connect/ca: add intermediate functions to Consul CA provider

agent/connect/ca/provider.go

@@ -22,6 +22,13 @@ type Provider interface {
 	// ActiveIntermediate()
 	ActiveRoot() (string, error)
 
+	// GenerateIntermediateCSR generates a CSR for an intermediate CA
+	// certificate, to be signed by the root of another datacenter. If isRoot is
+	// true, calling this is an error.
+	GenerateIntermediateCSR() (string, error)
+
+	SetIntermediate(intermediatePEM, rootPEM string) error
+
 	// ActiveIntermediate returns the current signing cert used by this provider
 	// for generating SPIFFE leaf certs. Note that this must not change except
 	// when Consul requests the change via GenerateIntermediate. Changing the
@@ -41,6 +48,12 @@ type Provider interface {
 	// intemediate and any cross-signed intermediates managed by Consul.
 	Sign(*x509.CertificateRequest) (string, error)
 
+	// SignIntermediate will validate the CSR to ensure the trust domain in the
+	// URI SAN matches the local one and that basic constraints for a CA certificate
+	// are met. It should return a signed CA certificate with a path length constraint
+	// of 0 to ensure that the certificate cannot be used to generate further CA certs.
+	SignIntermediate(*x509.CertificateRequest) (string, error)
+
 	// CrossSignCA must accept a CA certificate from another CA provider
 	// and cross sign it exactly as it is such that it forms a chain back the the
 	// CAProvider's current root. Specifically, the Distinguished Name, Subject

agent/connect/ca/provider_consul.go

@@ -25,9 +25,12 @@ var ErrNotInitialized = errors.New("provider not initialized")
 type ConsulProvider struct {
 	Delegate ConsulProviderStateDelegate
 
-	config *structs.ConsulCAProviderConfig
-	id     string
-	isRoot bool
+	config    *structs.ConsulCAProviderConfig
+	id        string
+	clusterID string
+	isRoot    bool
+	spiffeID  *connect.SpiffeIDSigning
+
 	sync.RWMutex
 }
 
@@ -44,9 +47,11 @@ func (c *ConsulProvider) Configure(clusterID string, isRoot bool, rawConfig map[
 		return err
 	}
 	c.config = config
-	c.isRoot = isRoot
 	hash := sha256.Sum256([]byte(fmt.Sprintf("%s,%s,%v", config.PrivateKey, config.RootCert, isRoot)))
 	c.id = strings.Replace(fmt.Sprintf("% x", hash), " ", ":", -1)
+	c.clusterID = clusterID
+	c.isRoot = isRoot
+	c.spiffeID = connect.SpiffeIDSigningForCluster(&structs.CAConfiguration{ClusterID: clusterID})
 
 	// Exit early if the state store has an entry for this provider's config.
 	_, providerState, err := c.Delegate.State().CAProviderState(c.id)
@@ -107,8 +112,7 @@ func (c *ConsulProvider) Configure(clusterID string, isRoot bool, rawConfig map[
 
 // ActiveRoot returns the active root CA certificate.
 func (c *ConsulProvider) ActiveRoot() (string, error) {
-	state := c.Delegate.State()
-	_, providerState, err := state.CAProviderState(c.id)
+	_, providerState, err := c.getState()
 	if err != nil {
 		return "", err
 	}
@@ -119,15 +123,11 @@ func (c *ConsulProvider) ActiveRoot() (string, error) {
 // GenerateRoot initializes a new root certificate and private key
 // if needed.
 func (c *ConsulProvider) GenerateRoot() error {
-	state := c.Delegate.State()
-	idx, providerState, err := state.CAProviderState(c.id)
+	idx, providerState, err := c.getState()
 	if err != nil {
 		return err
 	}
 
-	if providerState == nil {
-		return ErrNotInitialized
-	}
 	if !c.isRoot {
 		return fmt.Errorf("provider is not the root certificate authority")
 	}
@@ -170,10 +170,124 @@ func (c *ConsulProvider) GenerateRoot() error {
 	return nil
 }
 
