Allow CloudFront to access an S3 bucket with an origin access identity

Question

I have made a static website hosting S3 bucket that is served by CloudFront. I would like to restrict bucket access directly only from CloudFront through Origin Access Identity.

I have tried to update the S3 bucket policy but it's showing an error:

Error putting S3 policy: MalformedPolicy: Invalid principal in policy status code: 400, request id

I'm trying to use the following policy:

resource "aws_s3_bucket_policy" "default" {
  bucket = "${aws_s3_bucket.default.id}"
  policy = <<EOF
  {
"Version": "2008-10-17",
"Statement": [
    {
        "Sid": "2",
        "Effect": "Allow",
        "Principal": {
            "AWS": "arn:aws:iam::cloudfront:user/CloudFront Origin Access Identity ${aws_cloudfront_origin_access_identity.origin_access_identity.id}"
        },
        "Action": "s3:*",
        "Resource": "arn:aws:s3:::$/*"
    }
  ]
}
EOF 
}

Answer 1

As mentioned in the [aws_cloudfront_origin_access_identity documentation] 1 the best way to do this is by generating an IAM policy document with the aws_iam_policy_document data source and then attaching that directly.

An example would look something like this:

data "aws_iam_policy_document" "s3_policy" {
  statement {
    actions   = ["s3:GetObject"]
    resources = ["${module.names.s3_endpoint_arn_base}/*"]

    principals {
      type        = "AWS"
      identifiers = ["${aws_cloudfront_origin_access_identity.origin_access_identity.iam_arn}"]
    }
  }

  statement {
    actions   = ["s3:ListBucket"]
    resources = ["${module.names.s3_endpoint_arn_base}"]

    principals {
      type        = "AWS"
      identifiers = ["${aws_cloudfront_origin_access_identity.origin_access_identity.iam_arn}"]
    }
  }
}

resource "aws_s3_bucket" "bucket" {
  # ...
  policy = "${data.aws_iam_policy_document.s3_policy.json}"
}

If you really want to hand craft the IAM policy as you are in the question then you just need something like this:

resource "aws_s3_bucket_policy" "default" {
  bucket = "${aws_s3_bucket.default.id}"
  policy = <<EOF
{
"Version": "2008-10-17",
"Statement": [
    {
        "Sid": "2",
        "Effect": "Allow",
        "Principal": {
            "AWS": "${aws_cloudfront_origin_access_identity.origin_access_identity.iam_arn}"
        },
        "Action": "s3:*",
        "Resource": "${aws_s3_bucket.default.arn}""
    }
  ]
}
EOF 
}