NiFi access denied when trying list/put/delete objects from s3

I'm trying to move data between s3 and Nifi, and I'm getting access denied. The machine has the correct IAM permissions as all s3 cli commands work. I am running a RHEL instance with nifi running as root. I'm getting the following 403:

com.amazonaws.services.s3.model.AmazonS3Exception: Access Denied (Service: Amazon S3; Status Code: 403; Error Code: AccessDenied; Request ID: 019142B6C670F23C; S3 Extended Request ID: Zu18MItX7V7D6h5bhByZWaNfrPed/doXLW2Lw1jDVmyACeH5IfRdUi0Tqa5zfQmHCJWl+cn5HwI=)

Is there some sort of special setting required to get connectivity from the Nifi processors to s3? We are looking to use only the IAM role and avoid using keys.

The IAM role assigned to the EC2 instance is

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "s3:*",
            "Resource": "*"
        }
    ]
}