClient{},
-		&credentials.IAM{
-			Client: &http.Client{
-				Transport: http.DefaultTransport,
-			},
-		},
-	})
-
-	c, err := creds.Get()
 	if err != nil {
-		return nil, errors.Wrap(err, "creds.Get")
+		return nil, errors.Wrap(err, "s3.getCredentials")
-	}
-
-	if c.SignerType == credentials.SignatureAnonymous {
-		debug.Log("using anonymous access for %#v", cfg.Endpoint)
 	}
 
 	options := &minio.Options{
@@ -125,6 +94,93 @@ func open(ctx context.Context, cfg Config, rt http.RoundTripper) (*Backend, erro
 	return be, nil
 }
 
+// getCredentials -- runs through the various credential types and returns the first one that works.
+// additionally if the user has specified a role to assume, it will do that as well.
+func getCredentials(cfg Config) (*credentials.Credentials, error) {
+	// Chains all credential types, in the following order:
+	// 	- Static credentials provided by user
+	//	- AWS env vars (i.e. AWS_ACCESS_KEY_ID)
+	//  - Minio env vars (i.e. MINIO_ACCESS_KEY)
+	//  - AWS creds file (i.e. AWS_SHARED_CREDENTIALS_FILE or ~/.aws/credentials)
+	//  - Minio creds file (i.e. MINIO_SHARED_CREDENTIALS_FILE or ~/.mc/config.json)
+	//  - IAM profile based credentials. (performs an HTTP
+	//    call to a pre-defined endpoint, only valid inside
+	//    configured ec2 instances)
+	creds := credentials.NewChainCredentials([]credentials.Provider{
+		&credentials.Static{
+			Value: credentials.Value{
+				AccessKeyID:     cfg.KeyID,
+				SecretAccessKey: cfg.Secret.Unwrap(),
+			},
+		},
+		&credentials.EnvAWS{},
+		&credentials.EnvMinio{},
+		&credentials.FileAWSCredentials{},
+		&credentials.FileMinioClient{},
+		&credentials.IAM{
+			Client: &http.Client{
+				Transport: http.DefaultTransport,
+			},
+		},
+	})
+
+	c, err := creds.Get()
+	if err != nil {
+		return nil, errors.Wrap(err, "creds.Get")
+	}
+
+	if c.SignerType == credentials.SignatureAnonymous {
+		debug.Log("using anonymous access for %#v", cfg.Endpoint)
+	}
+
+	roleArn := os.Getenv("RESTIC_AWS_ASSUME_ROLE_ARN")
+	if roleArn != "" {
+		// use the region provided by the configuration by default
+		awsRegion := cfg.Region
+		// allow the region to be overridden if for some reason it is required
+		if len(os.Getenv("RESTIC_AWS_ASSUME_ROLE_REGION")) > 0 {
+			awsRegion = os.Getenv("RESTIC_AWS_ASSUME_ROLE_REGION")
+		}
+
+		sessionName := os.Getenv("RESTIC_AWS_ASSUME_ROLE_SESSION_NAME")
+		externalID := os.Getenv("RESTIC_AWS_ASSUME_ROLE_EXTERNAL_ID")
+		policy := os.Getenv("RESTIC_AWS_ASSUME_ROLE_POLICY")
+		stsEndpoint := os.Getenv("RESTIC_AWS_ASSUME_ROLE_STS_ENDPOINT")
+
+		if stsEndpoint == "" {
+			if len(awsRegion) > 0 {
+				if strings.HasPrefix(awsRegion, "cn-") {
+					stsEndpoint = "https://sts." + awsRegion + ".amazonaws.com.cn"
+				} else {
+					stsEndpoint = "https://sts." + awsRegion + ".amazonaws.com"
+				}
+			} else {
+				stsEndpoint = "https://sts.amazonaws.com"
+			}
+		}
+
+		opts := credentials.STSAssumeRoleOptions{
+			RoleARN:         roleArn,
+			AccessKey:       c.AccessKeyID,
+			SecretKey:       c.SecretAccessKey,
+			SessionToken:    c.SessionToken,
+			RoleSessionName: sessionName,
+			ExternalID:      externalID,
+			Policy:          policy,
+		}
+		if len(awsRegion) > 0 {
+			opts.Location = awsRegion
+		}
+
+		creds, err = credentials.NewSTSAssumeRole(stsEndpoint, opts)
+		if err != nil {
+			return nil, errors.Wrap(err, "creds.AssumeRole")
+		}
+	}
+
+	return creds, nil
+}
+
 // Open opens the S3 backend at bucket and region. The bucket is created if it
 // does not exist yet.
 func Open(ctx context.Context, cfg Config, rt http.RoundTripper) (backend.Backend, error) {