How to add additional security groups to EKS cluster?

I use terraform-aws-eks provision EKS cluster. Two security groups provisioned after "terraform apply". They are "Cluster security group" and "Additional security groups". I launched an EC2 instance. I would like to access EKS from that EC2. To do that, I need to add EC2 security group into "Additional security groups". As follows are my code. Two questions.

1. From my code, I have no idea how "Additional security groups" got created.
2. I added the "cluster_security_group_additional_rules" in my terraform code. I did not find the security group I added after "terraform apply". Seems like it is not created. At AWS console, it works if I manually added the EC2 security group into "Additional security groups". How to do it using Terraform code?

module "eks" {
  source  = "terraform-aws-modules/eks/aws"
  version = "18.30.1"

  cluster_name    = var.cluster_name
  cluster_version = var.cluster_version
  create_kms_key  = true
  kms_key_description = "KMS Secrets encryption for EKS Cluster"
  kms_key_enable_default_policy   = true

  cluster_endpoint_private_access = true
  cluster_endpoint_public_access  = true

  vpc_id                          = var.vpc_id
  subnet_ids                      = var.subnet_ids

  cluster_enabled_log_types       = var.cluster_enabled_log_types

  manage_aws_auth_configmap       = var.manage_aws_auth_configmap
  aws_auth_roles                  = var.aws_auth_roles
  aws_auth_users                  = var.aws_auth_users
  aws_auth_accounts               = var.aws_auth_accounts

  #Required for Karpenter role below
  enable_irsa                     = true

  create_cloudwatch_log_group            = var.create_cloudwatch_log_group
  cloudwatch_log_group_retention_in_days = var.cloudwatch_log_group_retention_in_days

  node_security_group_additional_rules = {
    ingress_nodes_karpenter_port = {
      description                   = "Cluster API to Node group for Karpenter webhook"
      protocol                      = "tcp"
      from_port                     = 8443
      to_port                       = 8443
      type                          = "ingress"
      source_cluster_security_group = true
    }
  }

  # Extend cluster security group rules
  cluster_security_group_additional_rules = {
    ingress_ec2_tcp = {
      description                = "Access EKS from EC2 instance."
      protocol                   = "tcp"
      from_port                  = 443
      to_port                    = 443
      type                       = "ingress"
      security_groups            = [var.ec2_sg_id]
      source_cluster_security_group = true
    }
  }

  node_security_group_tags = {
    # NOTE - if creating multiple security groups with this module, only tag the
    # security group that Karpenter should utilize with the following tag
    # (i.e. - at most, only one security group should have this tag in your account)
    "karpenter.sh/discovery/${var.cluster_name}" = var.cluster_name
  }

  # Need two nodes to get Karpenter up and running.
  # This ensures core services such as VPC CNI, CoreDNS, etc. are up and running
  # so that Karpenter can be deployed and start managing compute capacity as required
  eks_managed_node_groups = {
    "${var.cluster_name}" = {
      capacity_type  = "ON_DEMAND"

      instance_types = ["m5.large"]
      # Not required nor used - avoid tagging two security groups with same tag as well
      create_security_group = false

      # Ensure enough capacity to run 2 Karpenter pods
      min_size     = 2
      max_size     = 3
      desired_size = 2

      iam_role_additional_policies = [
        "arn:${local.partition}:iam::aws:policy/AmazonSSMManagedInstanceCore", # Required by Karpenter
        "arn:${local.partition}:iam::aws:policy/AmazonEKSWorkerNodePolicy",
        "arn:${local.partition}:iam::aws:policy/AmazonEKS_CNI_Policy",
        "arn:${local.partition}:iam::aws:policy/AmazonEC2ContainerRegistryReadOnly", #for access to ECR images
        "arn:${local.partition}:iam::aws:policy/CloudWatchAgentServerPolicy"
      ]

      labels = {
        AL2Nodes = "monitor"
      }
      tags = {
        # This will tag the launch template created for use by Karpenter
        "karpenter.sh/discovery/${var.cluster_name}" = var.cluster_name
      }
    }
  }
}

I believe you have 2 errors in your definition of the cluster_security_group_additional_rules field.

I'll repeat your definition here just to make it easier for future readers:

  cluster_security_group_additional_rules = {
    ingress_ec2_tcp = {
      description                = "Access EKS from EC2 instance."
      protocol                   = "tcp"
      from_port                  = 443
      to_port                    = 443
      type                       = "ingress"
      security_groups            = [var.ec2_sg_id]
      source_cluster_security_group = true
    }
  }

The first error is that you shouldn't be setting source_cluster_security_group = true. This isn't even a valid option for cluster_security_group_additional_rules, only for node_security_group_additional_rules. You can see that here and here.

But regardless of this, you shouldn't set any of these options to true because they mean you just want to use the Cluster SG or the Nodes SG as source for your rule. But in your case you have an external security group that you want to use as source (the EC2 instance SG)

The second error is that the security_groups field should actually be source_security_group_id, and shouldn't be an array. This borrows the same naming convention from the aws_security_group_rule resource. You can confirm that in the module source code.

So I would say the correct configuration for you is:

  cluster_security_group_additional_rules = {
    ingress_ec2_tcp = {
      description                = "Access EKS from EC2 instance."
      protocol                   = "tcp"
      from_port                  = 443
      to_port                    = 443
      type                       = "ingress"
      source_security_group_id   = var.ec2_sg_id
    }
  }