Can't access S3 bucket from within Fargate container (Bad Request and unable to locate credentials)

Question

I created a private s3 bucket and a fargate cluster with a simple task that attempts to read from that bucket using python 3 and boto3. I've tried this on 2 different docker images and on one I get a ClientError from boto saying HeadObject Bad request (400) and the other I get NoCredentialsError: Unable to locate credentials.

The only real different in the images is that the one saying bad request is being run normally and the other is being run manually by me via ssh to the task container. So I'm not sure why one image is saying "bad request" and the other "unable to locate credentials".

I have tried a couple different IAM policies, including (terraform) the following policies:

data "aws_iam_policy_document" "access_s3" {
  statement {
    effect    = "Allow"
    actions   = ["s3:ListBucket"]
    resources = ["arn:aws:s3:::bucket_name"]
  }

  statement {
    effect = "Allow"

    actions = [
      "s3:GetObject",
      "s3:GetObjectVersion",
      "s3:GetObjectTagging",
      "s3:GetObjectVersionTagging",
    ]

    resources = ["arn:aws:s3:::bucket_name/*"]
  }
}

Second try:

data "aws_iam_policy_document" "access_s3" {
  statement {
    effect    = "Allow"
    actions   = ["s3:*"]
    resources = ["arn:aws:s3:::*"]
  }
}

And the final one I tried was a build in policy:

resource "aws_iam_role_policy_attachment" "access_s3" {
  role       = "${aws_iam_role.ecstasks.name}"
  policy_arn = "arn:aws:iam::aws:policy/AmazonS3ReadOnlyAccess"
}

the bucket definition is very simple:

resource "aws_s3_bucket" "bucket" {
  bucket = "${var.bucket_name}"
  acl    = "private"
  region = "${var.region}"
}

Code used to access s3 bucket:

try:
    s3 = boto3.client('s3')
    tags = s3.head_object(Bucket='bucket_name', Key='filename')
    print(tags['ResponseMetadata']['HTTPHeaders']['etag'])
except ClientError:
    traceback.print_exc()

No matter what I do, I'm unable to use boto3 to access AWS resources from within a Fargate container task. I'm able to access the same s3 bucket with boto3 on an EC2 instance without providing any kind of credentials and only using the IAM roles/policies. What am I doing wrong? Is it not possible to access AWS resources in the same way from a Fargate container?

Forgot to mention that I am assigning the IAM roles to the task definition execution policy and task policy.

UPDATE: It turns out that the unable to find credentials error I was having is a red herring. The reason I could not get the credentials was because my direct ssh session did not have the AWS_CONTAINER_CREDENTIALS_RELATIVE_URI environment variable set.

AWS Fargate will inject an environment variable named AWS_CONTAINER_CREDENTIALS_RELATIVE_URI on your behalf which contains a url to what boto should use for grabbing API access credentials. So the Bad request error is the one I'm actually getting and need help resolving. I checked my environment variables inside the container and the AWS_CONTAINER_CREDENTIALS_RELATIVE_URI value is being set by Fargate.

Answer 1

I struggled quite a bit with this issue and constantly having AWS_CONTAINER_CREDENTIALS_RELATIVE_URI wrongly set to None, until I added a custom task role in addition to my current task execution role.

1) The task execution role is responsible for having access to the container in ECR and giving access to run the task itself, while 2) the task role is responsible for your docker container making API requests to other authorized AWS services.

1) For my task execution role I'm using AmazonECSTaskExecutionRolePolicy with the following JSON;

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "ecr:GetAuthorizationToken",
                "ecr:BatchCheckLayerAvailability",
                "ecr:GetDownloadUrlForLayer",
                "ecr:BatchGetImage",
                "logs:CreateLogStream",
                "logs:PutLogEvents"
            ],
            "Resource": "*"
        }
    ]
}

2) I finally got rid of the NoCredentialsError: Unable to locate credentials when I added a task role in addition to the task execution role, for instance, responsible of reading from a certain bucket;

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

In summary; make sure to both have a role for 1) executionRoleArn for access to run the task and 2) taskRoleArn for access to make API requests to authorized AWS services set in your task definition.