非VPC環境のLambdaからPublicかつAMI認証を許可したRDSにアクセスできない

以下のような設定です。なお、[]内は実際には正しい値が入っています

RDS

• エンジン MySQL
• クラス db.t2.micro
• リソースID [db-xxxxxxxx]
• パブリックアクセシビリティ あり
• IAM db 認証 有効
• ホスト名 [rdshost]

事前に以下の作業を実施済み

create database [dbname];
create user [dbuser] identified with AWSAuthenticationPlugin as 'RDS';
grant all privileges on [dbname].* to [dbuser] REQUIRE SSL;

Lambda

• 関数名 [funcname]
• ハンドラ lambda_function.lambda_handler
• 非VPC
• ロール service-role/[func-role-yyyyyy]
• メモリ 128MB
• タイムアウト 15分

Lambda環境変数

• DB_HOST [rdshost]
• DB_USER [dbuser]
• DB_NAME [dbname]

service-role/[func-role-yyyyyy]ロール設定

以下の4ポリシーをアタッチしています

• AWSLambdaVPCAccessExecutionRole 内容は以下の通りです

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "logs:CreateLogGroup",
                "logs:CreateLogStream",
                "logs:PutLogEvents",
                "ec2:CreateNetworkInterface",
                "ec2:DescribeNetworkInterfaces",
                "ec2:DeleteNetworkInterface"
            ],
            "Resource": "*"
        }
    ]
}

• AmazonRDSDataFullAccess 内容は以下の通りです

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "SecretsManagerDbCredentialsAccess",
            "Effect": "Allow",
            "Action": [
                "secretsmanager:GetSecretValue",
                "secretsmanager:PutResourcePolicy",
                "secretsmanager:PutSecretValue",
                "secretsmanager:DeleteSecret",
                "secretsmanager:DescribeSecret",
                "secretsmanager:TagResource"
            ],
            "Resource": "arn:aws:secretsmanager:*:*:secret:rds-db-credentials/*"
        },
        {
            "Sid": "RDSDataServiceAccess",
            "Effect": "Allow",
            "Action": [
                "dbqms:CreateFavoriteQuery",
                "dbqms:DescribeFavoriteQueries",
                "dbqms:UpdateFavoriteQuery",
                "dbqms:DeleteFavoriteQueries",
                "dbqms:GetQueryString",
                "dbqms:CreateQueryHistory",
                "dbqms:DescribeQueryHistory",
                "dbqms:UpdateQueryHistory",
                "dbqms:DeleteQueryHistory",
                "dbqms:DescribeQueryHistory",
                "rds-data:ExecuteSql",
                "rds-data:ExecuteStatement",
                "rds-data:BatchExecuteStatement",
                "rds-data:BeginTransaction",
                "rds-data:CommitTransaction",
                "rds-data:RollbackTransaction",
                "secretsmanager:CreateSecret",
                "secretsmanager:ListSecrets",
                "secretsmanager:GetRandomPassword",
                "tag:GetResources"
            ],
            "Resource": "*"
        }
    ]
}

• AWSLambdaBasicExecutionRole-zzzzzzz 内容は以下の通りです

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "logs:CreateLogGroup",
            "Resource": "arn:aws:logs:ap-northeast-1:[accid]:*"
        },
        {
            "Effect": "Allow",
            "Action": [
                "logs:CreateLogStream",
                "logs:PutLogEvents"
            ],
            "Resource": [
                "arn:aws:logs:ap-northeast-1:[accid]:log-group:/aws/lambda/[funcname]:*"
            ]
        }
    ]
}

• ConnectToRDS 内容は以下の通りです

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": "rds-data:*",
            "Resource": "*"
        },
        {
            "Sid": "VisualEditor1",
            "Effect": "Allow",
            "Action": [
                "rds-data:*",
                "rds-data:connect"
            ],
            "Resource": "arn:aws:rds:ap-northeast-1:[accid]:dbuser:[db-xxxxxxxx]/[dbuser]"
        }
    ]
}

Lambdaのプログラム

必要なライブラリおよび証明書は全てデプロイパッケージに入っています。

lambda_functions.py

import pymysql

import db_connector

def lambda_handler(event, context):
    db_host = os.environ['DB_HOST']
    db_name = os.environ['DB_NAME']
    db_user = os.environ['DB_USER']
    try:
        conn = db_connector.connect(db_host, db_user, db_name)
    except pymysql.MySQLError as e:
        return {
            'statusCode': 500,
            'body': json.dumps(e.args)
        }
    return {
        'statusCode': 200,
        'body': json.dumps('Hello, world')
    }

db_connector.py

import logging
import os

import boto3
import pymysql

logger = logging.getLogger()
logger.setLevel(logging.INFO)

def connect(host, user, db, port=3306, region='ap-northeast-1'):
    """Connect to database
    """
    rds = boto3.client('rds',region_name=region)
    passwd = rds.generate_db_auth_token(host, port, user)
    logger.info('Token: ' + passwd)
    return pymysql.connect(
        host=host,
        user=user,
        passwd=passwd,
        db=db,
        connect_timeout=5,
        ssl={
            'ca': '{}/rds-combined-ca-bundle.pem'.format(
                os.environ['LAMBDA_TASK_ROOT'])
        }
    )

起きたこと

Lambdaの出力が以下のようになりました。

{
  "statusCode": 500,
  "body": "[1045, \"Access denied for user '[dbuser]'@'[exechost]' (using password: YES)\"]"
}

アクセス権限がないってことです(パスワードが間違ってる)。

質問内容

どこをどう直せばよいのでしょうか？

15時34分追記

ConnectToRDSの権限を以下のように調整してもダメでした。

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": "rds-data:*",
            "Resource": "*"
        },
        {
            "Sid": "VisualEditor1",
            "Effect": "Allow",
            "Action": [
                "rds-data:connect"
            ],
            "Resource": [
                "arn:aws:rds-db:ap-northeast-1:[accid]:dbuser:[db-xxxxxxxx]/[dbuser]"
            ]
        }
    ]
}

16時17分追記

さらに、以下のような変更を行ってもダメでした。

1. AmazonRDSDataFullAccessをデタッチ
2. ConnectToRDS権限を以下のように変更

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": "rds-data:connect",
            "Resource": "arn:aws:rds-db:ap-northeast-1:[accid]:dbuser:[db-xxxxxxxx]/[dbuser]"
        }
    ]
}