How to add Policies to AWS Lambda function using the .yaml file?

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I am developing a REST API with AWS Lambda, API Gateway, RDS (MySQL). I am using the aws-sam tool to build, configure and publish my work to cloud.

Please check the below template.yaml file which I am using now.

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: >
  aaaa-restapi

  Sample SAM Template for aaaa-restapi

# More info about Globals: https://github.com/awslabs/serverless-application-model/blob/master/docs/globals.rst
Globals:
  Function:
    Timeout: 100

Resources:
  GetAllAccountTypesLambda:
    Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
    Properties:
      CodeUri: aaaa-restapi
      Handler: com.aaaa.dao.accountingtype.GetAllAccountTypesLambda::getAllAccountTypes
      Runtime: java11
      MemorySize: 1024
      Environment: # More info about Env Vars: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#environment-object
        Variables:
          PARAM1: VALUE
      Events:
        HelloWorld:
          Type: Api # More info about API Event Source: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#api
          Properties:
            Path: /accounttype
            Method: get

However to enable my lambda function to find the database, I had to enable some policies from the AWS Web console. I followed this link - https://ao.ms/the-provided-execution-role-does-not-have-permissions-to-call-createnetworkinterface-on-ec2/

Below is the policy I created for my Lambda function in AWS web console.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "ec2:DescribeNetworkInterfaces",
        "ec2:CreateNetworkInterface",
        "ec2:DeleteNetworkInterface",
        "ec2:DescribeInstances",
        "ec2:AttachNetworkInterface"
      ],
      "Resource": "*"
    }
  ]
}

However there is no way I can do this in web console, from function to function. I need to get this done in the yaml file.

With my yaml file provided above, how can I put these permissions to my Lambda function?

------------UPDATE---------------

Following Gaurauv's comment, I made the following changes to the yaml file.

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: >
  aaaa-restapi

  Sample SAM Template for aaaa-restapi

# More info about Globals: https://github.com/awslabs/serverless-application-model/blob/master/docs/globals.rst
Globals:
  Function:
    Timeout: 100

Resources:
  GetAllAccountTypesLambda:
    Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
    Properties:
      CodeUri: aaaa-restapi
      Handler: com.aaaa.dao.accountingtype.GetAllAccountTypesLambda::getAllAccountTypes
      Runtime: java11
      MemorySize: 1024
      Environment: # More info about Env Vars: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#environment-object
        Variables:
          PARAM1: VALUE
      Events:
        HelloWorld:
          Type: Api # More info about API Event Source: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#api
          Properties:
            Path: /accounttype
            Method: get
      Role: !GetAtt LambdaRole.Arn
  
  LambdaRole:
    Type: "AWS::IAM::Role"
    Properties:
      Path: "/"
      ManagedPolicyArns:
        - "arn:aws:iam::aws:policy/service-role/AWSLambdaBasicExecutionRole"
      Policies:
        - PolicyName: 'ec2-access-policy'
          PolicyDocument:
            Statement:
              - Effect: Allow
                Action:
                  - ec2:DescribeNetworkInterfaces
                  - ec2:CreateNetworkInterface
                  - ec2:DeleteNetworkInterface
                  - ec2:DescribeInstances
                  - ec2:AttachNetworkInterface
                Resource: '*'

However it failed to deploy, following error was produced.

CREATE_FAILED                           AWS::IAM::Role                          LambdaRole                              Property AssumeRolePolicyDocument
                                                                                                                        cannot be empty.
aws-sam aws-lambda amazon-web-services amazon-ec2 java
2 Answers

According to the IAM::Role Resource, you need an AssumeRolePolicyDocument when creating a Role. This property manages the trust policy that is associated with this role. Trust policies define which entities can assume the role. You can associate only one trust policy with a role.

Kindly find an updated Role resource for your use-case

AWSTemplateFormatVersion: "2010-09-09"
Resources:
  LambdaRole:
    Type: 'AWS::IAM::Role'
    Properties:
      AssumeRolePolicyDocument:
        Version: "2012-10-17"
        Statement:
          - Effect: Allow
            Principal:
              Service:
                - lambda.amazonaws.com
            Action:
              - 'sts:AssumeRole'
      Path: /
      ManagedPolicyArns:
        - arn:aws:iam::aws:policy/service-role/AWSLambdaBasicExecutionRole
      Policies:
        - PolicyName: root
          PolicyDocument:
            Version: "2012-10-17"
            Statement:
              - Effect: Allow
                Action:
                  - ec2:DescribeNetworkInterfaces
                  - ec2:CreateNetworkInterface
                  - ec2:DeleteNetworkInterface
                  - ec2:DescribeInstances
                  - ec2:AttachNetworkInterface
                Resource: '*'
user_53380230

You can attach a role to the lambda function with an inline policy. Something like this

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: >
  aaaa-restapi

  Sample SAM Template for aaaa-restapi

# More info about Globals: https://github.com/awslabs/serverless-application-model/blob/master/docs/globals.rst
Globals:
  Function:
    Timeout: 100

Resources:
  GetAllAccountTypesLambda:
    Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
    Properties:
      CodeUri: aaaa-restapi
      Handler: com.aaaa.dao.accountingtype.GetAllAccountTypesLambda::getAllAccountTypes
      Runtime: java11
      MemorySize: 1024
      Environment: # More info about Env Vars: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#environment-object
        Variables:
          PARAM1: VALUE
      Events:
        HelloWorld:
          Type: Api # More info about API Event Source: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#api
          Properties:
            Path: /accounttype
            Method: get
      Role: !GetAtt LambdaRole.Arn



  LambdaRole:
    Type: "AWS::IAM::Role"
    Properties:
      Path: "/"
      ManagedPolicyArns:
        - "arn:aws:iam::aws:policy/service-role/AWSLambdaBasicExecutionRole"
      Policies:
        - PolicyName: 's3-access-policy'
          PolicyDocument:
            Statement:
              - Effect: Allow
                Action:
                  - s3:GetBucketLocation
                  - s3:GetBucketCORS
                  - s3:GetObjectVersionForReplication
                  - s3:GetObject
                  - s3:GetBucketTagging
                  - s3:GetObjectVersion
                  - s3:GetObjectTagging
                  - s3:ListMultipartUploadParts
                  - s3:ListBucket
                  - s3:ListBucketMultipartUploads
                  - s3:PutObject
                  - s3:PutObjectTagging
                  - s3:DeleteObject
                Resource: '*'
user_28994310
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