readonly.json

{
  "Version":"2012-10-17",
  "Statement": [
    {
      "Action": [
        "s3:GetObject",
        "s3:ListBucket",
        "s3:ListAllMyBuckets"
      ],
      "Effect": "Allow",
      "Resource": [
        "arn:aws:s3:::*"
      ]
     }
  ]
}