Ensure Container-Optimized OS (cos) is used for Kubernetes Engine Clusters Node image

Explanation

GKE supports several OS image types but COS_CONTAINERD is the recommended OS image to use on cluster nodes for enhanced security

Possible Impact

COS_CONTAINERD is the recommended OS image to use on cluster nodes

Suggested Resolution

Use the COS_CONTAINERD image type

Insecure Example

The following example will fail the google-gke-node-pool-uses-cos check.

resource "google_service_account" "default" {
  account_id   = "service-account-id"
  display_name = "Service Account"
}

resource "google_container_cluster" "primary" {
  name     = "my-gke-cluster"
  location = "us-central1"

  # We can't create a cluster with no node pool defined, but we want to only use
  # separately managed node pools. So we create the smallest possible default
  # node pool and immediately delete it.
  remove_default_node_pool = true
  initial_node_count       = 1
}

resource "google_container_node_pool" "bad_example" {
  name       = "my-node-pool"
  cluster    = google_container_cluster.primary.id
  node_count = 1

  node_config {
    preemptible  = true
    machine_type = "e2-medium"

    # Google recommends custom service accounts that have cloud-platform scope and permissions granted via IAM Roles.
    service_account = google_service_account.default.email
    oauth_scopes = [
      "https://www.googleapis.com/auth/cloud-platform"
    ]
    image_type = "something"
  }
}

Secure Example

The following example will pass the google-gke-node-pool-uses-cos check.

resource "google_service_account" "default" {
  account_id   = "service-account-id"
  display_name = "Service Account"
}

resource "google_container_cluster" "primary" {
  name     = "my-gke-cluster"
  location = "us-central1"

  # We can't create a cluster with no node pool defined, but we want to only use
  # separately managed node pools. So we create the smallest possible default
  # node pool and immediately delete it.
  remove_default_node_pool = true
  initial_node_count       = 1
}

resource "google_container_node_pool" "good_example" {
  name       = "my-node-pool"
  cluster    = google_container_cluster.primary.id
  node_count = 1

  node_config {
    preemptible  = true
    machine_type = "e2-medium"

    # Google recommends custom service accounts that have cloud-platform scope and permissions granted via IAM Roles.
    service_account = google_service_account.default.email
    oauth_scopes = [
      "https://www.googleapis.com/auth/cloud-platform"
    ]
    image_type = "COS_CONTAINERD"
  }
}

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