kubeadm: add basic validation around kubelet.conf parsing

If the user has modified the kubelet.conf post TLS bootstrap
to become invalid, the function getNodeNameFromKubeletConfig() can
panic. This was observed to trigger in "kubeadm reset" use cases.

Add basic validation and unit tests around parsing the kubelet.conf
with the aforementioned function.

cmd/kubeadm/app/util/config/cluster.go

@@ -147,7 +147,14 @@ func getNodeNameFromKubeletConfig(kubeconfigDir string) (string, error) {
 	}
 
 	// gets the info about the current user
-	authInfo := config.AuthInfos[config.Contexts[config.CurrentContext].AuthInfo]
+	currentContext, exists := config.Contexts[config.CurrentContext]
+	if !exists {
+		return "", errors.Errorf("invalid kubeconfig file %s: missing context %s", fileName, config.CurrentContext)
+	}
+	authInfo, exists := config.AuthInfos[currentContext.AuthInfo]
+	if !exists {
+		return "", errors.Errorf("invalid kubeconfig file %s: missing AuthInfo %s", fileName, currentContext.AuthInfo)
+	}
 
 	// gets the X509 certificate with current user credentials
 	var certs []*x509.Certificate
@@ -162,7 +169,7 @@ func getNodeNameFromKubeletConfig(kubeconfigDir string) (string, error) {
 			return "", err
 		}
 	} else {
-		return "", errors.New("invalid kubelet.conf. X509 certificate expected")
+		return "", errors.Errorf("invalid kubeconfig file %s. x509 certificate expected", fileName)
 	}
 
 	// We are only putting one certificate in the certificate pem file, so it's safe to just pick the first one

cmd/kubeadm/app/util/config/cluster_test.go

@@ -143,6 +143,44 @@ current-context: system:node:mynode@kubernetes
 kind: Config
 preferences: {}
 users:
+- name: system:node:mynode
+  user:
+      client-certificate: kubelet.pem
+`),
+	"configWithInvalidContext": []byte(`
+apiVersion: v1
+clusters:
+- cluster:
+    server: https://10.0.2.15:6443
+  name: kubernetes
+contexts:
+- context:
+    cluster: kubernetes
+    user: system:node:mynode
+  name: system:node:mynode@kubernetes
+current-context: invalidContext
+kind: Config
+preferences: {}
+users:
+- name: system:node:mynode
+  user:
+      client-certificate: kubelet.pem
+`),
+	"configWithInvalidUser": []byte(`
+apiVersion: v1
+clusters:
+- cluster:
+    server: https://10.0.2.15:6443
+  name: kubernetes
+contexts:
+- context:
+    cluster: kubernetes
+    user: invalidUser
+  name: system:node:mynode@kubernetes
+current-context: system:node:mynode@kubernetes
+kind: Config
+preferences: {}
+users:
 - name: system:node:mynode
   user:
       client-certificate: kubelet.pem
@@ -204,6 +242,16 @@ func TestGetNodeNameFromKubeletConfig(t *testing.T) {
 			kubeconfigContent: kubeletConfFiles["withoutX509Cert"],
 			expectedError:     true,
 		},
+		{
+			name:              "invalid - the current context is invalid",
+			kubeconfigContent: kubeletConfFiles["configWithInvalidContext"],
+			expectedError:     true,
+		},
+		{
+			name:              "invalid - the user of the current context is invalid",
+			kubeconfigContent: kubeletConfFiles["configWithInvalidUser"],
+			expectedError:     true,
+		},
 	}
 
 	for _, rt := range tests {