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

Explanation

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

Possible Impact

COS is the recommended OS image to use on cluster nodes

Suggested Resolution

Use the COS 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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