やること

今回はPublic Subnetの中に置いたEC2にSessionManagerで接続できるCloudformationを作成します。（下図左）
今回と同様のやりかたで、NATに紐付けたPrivate Subnetの中に置いたEC2にも接続できます。（下図中）

ただし、NATに紐づかないPrivate Subnetの中のEC2に対しては、VPCエンドポイントの作成が必要です。（上図右）
この場合の対応は下記参照ください。

実装

ec2.yml

AWSTemplateFormatVersion: 2010-09-09
Parameters:
  EnvironmentName:
    Type: String
    Default: sample
  KeyName:
    Type: AWS::EC2::KeyPair::KeyName
    Default: sample-key-pair
    ConstraintDescription: must be the name of an existing EC2 KeyPair.
  InstanceType:
    Type: String
    Default: t2.micro
  LatestAmiId:
    Type: 'AWS::SSM::Parameter::Value<AWS::EC2::Image::Id>'
    Default: '/aws/service/ami-amazon-linux-latest/amzn2-ami-hvm-x86_64-gp2'
  VpcId:
    Type: String
  PublicSubnetId:
    Type: String

Resources:
  SessionManagerRole:
    Type: AWS::IAM::Role
    Properties:
      RoleName: !Sub ${EnvironmentName}-SessionManagerRole
      AssumeRolePolicyDocument:
        Version: 2012-10-17
        Statement:
          - Effect: Allow
            Action: sts:AssumeRole
            Principal:
              Service: ec2.amazonaws.com
      ManagedPolicyArns:
        - arn:aws:iam::aws:policy/AmazonSSMManagedInstanceCore
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName} SessionManagerRole

  SSMInstanceProfile:
    Type: AWS::IAM::InstanceProfile
    Properties:
      Path: /
      Roles:
        - !Ref SessionManagerRole

  InstanceSecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: Default setting
      VpcId: !Ref VpcId
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName}-EC2-sg

  EC2Instance:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: !Ref InstanceType
      KeyName: !Ref KeyName
      ImageId: !Ref LatestAmiId
      IamInstanceProfile: !Ref SSMInstanceProfile
      NetworkInterfaces:
        - AssociatePublicIpAddress: true
          DeviceIndex: 0
          GroupSet:
            - !Ref InstanceSecurityGroup
          SubnetId: !Ref PublicSubnetId
      UserData: !Base64 |
        #!/bin/bash
        sudo yum update
        sudo yum install -y https://s3.ap-northeast-1.amazonaws.com/amazon-ssm-ap-northeast-1/latest/linux_amd64/amazon-ssm-agent.rpm
        systemctl enable amazon-ssm-agent
        systemctl start amazon-ssm-agent
      Tags:
        - Key: Name
          Value: !Sub ${EnvironmentName} EC2 to connect via session manager

参考