connect/ca: update Vault provider to add cross-signing methods

agent/connect/ca/provider.go

@@ -32,7 +32,12 @@ type Provider interface {
 	// intemediate and any cross-signed intermediates managed by Consul.
 	Sign(*x509.CertificateRequest) (string, error)
 
-	// CrossSignCA must accept a CA certificate signed by another CA's key
+	// GetCrossSigningCSR returns a CSR that can be signed by another root
+	// to create an intermediate cert that forms a chain of trust back to
+	// that root CA.
+	GetCrossSigningCSR() (*x509.CertificateRequest, error)
+
+	// CrossSignCA must accept a CA CSR signed by another CA's key
 	// 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
 	// Alternative Name, SubjectKeyID and other relevant extensions must be kept.
@@ -40,7 +45,7 @@ type Provider interface {
 	// AuthorityKeyID set to the CAProvider's current signing key as well as the
 	// Issuer related fields changed as necessary. The resulting certificate is
 	// returned as a PEM formatted string.
-	CrossSignCA(*x509.Certificate) (string, error)
+	CrossSignCA(*x509.CertificateRequest) (string, error)
 
 	// Cleanup performs any necessary cleanup that should happen when the provider
 	// is shut down permanently, such as removing a temporary PKI backend in Vault

agent/connect/ca/provider_vault.go

@@ -181,7 +181,7 @@ func (v *VaultProvider) GenerateIntermediate() (string, error) {
 	if err != nil {
 		return "", err
 	}
-	if csr == nil || csr.Data["csr"] == nil {
+	if csr == nil || csr.Data["csr"] == "" {
 		return "", fmt.Errorf("got empty value when generating intermediate CSR")
 	}
 
@@ -193,7 +193,7 @@ func (v *VaultProvider) GenerateIntermediate() (string, error) {
 	if err != nil {
 		return "", err
 	}
-	if intermediate == nil || intermediate.Data["certificate"] == nil {
+	if intermediate == nil || intermediate.Data["certificate"] == "" {
 		return "", fmt.Errorf("got empty value when generating intermediate certificate")
 	}
 
@@ -240,14 +240,62 @@ func (v *VaultProvider) Sign(csr *x509.CertificateRequest) (string, error) {
 	return fmt.Sprintf("%s\n%s", cert, ca), nil
 }
 
-// todo(kyhavlov): decide which vault endpoint to use here
-func (v *VaultProvider) CrossSignCA(cert *x509.Certificate) (string, error) {
-	var pemBuf bytes.Buffer
-	if err := pem.Encode(&pemBuf, &pem.Block{Type: "CERTIFICATE", Bytes: cert.Raw}); err != nil {
+// GetCrossSigningCSR creates a CSR for an intermediate CA certificate to be signed
+// by another CA provider.
+func (v *VaultProvider) GetCrossSigningCSR() (*x509.CertificateRequest, error) {
+	// Generate a new intermediate CSR for the root to sign.
+	spiffeID := connect.SpiffeIDSigning{ClusterID: v.clusterId, Domain: "consul"}
+	csr, err := v.client.Logical().Write(v.config.IntermediatePKIPath+"intermediate/generate/internal", map[string]interface{}{
+		"common_name": "Vault CA Intermediate Authority",
+		"uri_sans":    spiffeID.URI().String(),
+	})
+	if err != nil {
+		return nil, err
+	}
+	if csr == nil || csr.Data["csr"] == "" {
+		return nil, fmt.Errorf("got empty value when generating intermediate CSR")
+	}
+
+	// Return the parsed CSR.
+	pem, ok := csr.Data["csr"].(string)
+	if !ok {
+		return nil, fmt.Errorf("CSR was not a string")
+	}
+	result, err := connect.ParseCSR(pem)
+	if err != nil {
+		return nil, err
+	}
+
+	return result, nil
+}
+
+// CrossSignCA creates a CSR from the given CA certificate and uses the
+// configured root PKI backend to issue a cert from the request.
+func (v *VaultProvider) CrossSignCA(cert *x509.CertificateRequest) (string, error) {
+	var csrBuf bytes.Buffer
+	err := pem.Encode(&csrBuf, &pem.Block{Type: "CERTIFICATE REQUEST", Bytes: cert.Raw})
+	if err != nil {
 		return "", err
 	}
 
-	return pemBuf.String(), nil
+	// Have the root PKI backend sign this CSR.
+	response, err := v.client.Logical().Write(v.config.RootPKIPath+"root/sign-intermediate", map[string]interface{}{
+		"csr":            csrBuf.String(),
+		"use_csr_values": true,
+	})
+	if err != nil {
+		return "", fmt.Errorf("error having Vault cross-sign cert: %v", err)
+	}
+	if response == nil || response.Data["certificate"] == "" {
+		return "", fmt.Errorf("certificate info returned from Vault was blank")
+	}
+
+	xcCert, ok := response.Data["certificate"].(string)
+	if !ok {
+		return "", fmt.Errorf("certificate was not a string")
+	}
+
+	return xcCert, nil
 }
 
 // Cleanup unmounts the configured intermediate PKI backend. It's fine to tear

agent/connect/ca/provider_vault_test.go

@@ -148,3 +148,43 @@ func TestVaultCAProvider_SignLeaf(t *testing.T) {
 		require.True(parsed.NotBefore.Before(time.Now()))
 	}
 }
+
+func TestVaultCAProvider_CrossSignCA(t *testing.T) {
+	t.Parallel()
+
+	require := require.New(t)
+	provider1, core1, listener1 := testVaultCluster(t)
+	defer core1.Shutdown()
+	defer listener1.Close()
+
+	provider2, core2, listener2 := testVaultCluster(t)
+	defer core2.Shutdown()
+	defer listener2.Close()
+
+	// Have provider2 generate a cross-signing CSR
+	csr, err := provider2.GetCrossSigningCSR()
+	require.NoError(err)
+	oldSubject := csr.Subject.CommonName
+
+	// Have the provider cross sign our new CA cert.
+	xcPEM, err := provider1.CrossSignCA(csr)
+	require.NoError(err)
+	xc, err := connect.ParseCert(xcPEM)
+	require.NoError(err)
+
+	rootPEM, err := provider1.ActiveRoot()
+	require.NoError(err)
+	root, err := connect.ParseCert(rootPEM)
+	require.NoError(err)
+
+	// AuthorityKeyID should be the signing root's, SubjectKeyId should be different.
+	require.Equal(root.AuthorityKeyId, xc.AuthorityKeyId)
+	require.NotEqual(root.SubjectKeyId, xc.SubjectKeyId)
+
+	// Subject name should not have changed.
+	require.NotEqual(root.Subject.CommonName, xc.Subject.CommonName)
+	require.Equal(oldSubject, xc.Subject.CommonName)
+
+	// Issuer should be the signing root.
+	require.Equal(root.Issuer.CommonName, xc.Issuer.CommonName)
+}