Dynamic identity file reloading support for API Client

api/client/credentials.go

@@ -17,18 +17,24 @@ limitations under the License.
 package client
 
 import (
+	"crypto"
 	"crypto/tls"
 	"crypto/x509"
+	"net"
 	"os"
+	"sync"
 
 	"github.com/gravitational/trace"
 	"golang.org/x/crypto/ssh"
 	"golang.org/x/net/http2"
 
 	"github.com/gravitational/teleport/api/constants"
+	"github.com/gravitational/teleport/api/defaults"
 	"github.com/gravitational/teleport/api/identityfile"
 	"github.com/gravitational/teleport/api/profile"
 	"github.com/gravitational/teleport/api/utils"
+	"github.com/gravitational/teleport/api/utils/keys"
+	"github.com/gravitational/teleport/api/utils/sshutils"
 )
 
 // Credentials are used to authenticate the API auth client. Some Credentials
@@ -394,3 +400,192 @@ func configureTLS(c *tls.Config) *tls.Config {
 
 	return tlsConfig
 }
+
+// DynamicIdentityFileCreds allows a changing identity file to be used as the
+// source of authentication for Client. It does not automatically watch the
+// identity file or reload on an interval, this is left as an exercise for the
+// consumer.
+type DynamicIdentityFileCreds struct {
+	// mu protects the fields that may change if the underlying identity file
+	// is reloaded.
+	mu            sync.RWMutex
+	tlsCert       *tls.Certificate
+	tlsRootCAs    *x509.CertPool
+	sshCert       *ssh.Certificate
+	sshKey        crypto.Signer
+	sshKnownHosts []ssh.PublicKey
+
+	// Path is the path to the identity file to load and reload.
+	Path string
+}
+
+// NewDynamicIdentityFileCreds returns a DynamicIdentityFileCreds which has
+// been initially loaded and is ready for use.
+func NewDynamicIdentityFileCreds(path string) (*DynamicIdentityFileCreds, error) {
+	d := &DynamicIdentityFileCreds{
+		Path: path,
+	}
+	if err := d.Reload(); err != nil {
+		return nil, trace.Wrap(err)
+	}
+	return d, nil
+}
+
+// Reload causes the identity file to be re-read from the disk. It will return
+// an error if loading the credentials fails.
+func (d *DynamicIdentityFileCreds) Reload() error {
+	id, err := identityfile.ReadFile(d.Path)
+	if err != nil {
+		return trace.Wrap(err)
+	}
+
+	// This section is essentially id.TLSConfig()
+	cert, err := keys.X509KeyPair(id.Certs.TLS, id.PrivateKey)
+	if err != nil {
+		return trace.Wrap(err)
+	}
+	pool := x509.NewCertPool()
+	for _, caCerts := range id.CACerts.TLS {
+		if !pool.AppendCertsFromPEM(caCerts) {
+			return trace.BadParameter("invalid CA cert PEM")
+		}
+	}
+
+	// This sections is essentially id.SSHClientConfig()
+	sshCert, err := sshutils.ParseCertificate(id.Certs.SSH)
+	if err != nil {
+		return trace.Wrap(err)
+	}
+	sshPrivateKey, err := keys.ParsePrivateKey(id.PrivateKey)
+	if err != nil {
+		return trace.Wrap(err)
+	}
+	knownHosts, err := sshutils.ParseKnownHosts(id.CACerts.SSH)
+	if err != nil {
+		return trace.Wrap(err)
+	}
+
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	d.tlsRootCAs = pool
+	d.tlsCert = &cert
+	d.sshCert = sshCert
+	d.sshKey = sshPrivateKey
+	d.sshKnownHosts = knownHosts
+	return nil
+}
+
+// Dialer returns a dialer for the client to use. This is not used, but is
+// needed to implement the Credentials interface.
+func (d *DynamicIdentityFileCreds) Dialer(
+	_ Config,
+) (ContextDialer, error) {
+	// Returning a dialer isn't necessary for this credential.
+	return nil, trace.NotImplemented("no dialer")
+}
+
+// TLSConfig returns TLS configuration. Implementing the Credentials interface.
+func (d *DynamicIdentityFileCreds) TLSConfig() (*tls.Config, error) {
+	// Build a "dynamic" tls.Config which can support a changing cert and root
+	// CA pool.
+	cfg := &tls.Config{
