metropolis/cli/metroctl/core/config.go

package core

import (
	"context"
	"crypto/ed25519"
	"crypto/rand"
	"crypto/tls"
	"crypto/x509"
	"encoding/pem"
	"errors"
	"fmt"
	"log"
	"net"
	"net/url"
	"os"
	"path/filepath"

	"golang.org/x/net/proxy"
	clientauthentication "k8s.io/client-go/pkg/apis/clientauthentication/v1"
	"k8s.io/client-go/tools/clientcmd"
	clientapi "k8s.io/client-go/tools/clientcmd/api"

	"source.monogon.dev/metropolis/node"
)

const (
	// OwnerKeyFileName is the filename of the owner key in a metroctl config
	// directory.
	OwnerKeyFileName = "owner-key.pem"
	// OwnerCertificateFileName is the filename of the owner certificate in a
	// metroctl config directory.
	OwnerCertificateFileName = "owner.pem"
	// CACertificateFileName is the filename of the cluster CA certificate in a
	// metroctl config directory.
	CACertificateFileName = "ca.pem"
)

var (
	// ErrNoCredentials indicates that the requested datum (eg. owner key or owner
	// certificate) is not present in the requested directory.
	ErrNoCredentials = errors.New("owner certificate or key does not exist")

	ErrNoCACertificate = errors.New("no cluster CA certificate while secure connection was requested")
)

// A PEM block type for a Metropolis initial owner private key
const ownerKeyType = "METROPOLIS INITIAL OWNER PRIVATE KEY"

// GetOrMakeOwnerKey returns the owner key for a given metroctl configuration
// directory path, generating and saving it first if it doesn't exist.
func GetOrMakeOwnerKey(path string) (ed25519.PrivateKey, error) {
	existing, err := GetOwnerKey(path)
	switch {
	case err == nil:
		return existing, nil
	case errors.Is(err, ErrNoCredentials):
	default:
		return nil, err
	}

	_, priv, err := ed25519.GenerateKey(rand.Reader)
	if err != nil {
		return nil, fmt.Errorf("when generating key: %w", err)
	}
	if err := WriteOwnerKey(path, priv); err != nil {
		return nil, err
	}
	return priv, nil
}

// WriteOwnerKey saves a given raw ED25519 private key as the owner key at a
// given metroctl configuration directory path.
func WriteOwnerKey(path string, priv ed25519.PrivateKey) error {
	pemPriv := pem.EncodeToMemory(&pem.Block{Type: ownerKeyType, Bytes: priv})
	if err := os.WriteFile(filepath.Join(path, OwnerKeyFileName), pemPriv, 0600); err != nil {
		return fmt.Errorf("when saving key: %w", err)
	}
	return nil
}

// WriteCACertificate writes the given der-encoded X509 certificate to the given
// metorctl configuration directory path.
func WriteCACertificate(path string, der []byte) error {
	pemCert := pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: der})
	if err := os.WriteFile(filepath.Join(path, CACertificateFileName), pemCert, 0600); err != nil {
		return fmt.Errorf("when saving CA certificate: %w", err)
	}
	return nil
}

// GetOwnerKey loads and returns a raw ED25519 private key from the saved owner
// key in a given metroctl configuration directory path. If the owner key doesn't
// exist, ErrNoCredentials will be returned.
func GetOwnerKey(path string) (ed25519.PrivateKey, error) {
	ownerPrivateKeyPEM, err := os.ReadFile(filepath.Join(path, OwnerKeyFileName))
	if os.IsNotExist(err) {
		return nil, ErrNoCredentials
	} else if err != nil {
		return nil, fmt.Errorf("failed to load owner private key: %w", err)
	}
	block, _ := pem.Decode(ownerPrivateKeyPEM)
	if block == nil {
		return nil, errors.New("owner-key.pem contains invalid PEM armoring")
	}
	if block.Type != ownerKeyType {
		return nil, fmt.Errorf("owner-key.pem contains a PEM block that's not a %v", ownerKeyType)
	}
	if len(block.Bytes) != ed25519.PrivateKeySize {
		return nil, errors.New("owner-key.pem contains a non-Ed25519 key")
	}
	return block.Bytes, nil
}

// WriteOwnerCertificate saves a given DER-encoded X509 certificate as the owner
// key for a given metroctl configuration directory path.
func WriteOwnerCertificate(path string, cert []byte) error {
	ownerCertPEM := pem.Block{
		Type:  "CERTIFICATE",
		Bytes: cert,
	}
	if err := os.WriteFile(filepath.Join(path, OwnerCertificateFileName), pem.EncodeToMemory(&ownerCertPEM), 0644); err != nil {
		return err
	}
	return nil
}

