こんな感じのテンプレートで表題のエラーが発生。

    LaboLambdaRole:
      Type: AWS::IAM::Role
      Properties:
        Path: /
        RoleName: "LaboLambdaRole"
        AssumeRolePolicyDocument:
          Version: '2012-10-17'
          Statement:
            - Effect: Allow
              Action: sts:AssumeRole
              Principal:
                Service:
                  - lambda.amazonaws.com
            - Effect: Allow
              Action: s3:ListAllMyBuckets
              Resource: "arn:aws:s3:::*"
        ManagedPolicyArns:
          - arn:aws:iam::aws:policy/AmazonDynamoDBReadOnlyAccess

エラー

An error occurred: LaboLambdaRole - Has prohibited field Resource (Service: AmazonIdentityManagement; Status Code: 400; Error Code: MalformedPolicyDocument; Request ID: 11ee95fd-xxxx-xxxx-xxxx).

結論

AssumeRolePolicyDocument は AssumeRole のためのセクションであり Resource は定義できない。
AWS::IAM::Policy を利用する。

    LaboLambdaRole:
      Type: AWS::IAM::Role
      Properties:
        Path: /
        RoleName: "LaboLambdaRole"
        AssumeRolePolicyDocument:
          Version: '2012-10-17'
          Statement:
            - Effect: Allow
              Action: sts:AssumeRole
              Principal:
                Service:
                  - lambda.amazonaws.com
        ManagedPolicyArns:
          - arn:aws:iam::aws:policy/AmazonDynamoDBReadOnlyAccess

    LaboLambdaRolePolicies:
      Type: "AWS::IAM::Policy"
      Properties:
        PolicyName: "root"
        PolicyDocument:
          Version: "2012-10-17"
          Statement:
            -
              Effect: "Allow"
              Action: s3:ListAllMyBuckets
              Resource: "arn:aws:s3:::*"
        Roles:
          -
            Ref: "LaboLambdaRole"