+		// Set the default NextProto of "h2". Based on the value in
+		// configureTLS()
+		NextProtos: []string{http2.NextProtoTLS},
+
+		// GetClientCertificate is used instead of the static Certificates
+		// field.
+		Certificates: nil,
+		GetClientCertificate: func(
+			_ *tls.CertificateRequestInfo,
+		) (*tls.Certificate, error) {
+			// GetClientCertificate callback is used to allow us to dynamically
+			// change the certificate when reloaded.
+			d.mu.RLock()
+			defer d.mu.RUnlock()
+			return d.tlsCert, nil
+		},
+
+		// VerifyConnection is used instead of the static RootCAs field.
+		RootCAs: nil,
+		// InsecureSkipVerify is forced true to ensure that only our
+		// VerifyConnection callback is used to verify the server's presented
+		// certificate.
+		InsecureSkipVerify: true,
+		VerifyConnection: func(state tls.ConnectionState) error {
+			// This VerifyConnection callback is based on the standard library
+			// implementation of verifyServerCertificate in the `tls` package.
+			// We provide our own implementation so we can dynamically handle
+			// a changing CA Roots pool.
+			d.mu.RLock()
+			defer d.mu.RUnlock()
+			opts := x509.VerifyOptions{
+				DNSName:       state.ServerName,
+				Intermediates: x509.NewCertPool(),
+				Roots:         d.tlsRootCAs,
+			}
+			for _, cert := range state.PeerCertificates[1:] {
+				// Whilst we don't currently use intermediate certs at
+				// Teleport, including this here means that we are
+				// future-proofed in case we do.
+				opts.Intermediates.AddCert(cert)
+			}
+			_, err := state.PeerCertificates[0].Verify(opts)
+			return err
+		},
+		// Set ServerName for SNI & Certificate Validation to the sentinel
+		// teleport.cluster.local which is included on all Teleport Auth Server
+		// certificates. Based on the value in configureTLS()
+		ServerName: constants.APIDomain,
+	}
+
+	return cfg, nil
+}
+
+// SSHClientConfig returns SSH configuration, implementing the Credentials
+// interface.
+func (d *DynamicIdentityFileCreds) SSHClientConfig() (*ssh.ClientConfig, error) {
+	// This lock is necessary for `d.sshCert` used in `cfg.User`.
+	d.mu.RLock()
+	defer d.mu.RUnlock()
+	// Build a "dynamic" ssh config. Based roughly on
+	// `sshutils.ProxyClientSSHConfig` with modifications to make it work with
+	// dynamically changing credentials and CAs.
+	cfg := &ssh.ClientConfig{
+		Auth: []ssh.AuthMethod{
+			ssh.PublicKeysCallback(func() (signers []ssh.Signer, err error) {
+				d.mu.RLock()
+				defer d.mu.RUnlock()
+				sshSigner, err := sshutils.SSHSigner(d.sshCert, d.sshKey)
+				if err != nil {
+					return nil, trace.Wrap(err)
+				}
+				return []ssh.Signer{sshSigner}, nil
+			}),
+		},
+		HostKeyCallback: func(hostname string, remote net.Addr, key ssh.PublicKey) error {
+			d.mu.RLock()
+			defer d.mu.RUnlock()
+			hostKeyCallback, err := sshutils.HostKeyCallback(
+				d.sshKnownHosts,
+				false,
+			)
+			if err != nil {
+				return trace.Wrap(err)
+			}
+			return hostKeyCallback(hostname, remote, key)
+		},
+		Timeout: defaults.DefaultIOTimeout,
+		User:    userFromSSHCert(d.sshCert),
+	}
+	return cfg, nil
+}
+
+func userFromSSHCert(c *ssh.Certificate) string {
+	// The KeyId is not always a valid principal, so we prefer the first valid
+	// principal.
+	if len(c.ValidPrincipals) > 0 {
+		return c.ValidPrincipals[0]
+	}
+	return c.KeyId
+}

api/client/credentials_test.go

@@ -17,9 +17,15 @@ limitations under the License.
 package client
 
 import (
+	"crypto/rand"
+	"crypto/rsa"
 	"crypto/tls"
 	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/pem"
+	"math/big"
 	"os"
+	"path"
 	"path/filepath"
 	"testing"
 