+// GenerateIntermediateCSR creates a private key and generates a CSR
+// for another datacenter's root to sign.
+func (c *ConsulProvider) GenerateIntermediateCSR() (string, error) {
+	_, providerState, err := c.getState()
+	if err != nil {
+		return "", err
+	}
+
+	if c.isRoot {
+		return "", fmt.Errorf("provider is the root certificate authority, " +
+			"cannot generate an intermediate CSR")
+	}
+
+	// Create a new private key and CSR.
+	signer, pk, err := connect.GeneratePrivateKey()
+	if err != nil {
+		return "", err
+	}
+
+	csr, err := connect.CreateCSR(c.spiffeID, signer)
+	if err != nil {
+		return "", err
+	}
+
+	// Write the new provider state to the store.
+	newState := *providerState
+	newState.PrivateKey = pk
+	args := &structs.CARequest{
+		Op:            structs.CAOpSetProviderState,
+		ProviderState: &newState,
+	}
+	if err := c.Delegate.ApplyCARequest(args); err != nil {
+		return "", err
+	}
+
+	return csr, nil
+}
+
+func (c *ConsulProvider) SetIntermediate(intermediatePEM, rootPEM string) error {
+	_, providerState, err := c.getState()
+	if err != nil {
+		return err
+	}
+
+	if c.isRoot {
+		return fmt.Errorf("cannot set an intermediate using another root in the primary datacenter")
+	}
+
+	// Get the key from the incoming intermediate cert so we can compare it
+	// to the currently stored key.
+	intermediate, err := connect.ParseCert(intermediatePEM)
+	if err != nil {
+		return fmt.Errorf("error parsing intermediate PEM: %v", err)
+	}
+	privKey, err := connect.ParseSigner(providerState.PrivateKey)
+	if err != nil {
+		return err
+	}
+
+	// Compare the two keys to make sure they match.
+	b1, err := x509.MarshalPKIXPublicKey(intermediate.PublicKey)
+	if err != nil {
+		return err
+	}
+	b2, err := x509.MarshalPKIXPublicKey(privKey.Public())
+	if err != nil {
+		return err
+	}
+	if !bytes.Equal(b1, b2) {
+		return fmt.Errorf("intermediate cert is for a different private key")
+	}
+
+	// Validate the remaining fields and make sure the intermediate validates against
+	// the given root cert.
+	if !intermediate.IsCA {
+		return fmt.Errorf("intermediate is not a CA certificate")
+	}
+	if got, want := intermediate.URIs[0].String(), c.spiffeID.URI().String(); got != want {
+		return fmt.Errorf("incoming cert URI %q does not match current URI: %q", got, want)
+	}
+
+	pool := x509.NewCertPool()
+	pool.AppendCertsFromPEM([]byte(rootPEM))
+	_, err = intermediate.Verify(x509.VerifyOptions{
+		Roots: pool,
+	})
+	if err != nil {
+		return fmt.Errorf("could not verify intermediate cert against root: %v", err)
+	}
+
+	// Update the state
+	newState := *providerState
+	newState.IntermediateCert = intermediatePEM
+	newState.RootCert = rootPEM
+	args := &structs.CARequest{
+		Op:            structs.CAOpSetProviderState,
+		ProviderState: &newState,
+	}
+	if err := c.Delegate.ApplyCARequest(args); err != nil {
+		return err
+	}
+
+	return nil
+}
+
 // We aren't maintaining separate root/intermediate CAs for the builtin
 // provider, so just return the root.
 func (c *ConsulProvider) ActiveIntermediate() (string, error) {
-	return c.ActiveRoot()
+	if c.isRoot {
+		return c.ActiveRoot()
+	}
+
+	_, providerState, err := c.getState()
+	if err != nil {
+		return "", err
+	}
+
+	return providerState.IntermediateCert, nil
 }
 
 // We aren't maintaining separate root/intermediate CAs for the builtin
@@ -216,7 +330,7 @@ func (c *ConsulProvider) Sign(csr *x509.CertificateRequest) (string, error) {
 		return "", err
 	}
 	if signer == nil {
-		return "", fmt.Errorf("error signing cert: Consul CA not initialized yet")
+		return "", ErrNotInitialized
 	}
 	keyId, err := connect.KeyId(signer.Public())
 	if err != nil {
@@ -234,7 +348,11 @@ func (c *ConsulProvider) Sign(csr *x509.CertificateRequest) (string, error) {
 	}
 
 	// Parse the CA cert
-	caCert, err := connect.ParseCert(providerState.RootCert)
+	certPEM, err := c.ActiveIntermediate()
+	if err != nil {
+		return "", err
+	}
+	caCert, err := connect.ParseCert(certPEM)
 	if err != nil {
 		return "", fmt.Errorf("error parsing CA cert: %s", err)
 	}
@@ -290,6 +408,96 @@ func (c *ConsulProvider) Sign(csr *x509.CertificateRequest) (string, error) {
 	return buf.String(), nil
 }
 