// GetOwnerCredentials loads and returns a raw ED25519 private key alongside a
// DER-encoded X509 certificate from the saved owner key and certificate in a
// given metroctl configuration directory path. If either the key or certificate
// doesn't exist, ErrNoCredentials will be returned.
func GetOwnerCredentials(path string) (cert *x509.Certificate, key ed25519.PrivateKey, err error) {
	key, err = GetOwnerKey(path)
	if err != nil {
		return nil, nil, err
	}

	ownerCertPEM, err := os.ReadFile(filepath.Join(path, OwnerCertificateFileName))
	if os.IsNotExist(err) {
		return nil, nil, ErrNoCredentials
	} else if err != nil {
		return nil, nil, fmt.Errorf("failed to load owner certificate: %w", err)
	}
	block, _ := pem.Decode(ownerCertPEM)
	if block == nil {
		return nil, nil, errors.New("owner.pem contains invalid PEM armoring")
	}
	if block.Type != "CERTIFICATE" {
		return nil, nil, fmt.Errorf("owner.pem contains a PEM block that's not a CERTIFICATE")
	}
	cert, err = x509.ParseCertificate(block.Bytes)
	if err != nil {
		return nil, nil, fmt.Errorf("owner.pem contains an invalid X.509 certificate: %w", err)
	}
	return
}

// GetOwnerTLSCredentials returns a client TLS Certificate for authenticating to
// the metropolis cluster, based on metroctl configuration at a given path.
func GetOwnerTLSCredentials(path string) (*tls.Certificate, error) {
	ocert, opkey, err := GetOwnerCredentials(path)
	if err != nil {
		return nil, err
	}
	return &tls.Certificate{
		Certificate: [][]byte{ocert.Raw},
		PrivateKey:  opkey,
	}, nil
}

// GetClusterCA returns the saved cluster CA certificate at the given metoctl
// configuration path. This does not perform TOFU if the certificate is not
// present.
func GetClusterCA(path string) (cert *x509.Certificate, err error) {
	caCertPEM, err := os.ReadFile(filepath.Join(path, CACertificateFileName))
	if os.IsNotExist(err) {
		return nil, ErrNoCACertificate
	} else if err != nil {
		return nil, fmt.Errorf("failed to load CA certificate: %w", err)
	}
	block, _ := pem.Decode(caCertPEM)
	if block == nil {
		return nil, errors.New("ca.pem contains invalid PEM armoring")
	}
	if block.Type != "CERTIFICATE" {
		return nil, fmt.Errorf("ca.pem contains a PEM block that's not a CERTIFICATE")
	}
	cert, err = x509.ParseCertificate(block.Bytes)
	if err != nil {
		return nil, fmt.Errorf("ca.pem contains an invalid X.509 certificate: %w", err)
	}
	return
}

// InstallKubeletConfig modifies the default kubelet kubeconfig of the host
// system to be able to connect via a metroctl (and an associated ConnectOptions)
// to a Kubernetes apiserver at IP address/hostname 'server'.
//
// The kubelet's kubeconfig changes will be limited to contexts/configs/... named
// configName. The configName context will be made the default context only if
// there is no other default context in the current subconfig.
//
// Kubeconfigs can only take a single Kubernetes server address, so this function
// similarly only allows you to specify only a single server address.
func InstallKubeletConfig(ctx context.Context, metroctlPath string, opts *ConnectOptions, configName, server string) error {
	po := clientcmd.NewDefaultPathOptions()
	config, err := po.GetStartingConfig()
	if err != nil {
		return fmt.Errorf("getting initial config failed: %w", err)
	}

	args := []string{
		"k8scredplugin",
	}
	args = append(args, opts.ToFlags()...)

	config.AuthInfos[configName] = &clientapi.AuthInfo{
		Exec: &clientapi.ExecConfig{
			APIVersion: clientauthentication.SchemeGroupVersion.String(),
			Command:    metroctlPath,
			Args:       args,
			InstallHint: `Authenticating to Metropolis clusters requires metroctl to be present.
Running metroctl takeownership creates this entry and either points to metroctl as a command in
PATH if metroctl is in PATH at that time or to the absolute path to metroctl at that time.
If you moved metroctl afterwards or want to switch to PATH resolution, edit $HOME/.kube/config and
change users.metropolis.exec.command to the required path (or just metroctl if using PATH resolution).`,
			InteractiveMode: clientapi.NeverExecInteractiveMode,
		},
	}

	var u url.URL
	u.Scheme = "https"
	u.Host = net.JoinHostPort(server, node.KubernetesAPIWrappedPort.PortString())

	// HACK: the Metropolis node certificates only contain the node ID as a SAN. This
	// means that we can't use some 'global' identifier as the TLSServerName below
	// that would be the same across all cluster nodes. Unfortunately the Kubeconfig
	// system only allows for specifying a concrete name, not a regexp or some more
	// complex validation mechanism for certs.
	//
	// The correct fix for this is to issue a new set of certs for the nodes to use,
	// but that would require implementing a migration mechanism which we don't want
	// to do as that entire system is getting replaced with SPIFFE based certificates
	// very soon.
	//
	// To get around this, we thus pin the TLSServerName. This works because current
	// production deployments only use a single node as the Kubernetes endpoint. To
	// actually get the cert we connect here to the given server and retrieve its
	// node ID.
	//
	// TODO(lorenz): replace as part of SPIFFE authn work

