AWS IAM Execution role does not have permissions to call CreateNetworkInterface on EC2 for a specific VPC

Question

I'm getting this error: Error: Error creating Lambda function: InvalidParameterValueException: The provided execution role does not have permissions to call CreateNetworkInterface on EC2 when trying to create a lambda with IAM permissions like this with custom Lambda role:

  ...
  statement {
    sid = "MyCustomLamdaStatementDescribe"
    actions = [
      "ec2:DescribeNetworkInterfaces",
    ]
    resources = ["*"]
  }
  statement {
    sid = "MyCustomLamdaStatementCreateDelete"
    actions = [
        "ec2:AttachNetworkInterface",
        "ec2:CreateNetworkInterface",
        "ec2:DeleteNetworkInterface",
        "ec2:AssignPrivateIpAddresses",
        "ec2:UnassignPrivateIpAddresses",
        "ec2:DescribeVpcs"
    ]
    resources = [
      "*"
    ]
    condition {
      test     = "ArnEquals"
      variable = "ec2:vpc"
      values = [
        "arn:aws:ec2:${var.my_region}:${var.my_account_id}:vpc/${var.my_vpc_id}",
      ]
    }
  }
  ...

Creating the lambda works perfectly without any condition (as pointed out in AWS Lambda:The provided execution role does not have permissions to call DescribeNetworkInterfaces on EC2) but I need the role to be able to match the VPC (or ec2:Subnet arn).

Note: I tried the condition.test with ArnEquals and StringEquals.

score 1 · Answer 1 · answered Dec 29 '21 at 06:32

If you want to restrict this to just one VPC, you have to split up each action.

ec2:DescribeNetworkInterfaces can only be used with Resource: * and no Conditions (see docs). But that's relatively harmless on it's own. The others can be restricted.

Here's a solution, in YAML (CloudFormation). It's not perfect. In particular I can't figure out how to restrict the resource or apply a condition to ec2:DeleteNetworkInterface. When I try, I get the same error.

- Effect: Allow
  Action:
    - 'ec2:CreateNetworkInterface'
  Resource:
    - !Sub 'arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:security-group/${SecGrp}'
    - !Sub 'arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:subnet/${SubnetA}'
    - !Sub 'arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:subnet/${SubnetB}'
    - !Sub 'arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:subnet/${SubnetC}'
  Condition:
    StringEquals:
      'ec2:Vpc': !Sub 'arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:vpc/${VPC}'
- Effect: Allow
  Action:
    - 'ec2:CreateNetworkInterface'
  Resource:
    - !Sub 'arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:network-interface/*'
    # VPC condition not supported for this type of resource for this action
- Effect: Allow
  Action:
    - 'ec2:DeleteNetworkInterface'
  Resource:
    # I don't know why we need the first
    # the docs say the second is sufficient, but it doesn't work
    - !Sub 'arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:*'
    - !Sub 'arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:network-interface/*'
  # the docs say this is supported, but it's not
  # Condition:
  #   StringEquals:
  #     'ec2:Vpc': !Sub 'arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:vpc/${VPC}'

- Effect: Allow
  Action:
    # this action must have resource: * and no conditions
    # cannot be restricted
    - 'ec2:DescribeNetworkInterfaces'
  Resource:
    - '*'
- Effect: Allow
  Action:
    - 'ec2:AssignPrivateIpAddresses'
    - 'ec2:UnassignPrivateIpAddresses'
  Resource:
    - !Sub 'arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:network-interface/*'

  Condition:
    StringEquals:
      'ec2:Vpc': !Sub 'arn:aws:ec2:${AWS::Region}:${AWS::AccountId}:vpc/${VPC}'

AWS IAM Execution role does not have permissions to call CreateNetworkInterface on EC2 for a specific VPC

1 Answers1