+// SignIntermediate will validate the CSR to ensure the trust domain in the
+// URI SAN matches the local one and that basic constraints for a CA certificate
+// are met. It should return a signed CA certificate with a path length constraint
+// of 0 to ensure that the certificate cannot be used to generate further CA certs.
+func (c *ConsulProvider) SignIntermediate(csr *x509.CertificateRequest) (string, error) {
+	idx, providerState, err := c.getState()
+	if err != nil {
+		return "", err
+	}
+
+	if len(csr.URIs) < 1 {
+		return "", fmt.Errorf("intermediate CSR has no URIs")
+	}
+	certURI, err := connect.ParseCertURI(csr.URIs[0])
+	if err != nil {
+		return "", err
+	}
+
+	// Verify that the trust domain is valid
+	if !c.spiffeID.CanSign(certURI) {
+		return "", fmt.Errorf("incoming CSR domain %q is not valid for our domain %q",
+			certURI.URI().String(), c.spiffeID.URI().String())
+	}
+
+	// Get the signing private key.
+	signer, err := connect.ParseSigner(providerState.PrivateKey)
+	if err != nil {
+		return "", err
+	}
+	if signer == nil {
+		return "", ErrNotInitialized
+	}
+	subjectKeyId, err := connect.KeyId(csr.PublicKey)
+	if err != nil {
+		return "", err
+	}
+
+	// Parse the CA cert
+	caCert, err := connect.ParseCert(providerState.RootCert)
+	if err != nil {
+		return "", fmt.Errorf("error parsing CA cert: %s", err)
+	}
+
+	// Cert template for generation
+	sn := &big.Int{}
+	sn.SetUint64(idx + 1)
+	// Sign the certificate valid from 1 minute in the past, this helps it be
+	// accepted right away even when nodes are not in close time sync accross the
+	// cluster. A minute is more than enough for typical DC clock drift.
+	effectiveNow := time.Now().Add(-1 * time.Minute)
+	template := x509.Certificate{
+		SerialNumber:          sn,
+		Subject:               csr.Subject,
+		URIs:                  csr.URIs,
+		Signature:             csr.Signature,
+		SignatureAlgorithm:    csr.SignatureAlgorithm,
+		PublicKeyAlgorithm:    csr.PublicKeyAlgorithm,
+		PublicKey:             csr.PublicKey,
+		BasicConstraintsValid: true,
+		KeyUsage: x509.KeyUsageCertSign |
+			x509.KeyUsageCRLSign |
+			x509.KeyUsageDigitalSignature,
+		IsCA:           true,
+		MaxPathLenZero: true,
+		NotAfter:       effectiveNow.Add(365 * 24 * time.Hour),
+		NotBefore:      effectiveNow,
+		SubjectKeyId:   subjectKeyId,
+	}
+
+	// Create the certificate, PEM encode it and return that value.
+	var buf bytes.Buffer
+	bs, err := x509.CreateCertificate(
+		rand.Reader, &template, caCert, csr.PublicKey, signer)
+	if err != nil {
+		return "", fmt.Errorf("error generating certificate: %s", err)
+	}
+	err = pem.Encode(&buf, &pem.Block{Type: "CERTIFICATE", Bytes: bs})
+	if err != nil {
+		return "", fmt.Errorf("error encoding certificate: %s", err)
+	}
+
+	err = c.incrementProviderIndex(providerState)
+	if err != nil {
+		return "", err
+	}
+
+	// Set the response
+	return buf.String(), nil
+}
+
 // CrossSignCA returns the given CA cert signed by the current active root.
 func (c *ConsulProvider) CrossSignCA(cert *x509.Certificate) (string, error) {
 	c.Lock()
@@ -356,6 +564,22 @@ func (c *ConsulProvider) CrossSignCA(cert *x509.Certificate) (string, error) {
 	return buf.String(), nil
 }
 
+// getState returns the current provider state from the state delegate, and returns
+// ErrNotInitialized if no entry is found.
+func (c *ConsulProvider) getState() (uint64, *structs.CAConsulProviderState, error) {
+	state := c.Delegate.State()
+	idx, providerState, err := state.CAProviderState(c.id)
+	if err != nil {
+		return 0, nil, err
+	}
+
+	if providerState == nil {
+		return 0, nil, ErrNotInitialized
+	}
+
+	return idx, providerState, nil
+}
+
 // incrementProviderIndex does a write to increment the provider state store table index
 // used for serial numbers when generating certificates.
 func (c *ConsulProvider) incrementProviderIndex(providerState *structs.CAConsulProviderState) error {

agent/connect/ca/provider_consul_test.go

@@ -275,6 +275,69 @@ func testCrossSignProviders(t *testing.T, provider1, provider2 Provider) {
 	}
 }
 
