add provider ca auth support for kubernetes

Adds support for Kubernetes jwt/token file based auth. Only needs to
read the file and save the contents as the jwt/token.

.changelog/16262.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+ca: support Vault agent auto-auth config for Vault CA provider using Kubernetes authentication.
+```

agent/connect/ca/provider_vault.go

@@ -8,7 +8,6 @@ import (
 	"fmt"
 	"io"
 	"net/http"
-	"os"
 	"strings"
 	"sync"
 	"time"
@@ -922,6 +921,14 @@ func vaultLogin(client *vaultapi.Client, authMethod *structs.VaultAuthMethod) (*
 	return resp, nil
 }
 
+// Note the authMethod's parameters (Params) is populated from a freeform map
+// in the configuration where they could hardcode values to be passed directly
+// to the `auth/*/login` endpoint. Each auth method's authentication code
+// needs to handle two cases:
+// - The legacy case (which should be deprecated) where the user has
+// hardcoded login values directly (eg. a `jwt` string)
+// - The case where they use the configuration option used in the
+// vault agent's auth methods.
 func configureVaultAuthMethod(authMethod *structs.VaultAuthMethod) (VaultAuthenticator, error) {
 	if authMethod.MountPath == "" {
 		authMethod.MountPath = authMethod.Type
@@ -938,17 +945,7 @@ func configureVaultAuthMethod(authMethod *structs.VaultAuthMethod) (VaultAuthent
 	case VaultAuthMethodTypeJWT:
 		return NewJwtAuthClient(authMethod)
 	case VaultAuthMethodTypeKubernetes:
-		// For the Kubernetes Auth method, we will try to read the JWT token
-		// from the default service account file location if jwt was not provided.
-		if jwt, ok := authMethod.Params["jwt"]; !ok || jwt == "" {
-			serviceAccountToken, err := os.ReadFile(defaultK8SServiceAccountTokenPath)
-			if err != nil {
-				return nil, err
-			}
-
-			authMethod.Params["jwt"] = string(serviceAccountToken)
-		}
-		return NewVaultAPIAuthClient(authMethod, loginPath), nil
+		return NewK8sAuthClient(authMethod)
 	// These auth methods require a username for the login API path.
 	case VaultAuthMethodTypeLDAP, VaultAuthMethodTypeUserpass, VaultAuthMethodTypeOkta, VaultAuthMethodTypeRadius:
 		// Get username from the params.

agent/connect/ca/provider_vault_auth_k8s.go

@@ -0,0 +1,47 @@
+package ca
+
+import (
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/hashicorp/consul/agent/structs"
+)
+
+func NewK8sAuthClient(authMethod *structs.VaultAuthMethod) (*VaultAuthClient, error) {
+	params := authMethod.Params
+	role, ok := params["role"].(string)
+	if !ok || strings.TrimSpace(role) == "" {
+		return nil, fmt.Errorf("missing 'role' value")
+	}
+	// don't check for `token_path` as it is optional
+
+	authClient := NewVaultAPIAuthClient(authMethod, "")
+	// Note the `jwt` can be passed directly in the authMethod as a Param value
+	// is a freeform map in the config where they could hardcode it.
+	if legacyCheck(params, "jwt") {
+		return authClient, nil
+	}
+
+	authClient.LoginDataGen = K8sLoginDataGen
+	return authClient, nil
+}
+
+func K8sLoginDataGen(authMethod *structs.VaultAuthMethod) (map[string]any, error) {
+	params := authMethod.Params
+	role := params["role"].(string)
+
+	// read token from file on path
+	tokenPath, ok := params["token_path"].(string)
+	if !ok || strings.TrimSpace(tokenPath) == "" {
+		tokenPath = defaultK8SServiceAccountTokenPath
+	}
+	rawToken, err := os.ReadFile(tokenPath)
+	if err != nil {
+		return nil, err
+	}
+	return map[string]any{
+		"role": role,
+		"jwt":  strings.TrimSpace(string(rawToken)),
+	}, nil
+}

agent/connect/ca/provider_vault_auth_test.go

@@ -502,3 +502,69 @@ func TestVaultCAProvider_JwtAuthClient(t *testing.T) {
 		})
 	}
 }
+
+func TestVaultCAProvider_K8sAuthClient(t *testing.T) {
+	tokenF, err := os.CreateTemp("", "token-path")
+	require.NoError(t, err)
+	defer func() { os.Remove(tokenF.Name()) }()
+	_, err = tokenF.WriteString("test-token")
+	require.NoError(t, err)
+	err = tokenF.Close()
+	require.NoError(t, err)
+
+	cases := map[string]struct {
+		authMethod *structs.VaultAuthMethod
+		expData    map[string]any
+		expErr     error
+	}{
+		"base-case": {
+			authMethod: &structs.VaultAuthMethod{
+				Type: "kubernetes",
+				Params: map[string]any{
+					"role":       "test-role",
+					"token_path": tokenF.Name(),
+				},
+			},
+			expData: map[string]any{
+				"role": "test-role",
+				"jwt":  "test-token",
+			},
+		},
+		"legacy-case": {
+			authMethod: &structs.VaultAuthMethod{
+				Type: "kubernetes",
+				Params: map[string]any{
+					"role": "test-role",
+					"jwt":  "test-token",
+				},
+			},
+			expData: map[string]any{
+				"role": "test-role",
+				"jwt":  "test-token",
+			},
+		},
+		"no-role": {
+			authMethod: &structs.VaultAuthMethod{
+				Type:   "kubernetes",
+				Params: map[string]any{},
+			},
+			expErr: fmt.Errorf("missing 'role' value"),
+		},
+	}
+	for name, c := range cases {
+		t.Run(name, func(t *testing.T) {
+			auth, err := NewK8sAuthClient(c.authMethod)
+			if c.expErr != nil {
+				require.Error(t, err)
+				require.EqualError(t, c.expErr, err.Error())
+				return
+			}
+			require.NoError(t, err)
+			if auth.LoginDataGen != nil {
+				data, err := auth.LoginDataGen(c.authMethod)
+				require.NoError(t, err)
+				require.Equal(t, c.expData, data)
+			}
+		})
+	}
+}

agent/connect/ca/provider_vault_test.go

@@ -113,7 +113,7 @@ func TestVaultCAProvider_configureVaultAuthMethod(t *testing.T) {
 		"gcp":         {expLoginPath: "auth/gcp/login", params: map[string]interface{}{"type": "iam", "role": "test-role"}},
 		"jwt":         {expLoginPath: "auth/jwt/login", params: map[string]any{"role": "test-role", "path": "test-path"}, hasLDG: true},
 		"kerberos":    {expLoginPath: "auth/kerberos/login"},
-		"kubernetes":  {expLoginPath: "auth/kubernetes/login", params: map[string]interface{}{"jwt": "fake"}},
+		"kubernetes":  {expLoginPath: "auth/kubernetes/login", params: map[string]interface{}{"role": "test-role"}, hasLDG: true},
 		"ldap":        {expLoginPath: "auth/ldap/login/foo", params: map[string]interface{}{"username": "foo"}},
 		"oci":         {expLoginPath: "auth/oci/login/foo", params: map[string]interface{}{"role": "foo"}},
 		"okta":        {expLoginPath: "auth/okta/login/foo", params: map[string]interface{}{"username": "foo"}},