@@ -28,6 +34,7 @@ import (
 	"github.com/stretchr/testify/require"
 	"golang.org/x/crypto/ssh"
 
+	"github.com/gravitational/teleport/api/constants"
 	"github.com/gravitational/teleport/api/identityfile"
 	"github.com/gravitational/teleport/api/profile"
 	"github.com/gravitational/teleport/api/utils/keys"
@@ -382,3 +389,81 @@ AIBV1ZA8WqvC+xZrPwmtmN87BHwGjqpE52kbUfcD94k8IqqhPR9oN9uOlcoBzZiS
 k53lH1qmEOm9+vrhNwNzpHk4AqDkP+0YDG++B4n0BtJJpw==
 Private-MAC: 8951bbe929e0714a61df01bc8fbc5223e3688f174aee29339931984fb9224c7d`)
 )
+
+func TestDynamicIdentityFileCreds(t *testing.T) {
+	dir := t.TempDir()
+	identityPath := path.Join(dir, "identity")
+
+	idFile := &identityfile.IdentityFile{
+		PrivateKey: keyPEM,
+		Certs: identityfile.Certs{
+			TLS: tlsCert,
+			SSH: sshCert,
+		},
+		CACerts: identityfile.CACerts{
+			TLS: [][]byte{tlsCACert},
+			SSH: [][]byte{sshCACert},
+		},
+	}
+	require.NoError(t, identityfile.Write(idFile, identityPath))
+
+	cred, err := NewDynamicIdentityFileCreds(identityPath)
+	require.NoError(t, err)
+
+	// Check the initial TLS certificate/key has been loaded.
+	tlsConfig, err := cred.TLSConfig()
+	require.NoError(t, err)
+	gotTLSCert, err := tlsConfig.GetClientCertificate(&tls.CertificateRequestInfo{
+		// We always return the same cert so this can be empty
+	})
+	require.NoError(t, err)
+	wantTLSCert, err := tls.X509KeyPair(tlsCert, keyPEM)
+	require.NoError(t, err)
+	require.Equal(t, wantTLSCert, *gotTLSCert)
+
+	// Generate a new TLS certificate that contains the same private key as
+	// the original.
+	template := &x509.Certificate{
+		SerialNumber: big.NewInt(0),
+		Subject: pkix.Name{
+			CommonName: "example",
+		},
+		KeyUsage:              x509.KeyUsageDigitalSignature,
+		BasicConstraintsValid: true,
+		DNSNames:              []string{constants.APIDomain},
+	}
+	secondTLSCert, err := x509.CreateCertificate(
+		rand.Reader, template, template, &wantTLSCert.PrivateKey.(*rsa.PrivateKey).PublicKey, wantTLSCert.PrivateKey,
+	)
+	require.NoError(t, err)
+	secondTLSCertPem := pem.EncodeToMemory(&pem.Block{
+		Type:  "CERTIFICATE",
+		Bytes: secondTLSCert,
+	})
+
+	// Write the new TLS certificate as part of the identity file and reload.
+	secondIDFile := &identityfile.IdentityFile{
+		PrivateKey: keyPEM,
+		Certs: identityfile.Certs{
+			TLS: secondTLSCertPem,
+			SSH: sshCert,
+		},
+		CACerts: identityfile.CACerts{
+			TLS: [][]byte{tlsCACert},
+			SSH: [][]byte{sshCACert},
+		},
+	}
+	require.NoError(t, identityfile.Write(secondIDFile, identityPath))
+	require.NoError(t, cred.Reload())
+
+	// Test that calling GetClientCertificate on the original tls.Config now
+	// returns the new certificate we wrote and reloaded.
+	gotTLSCert, err = tlsConfig.GetClientCertificate(&tls.CertificateRequestInfo{
+		// We always return the same cert so this can be empty
+	})
+	require.NoError(t, err)
+	wantTLSCert, err = tls.X509KeyPair(secondTLSCertPem, keyPEM)
+	require.NoError(t, err)
+	require.Equal(t, wantTLSCert, *gotTLSCert)
+
+}