	ca, err := GetClusterCAWithTOFU(ctx, opts)
	if err != nil {
		return fmt.Errorf("failed to retrieve CA certificate: %w", err)
	}

	pinnedNameC := make(chan string, 1)
	connLower, err := opts.Dial("tcp", u.Host)
	if err != nil {
		return fmt.Errorf("failed to dial to retrieve server cert: %w", err)
	}
	conn := tls.Client(connLower, &tls.Config{
		InsecureSkipVerify: true,
		VerifyPeerCertificate: func(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error {
			if ncerts := len(rawCerts); ncerts != 1 {
				return fmt.Errorf("expected 1 server cert, got %d", ncerts)
			}
			cert, err := x509.ParseCertificate(rawCerts[0])
			if err != nil {
				return fmt.Errorf("parsing server certificate failed: %w", err)
			}
			if err := cert.CheckSignatureFrom(ca); err != nil {
				return fmt.Errorf("server certificate verification failed: %w", err)
			}
			if nnames := len(cert.DNSNames); nnames != 1 {
				return fmt.Errorf("expected 1 DNS SAN, got %q", cert.DNSNames)
			}
			pinnedNameC <- cert.DNSNames[0]
			return nil
		},
	})
	if err := conn.Handshake(); err != nil {
		return fmt.Errorf("failed to connect to retrieve server cert: %w", err)
	}
	var pinnedName string
	select {
	case pinnedName = <-pinnedNameC:
	case <-ctx.Done():
		return ctx.Err()
	}

	log.Printf("Pinning Kubernetes server certificate to %q", pinnedName)

	// Actually configure Kubernetes now.

	config.Clusters[configName] = &clientapi.Cluster{
		CertificateAuthorityData: pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: ca.Raw}),
		TLSServerName:            pinnedName,
		Server:                   u.String(),
		ProxyURL:                 opts.ProxyURL(),
	}

	config.Contexts[configName] = &clientapi.Context{
		AuthInfo:  configName,
		Cluster:   configName,
		Namespace: "default",
	}

	// Only set us as the current context if no other exists. Changing that
	// unprompted would be kind of rude.
	if config.CurrentContext == "" {
		config.CurrentContext = configName
	}

	if err := clientcmd.ModifyConfig(po, *config, true); err != nil {
		return fmt.Errorf("modifying config failed: %w", err)
	}
	return nil
}

// ConnectOptions define how to reach a Metropolis cluster from metroctl.
//
// This structure can be built directly. All unset fields mean 'default'. It can
// then be used to generate the equivalent flags to passs to metroctl.
//
// Nil pointers to ConnectOptions are equivalent to an empty ConneectOptions when
// methods on it are called.
type ConnectOptions struct {
	// ConfigPath is the path at which the metroctl configuration/credentials live.
	// If not set, the default will be used.
	ConfigPath string
	// ProxyServer is a host:port pair that indicates the metropolis cluster should
	// be reached via the given SOCKS5 proxy. If not set, the cluster can be reached
	// directly from the host networking stack.
	ProxyServer string
	// Endpoints are the IP addresses/hostnames (without port part) of the Metropolis
	// instances that metroctl should use to establish connectivity to a cluster.
	// These instances should have the ControlPlane role set.
	Endpoints []string
	// ResolverLogger can be set to enable verbose logging of the Metropolis RPC
	// resolver layer.
	ResolverLogger ResolverLogger
	// TOFU overrides the trust-on-first-use behaviour for CA certificates for the
	// connection. If not set, TerminalTOFU is used which will interactively ask the
	// user to accept a CA certificate using os.Stdin/Stdout.
	TOFU CertificateTOFU
}

// ToFlags returns the metroctl flags corresponding to the options described by
// this ConnectionOptions struct.
func (c *ConnectOptions) ToFlags() []string {
	var res []string

	if c == nil {
		return res
	}

	if c.ConfigPath != "" {
		res = append(res, "--config", c.ConfigPath)
	}
	if c.ProxyServer != "" {
		res = append(res, "--proxy", c.ProxyServer)
	}
	for _, ep := range c.Endpoints {
		res = append(res, "--endpoints", ep)
	}

	return res
}

// ProxyURL returns a kubeconfig-compatible URL of the proxy server configured by
// ConnectOptions, or an empty string if not set.
func (c *ConnectOptions) ProxyURL() string {
	if c == nil {
		return ""
	}
	if c.ProxyServer == "" {
		return ""
	}
	var u url.URL
	u.Scheme = "socks5"
	u.Host = c.ProxyServer
	return u.String()
}

func (c *ConnectOptions) Dial(network, addr string) (net.Conn, error) {
	if c.ProxyServer != "" {
		socksDialer, err := proxy.SOCKS5("tcp", c.ProxyServer, nil, proxy.Direct)
		if err != nil {
			return nil, fmt.Errorf("failed to build a SOCKS dialer: %w", err)
		}
		return socksDialer.Dial(network, addr)
	} else {
		return net.Dial(network, addr)
	}
}