kubectl and seeing (cluster)roles assigned to subjects - kubernetes

Question

I can use kubectl to see to which subjects a cluster role is applied, eg:
kubectl get clusterrolebindings system:node --all-namespaces -o json
{
"apiVersion": "rbac.authorization.k8s.io/v1beta1",
"kind": "ClusterRoleBinding",
....
....
"subjects": [
{
"apiGroup": "rbac.authorization.k8s.io",
"kind": "Group",
"name": "system:nodes"
}
]
}
I would like to get this info the other way around, eg: I want to list all policies applied to the "system:nodes" subject.
How can I do that?

Answer 1

There is no API for the reverse index. You can look up bindings and filter on ones containing the expected subject. For example, using bash, jq, and kubectl:
# $1 is kind (User, Group, ServiceAccount)
# $2 is name ("system:nodes", etc)
# $3 is namespace (optional, only applies to kind=ServiceAccount)
function getRoles() {
local kind="${1}"
local name="${2}"
local namespace="${3:-}"
kubectl get clusterrolebinding -o json | jq -r "
.items[]
|
select(
.subjects[]?
|
select(
.kind == \"${kind}\"
and
.name == \"${name}\"
and
(if .namespace then .namespace else \"\" end) == \"${namespace}\"
)
)
|
(.roleRef.kind + \"/\" + .roleRef.name)
"
}
$ getRoles Group system:authenticated
ClusterRole/system:basic-user
ClusterRole/system:discovery
$ getRoles ServiceAccount attachdetach-controller kube-system
ClusterRole/system:controller:attachdetach-controller