Terraform: Error creating IAM Role. MalformedPolicyDocument: Has prohibited field Resource

Question

My TF code is giving me an error:

  /*
   * Policy: AmazonEC2ReadOnlyAccess
   */
  assume_role_policy = <<EOF
{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "ec2:Describe*",
            "Resource": "*"
        },
        {
            "Effect": "Allow",
            "Action": "elasticloadbalancing:Describe*",
            "Resource": "*"
        },
        {
            "Effect": "Allow",
            "Action": [
                "cloudwatch:ListMetrics",
                "cloudwatch:GetMetricStatistics",
                "cloudwatch:Describe*"
            ],
            "Resource": "*"
        },
        {
            "Effect": "Allow",
            "Action": "autoscaling:Describe*",
            "Resource": "*"
        }
    ]
}
EOF

I copied and pasted the Policy from https://console.aws.amazon.com/iam/home?region=us-west-2#/policies/arn:aws:iam::aws:policy/AmazonEC2ReadOnlyAccess$jsonEditor

* aws_iam_role.<role name>: Error creating IAM Role <role name>: MalformedPolicyDocument: Has prohibited field Resource
status code: 400, request id: <request id>

Not sure why it's saying Resource is prohibited.

Answer 1

You need to mention sts:AssumeRole. You can directly attach policy using aws_iam_role_policy_attachment instead of duplicating existing policy.

resource "aws_iam_role" "ec2_iam_role" {
  name = "ec2_iam_role"
  assume_role_policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "",
      "Effect": "Allow",
      "Principal": {
        "Service": [
          "ec2.amazonaws.com"
        ]
      },
      "Action": "sts:AssumeRole"
    }
  ]
}
EOF
}

resource "aws_iam_role_policy_attachment" "ec2-read-only-policy-attachment" {
    role = "${aws_iam_role.ec2_iam_role.name}"
    policy_arn = "arn:aws:iam::aws:policy/AmazonEC2ReadOnlyAccess"
}