Granting a User Permissions to Pass a Role to an AWS Service

To configure many AWS services, you must pass an IAM role to the service. This allows the service to later assume the role and perform actions on your behalf. You only have to pass the role to the service once during set-up, and not every time that the service assumes the role. For example, assume that you have an application running on an Amazon EC2 instance. That application requires temporary credentials for authentication, and permissions to authorize the application to perform actions in AWS. When you set up the application, you must pass a role to EC2 to use with the instance that provides those credentials. You define the permissions for the applications running on the instance by attaching an IAM policy to the role. The application assumes the role every time it needs to perform the actions that are allowed by the role.

To pass a role (and its permissions) to an AWS service, a user must have permissions to pass the role to the service. This helps administrators ensure that only approved users can configure a service with a role that grants permissions. To allow a user to pass a role to an AWS service, you must grant the PassRole permission to the user's IAM user, role, or group.

When you create a service-linked role, you must also have permission to pass that role to the service. Some services automatically create a service-linked role in your account when you perform an action in that service. For example, Amazon EC2 Auto Scaling creates the AWSServiceRoleForAutoScaling service-linked role for you the first time that you create an Auto Scaling group. If you try to create an Auto Scaling group without the PassRole permission, you receive an error. To learn which services support service-linked roles, see AWS Services That Work with IAM. To learn which services automatically create a service-linked role when you perform an action in that service, choose the Yes link and view the service-linked role documentation for the service.

A user can pass a role ARN as a parameter in any API operation that uses the role to assign permissions to the service. The service then checks whether that user has the iam:PassRole permission. To limit the user to passing only approved roles, you can filter the iam:PassRole permission with the Resources element of the IAM policy statement.

Example 1

Imagine that you want to grant a user the ability to pass any of an approved set of roles to the Amazon EC2 service upon launching an instance. You need three elements:

An IAM permissions policy attached to the role that determines what the role can do. Scope permissions to only the actions that the role must perform, and to only the resources that the role needs for those actions. You can use AWS managed or customer-created IAM permissions policy.
```
{
    "Version": "2012-10-17",
    "Statement": {
        "Effect": "Allow",
        "Action": [ "A list of the permissions the role is allowed to use" ],
        "Resource": [ "A list of the resources the role is allowed to access" ]
    }
}      
```
A trust policy for the role that allows the service to assume the role. For example, you could attach the following trust policy to the role with the UpdateAssumeRolePolicy action. This trust policy allows Amazon EC2 to use the role and the permissions attached to the role.
```
{
    "Version": "2012-10-17",
    "Statement": {
        "Sid": "TrustPolicyStatementThatAllowsEC2ServiceToAssumeTheAttachedRole",
        "Effect": "Allow",
        "Principal": { "Service": "ec2.amazonaws.com" },
       "Action": "sts:AssumeRole"
    }
}                    
```
An IAM permissions policy attached to the IAM user that allows the user to pass only those roles that are approved. iam:PassRole usually is accompanied by iam:GetRole so that the user can get the details of the role to be passed. In this example, the user can pass only roles that exist in the specified account with names that begin with EC2-roles-for-XYZ-:
```
{
    "Version": "2012-10-17",
    "Statement": [{
        "Effect": "Allow",
        "Action": [
            "iam:GetRole",
            "iam:PassRole"
        ],
        "Resource": "arn:aws:iam::<account-id>:role/EC2-roles-for-XYZ-*"
    }]
}
```

Now the user can start an Amazon EC2 instance with an assigned role. Applications running on the instance can access temporary credentials for the role through the instance profile metadata. The permission policies attached to the role determine what the instance can do.

Example 2

Amazon Relational Database Service (Amazon RDS) supports a feature called Enhanced Monitoring. This feature enables Amazon RDS to monitor a database instance using an agent. It also allows Amazon RDS to log metrics to Amazon CloudWatch Logs. To enable this feature, you must create a service role to give Amazon RDS permissions to monitor and write metrics to your logs.

To create a role for Amazon RDS Enhanced Monitoring

Sign in to the AWS Management Console and open the IAM console at https://console.aws.amazon.com/iam/.
Choose Roles, and then choose Create role.
Choose the AWS Service role type, and then choose the Amazon RDS Role for Enhanced Monitoring service. Then choose Next: Permissions.
Choose the AmazonRDSEnhancedMonitoringRole, permissions policy.
Choose Next: Tagging.
(Optional) Add metadata to the user by attaching tags as key-value pairs. For more information about using tags in IAM, see Tagging IAM Entities.
Choose Next: Review.
For Role name, type a role name that helps you identify the purpose of this role. Role names must be unique within your AWS account. They are not distinguished by case. For example, you cannot create roles named both PRODROLE and prodrole. Because various entities might reference the role, you cannot edit the name of the role after it has been created.
(Optional) For Role description, type a description for the new role.
Review the role and then choose Create role.

The role automatically gets a trust policy that grants the monitoring.rds.amazonaws.com service permissions to assume the role. After it does, Amazon RDS can perform all of the actions that the AmazonRDSEnhancedMonitoringRole policy allows.

The user that you want to enable Enhanced Monitoring needs a policy that includes a statement that allows the user to pass the role, like the following. Use your account number and replace the role name with the name you provided in step 3:


    {
        "Sid": "PolicyStatementToAllowUserToPassOneSpecificRole",
        "Effect": "Allow",
        "Action": [ "iam:PassRole" ],
        "Resource": "arn:aws:iam:::role/RDS-Monitoring-Role"
    }

You can combine this statement with statements in another policy or put it in its own policy. To instead specify that the user can pass any role that begins with RDS-, you can replace the role name in the resource ARN with a wildcard, for example:


        "Resource": "arn:aws:iam:::role/RDS-*"

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