Understanding AWS IAM Roles and useful scripts to assume a role. See http://blog.schlomo.schapiro.org/2017/06/understanding-iam-roles-in-amazon-aws.html for details.

aws-assume-role.sh

#!/bin/bash -eu
die() { echo 1>&2 "ERROR: $*"  ; exit 1 ; }
info() { echo 1>&2 "INFO: $*" ; }

test "${1:-}" || die "Usage: $0 <role-name | role ARN> [<role-name | role ARN> ...]"

while test "${1:-}" ; do
  role="$1"
  shift

  if [[ "$role" != */* ]] ; then
    role=arn:aws:iam::$(aws sts get-caller-identity --output text --query Account):role/"$role" || \
      die "Could not get your AWS account ID"
  fi
  read -r \
    AWS_ACCESS_KEY_ID \
    AWS_SECRET_ACCESS_KEY \
    AWS_SESSION_TOKEN \
    < <(
      aws sts assume-role --role-arn "$role" --role-session-name "$USER@$HOSTNAME" \
        --query 'Credentials.[AccessKeyId,SecretAccessKey,SessionToken]' --output text
      ) \
    || \
      die "Could not assume role $role"
  info "Switched to role $role"
  export AWS_ACCESS_KEY_ID AWS_SECRET_ACCESS_KEY AWS_SESSION_TOKEN
done
echo AWS_SECRET_ACCESS_KEY="$AWS_SECRET_ACCESS_KEY"
echo AWS_SESSION_TOKEN="$AWS_SESSION_TOKEN"
echo AWS_ACCESS_KEY_ID="$AWS_ACCESS_KEY_ID"

aws-iam-get-role-policy.json

{
    "PolicyDocument": {
        "Statement": [
            {
                "Resource": [
                    "arn:aws:s3:::my-backups",
                    "arn:aws:s3:::my-backups/*"
                ],
                "Action": [
                    "s3:GetObject",
                    "s3:ListBucket",
                    "s3:ListBucketVersions",
                    "s3:ListObjects",
                    "s3:GetBucketLocation",
                    "s3:PutObject"
                ],
                "Effect": "Allow"
            }
        ],
        "Version": "2012-10-17"
    },
    "RoleName": "backup-role",
    "PolicyName": "read-write-backupbucket"
}

aws-iam-get-role.json

{
    "Role": {
        "Arn": "arn:aws:iam::123456789:role/backup-role",
        "CreateDate": "2017-05-18T13:47:44Z",
        "RoleId": "ABCDEF0123456789ABCDEF",
        "Path": "/",
        "RoleName": "backup-role",
        "AssumeRolePolicyDocument": {
            "Statement": [
                {
                    "Action": "sts:AssumeRole",
                    "Effect": "Allow",
                    "Principal": {
                        "AWS": [
                            "arn:aws:iam::123456789:role/some-instances-role",
                            "arn:aws:iam::123456789:role/PowerUser"
                        ]
                    }
                }
            ],
            "Version": "2012-10-17"
        }
    }
}

s3-bucket-iam-role-cf.yaml

AWSTemplateFormatVersion: "2010-09-09"

Resources:
  bucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: my-backups
      LifecycleConfiguration:
        Rules:
        - Id: DeleteEverythingInThreeMonths
          Prefix: ''
          Status: Enabled
          ExpirationInDays: '180'
  role:
    Type: AWS::IAM::Role
    Properties:
      RoleName: backup-role
      AssumeRolePolicyDocument:
        Version: "2012-10-17"
        Statement:
        - Effect: Allow
          Principal:
            Service: ec2.amazonaws.com
            AWS: arn:aws:iam::123456789:role/PowerUser
          Action:
          - sts:AssumeRole
      Policies:
      - PolicyName: read-write-backupbucket
        PolicyDocument:
          Version: "2012-10-17"
          Statement:
          - Effect: Allow
            Action:
            - s3:GetObject
            - s3:ListBucket
            - s3:ListBucketVersions
            - s3:ListObjects
            - s3:GetBucketLocation
            - s3:PutObject
            Resource:
            - "arn:aws:s3:::my-backups"
            - "arn:aws:s3:::my-backups/*"

~ LICENSE.txt

Copyright 2017 Schlomo Schapiro / Zalando SE

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.