+func TestConsulProvider_SignIntermediate(t *testing.T) {
+	t.Parallel()
+	require := require.New(t)
+
+	conf1 := testConsulCAConfig()
+	delegate1 := newMockDelegate(t, conf1)
+	provider1 := &ConsulProvider{Delegate: delegate1}
+	require.NoError(provider1.Configure(conf1.ClusterID, true, conf1.Config))
+	require.NoError(provider1.GenerateRoot())
+
+	conf2 := testConsulCAConfig()
+	conf2.CreateIndex = 10
+	delegate2 := newMockDelegate(t, conf2)
+	provider2 := &ConsulProvider{Delegate: delegate2}
+	require.NoError(provider2.Configure(conf2.ClusterID, false, conf2.Config))
+
+	// Get the intermediate CSR from provider2.
+	csrPEM, err := provider2.GenerateIntermediateCSR()
+	require.NoError(err)
+	csr, err := connect.ParseCSR(csrPEM)
+	require.NoError(err)
+
+	// Sign the CSR with provider1.
+	intermediatePEM, err := provider1.SignIntermediate(csr)
+	require.NoError(err)
+	rootPEM, err := provider1.ActiveRoot()
+	require.NoError(err)
+
+	// Give the new intermediate to provider2 to use.
+	require.NoError(provider2.SetIntermediate(intermediatePEM, rootPEM))
+
+	// Have provider2 sign a leaf cert and make sure the chain is correct.
+	spiffeService := &connect.SpiffeIDService{
+		Host:       "node1",
+		Namespace:  "default",
+		Datacenter: "dc1",
+		Service:    "foo",
+	}
+	raw, _ := connect.TestCSR(t, spiffeService)
+
+	leafCsr, err := connect.ParseCSR(raw)
+	require.NoError(err)
+
+	leafPEM, err := provider2.Sign(leafCsr)
+	require.NoError(err)
+
+	cert, err := connect.ParseCert(leafPEM)
+	require.NoError(err)
+
+	// Check that the leaf signed by the new cert can be verified by either root
+	// certificate by using the new intermediate + cross-signed cert.
+	intermediatePool := x509.NewCertPool()
+	intermediatePool.AppendCertsFromPEM([]byte(intermediatePEM))
+	rootPool := x509.NewCertPool()
+	rootPool.AppendCertsFromPEM([]byte(rootPEM))
+
+	_, err = cert.Verify(x509.VerifyOptions{
+		Intermediates: intermediatePool,
+		Roots:         rootPool,
+	})
+	require.NoError(err)
+}
+
 func TestConsulCAProvider_MigrateOldID(t *testing.T) {
 	t.Parallel()
 

agent/connect/ca/provider_vault.go

@@ -102,6 +102,14 @@ func (v *VaultProvider) GenerateRoot() error {
 	return nil
 }
 
+func (v *VaultProvider) GenerateIntermediateCSR() (string, error) {
+	return "", nil
+}
+
+func (v *VaultProvider) SetIntermediate(intermediatePEM, rootPEM string) error {
+	return nil
+}
+
 // ActiveIntermediate returns the current intermediate certificate.
 func (v *VaultProvider) ActiveIntermediate() (string, error) {
 	return v.getCA(v.config.IntermediatePKIPath)
@@ -249,6 +257,10 @@ func (v *VaultProvider) Sign(csr *x509.CertificateRequest) (string, error) {
 	return fmt.Sprintf("%s\n%s", cert, ca), nil
 }
 
+func (v *VaultProvider) SignIntermediate(*x509.CertificateRequest) (string, error) {
+	return "", nil
+}
+
 // CrossSignCA takes a CA certificate and cross-signs it to form a trust chain
 // back to our active root.
 func (v *VaultProvider) CrossSignCA(cert *x509.Certificate) (string, error) {

agent/connect/uri_signing.go

@@ -49,7 +49,7 @@ func (id *SpiffeIDSigning) CanSign(cu CertURI) bool {
 		// that we could open this up later for example to support external
 		// federation of roots and cross-signing external roots that have different
 		// URI structure but it's simpler to start off restrictive.
-		return id == other
+		return id.URI().String() == other.URI().String()
 	case *SpiffeIDService:
 		// The host component of the service must be an exact match for now under
 		// ascii case folding (since hostnames are case-insensitive). Later we might