This is the default bucket policy I'm using for IAM accounts with read/write access to an S3 bucket:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": ["s3:GetBucketLocation", "s3:ListBucket"],
      "Resource": "arn:aws:s3:::<bucketname>"
    },
    {
      "Effect": "Allow",
      "Action": ["s3:PutObject", "s3:DeleteObject", "s3:GetObject"],
      "Resource": ["arn:aws:s3:::<bucketname>/*", "arn:aws:s3:::<bucketname>"]
    }
  ]
}