AmazonS3FullAccess managed policy on a group doesn't give S3 permission?

Question

I have an S3 bucket that has in its policy permission for my CloudFront origin access identity:

{
  "Version": "2008-10-17",
  "Id": "PolicyForCloudFrontPrivateContent",
  "Statement": [
    {
      "Sid": "1",
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::cloudfront:user/CloudFront Origin Access Identity <mine>"
      },
      "Action": "s3:GetObject",
      "Resource": "arn:aws:s3:::<my-bucket>/*"
    }
  ]
}

Additionally I've created a group and attached the AmazonS3FullAccess managed policy to it and added an IAM user to that group. The managed policy looks like this:

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

However when that user tries to add anything to the bucket, I get a 403 (access denied). I'm unsure if any other operations work, I haven't written code to try them. It's only when I specifically allow that user's ARN access to the bucket directly in the bucket policy that they're allowed to add objects. What am I missing? It seems like the above group policy should allow members of that group access to all operations in all buckets, but it doesn't do that.

EDIT: After a whole bunch more poring over documentation, I think I've figured out that Amazon doesn't intend for groups to be used this way. They want you to use roles instead, which you can assign to an EC2 instance for automagic credential management. That's fine, but then I can't figure out how to test my code (using the aws-sdk Ruby gem) locally. Amazon says to define environment variables for your access ID and key - but what access ID and key? There's no such thing for a role...

Answer 1

Try replacing (in your policy):

"Resource": "arn:aws:s3:::<my-bucket>/*"

with:

"Resource": ["arn:aws:s3:::<my-bucket>", "arn:aws:s3:::<my-bucket>/*",]

and:

"Principal": {"AWS": "arn:aws:iam::cloudfront:user/CloudFront Origin Access Identity <mine>"},

with:

"Principal": {"AWS": "arn:aws:iam::ACCOUNT_ID:user/USERNAME"},