Reuse existing SAMLConnector signing key when possible (#46887)

\
Signed-off-by: Tiago Silva <tiago.silva@goteleport.com>

lib/auth/saml.go

@@ -56,6 +56,16 @@ func (a *Server) UpsertSAMLConnector(ctx context.Context, connector types.SAMLCo
 	if err := services.ValidateSAMLConnector(connector, a); err != nil {
 		return trace.Wrap(err)
 	}
+
+	// If someone is applying a SAML Connector obtained with `tctl get` without secrets, the signing key pair is
+	// not empty (cert is set) but the private key is missing. Such a SAML resource is invalid and not usable.
+	if connector.GetSigningKeyPair().PrivateKey == "" {
+		err := services.FillSAMLSigningKeyFromExisting(ctx, connector, a.Services)
+		if err != nil {
+			return trace.Wrap(err)
+		}
+	}
+
 	if err := a.Services.UpsertSAMLConnector(ctx, connector); err != nil {
 		return trace.Wrap(err)
 	}

lib/services/saml.go

@@ -39,6 +39,12 @@ import (
 	"github.com/gravitational/teleport/lib/utils"
 )
 
+type SAMLConnectorGetter interface {
+	GetSAMLConnector(ctx context.Context, id string, withSecrets bool) (types.SAMLConnector, error)
+}
+
+const ErrMsgHowToFixMissingPrivateKey = "You must either specify the singing key pair (obtain the existing one with `tctl get saml --with-secrets`) or let Teleport generate a new one (remove singing_key_pair in the resource you're trying to create)."
+
 // ValidateSAMLConnector validates the SAMLConnector and sets default values.
 // If a remote to fetch roles is specified, roles will be validated to exist.
 func ValidateSAMLConnector(sc types.SAMLConnector, rg RoleGetter) error {
@@ -339,3 +345,23 @@ func MarshalSAMLConnector(samlConnector types.SAMLConnector, opts ...MarshalOpti
 		return nil, trace.BadParameter("unrecognized SAML connector version %T", samlConnector)
 	}
 }
+
+// FillSAMLSigningKeyFromExisting looks up the existing SAML connector and populates the signing key if it's missing.
+// This must be called only if the SAML Connector signing key pair has been initialized (ValidateSAMLConnector) and
+// the private key is still empty.
+func FillSAMLSigningKeyFromExisting(ctx context.Context, connector types.SAMLConnector, sg SAMLConnectorGetter) error {
+	existing, err := sg.GetSAMLConnector(ctx, connector.GetName(), true /* with secrets */)
+	switch {
+	case trace.IsNotFound(err):
+		return trace.BadParameter("failed to create SAML connector, the SAML connector has no signing key set. " + ErrMsgHowToFixMissingPrivateKey)
+	case err != nil:
+		return trace.BadParameter("failed to update SAML connector, the SAML connector has no signing key set and looking up the existing connector failed with the error: %s. %s", err.Error(), ErrMsgHowToFixMissingPrivateKey)
+	}
+
+	existingSkp := existing.GetSigningKeyPair()
+	if existingSkp == nil || existingSkp.Cert != connector.GetSigningKeyPair().Cert {
+		return trace.BadParameter("failed to update the SAML connector, the SAML connector has no signing key and its signing certificate does not match the existing one. " + ErrMsgHowToFixMissingPrivateKey)
+	}
+	connector.SetSigningKeyPair(existingSkp)
+	return nil
+}

lib/services/saml_test.go

@@ -18,8 +18,10 @@ package services
 
 import (
 	"context"
+	"crypto/x509/pkix"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/gravitational/trace"
@@ -29,6 +31,7 @@ import (
 	"github.com/gravitational/teleport/api/types"
 	"github.com/gravitational/teleport/lib/defaults"
 	"github.com/gravitational/teleport/lib/fixtures"
+	"github.com/gravitational/teleport/lib/utils"
 )
 
 func TestParseFromMetadata(t *testing.T) {
@@ -137,3 +140,112 @@ func (rs roleSet) GetRole(ctx context.Context, name string) (types.Role, error)
 	}
 	return nil, trace.NotFound("unknown role: %s", name)
 }
+
+type mockSAMLGetter map[string]types.SAMLConnector
+
+func (m mockSAMLGetter) GetSAMLConnector(_ context.Context, id string, withSecrets bool) (types.SAMLConnector, error) {
+	connector, ok := m[id]
+	if !ok {
+		return nil, trace.NotFound("%s not found", id)
+	}
+	return connector, nil
+}
+
+func TestFillSAMLSigningKeyFromExisting(t *testing.T) {
+	t.Parallel()
+
+	// Test setup: generate the fixtures
+	existingKeyPEM, existingCertPEM, err := utils.GenerateSelfSignedSigningCert(pkix.Name{
+		Organization: []string{"Teleport OSS"},
+		CommonName:   "teleport.localhost.localdomain",
+	}, nil, 10*365*24*time.Hour)
+	require.NoError(t, err)
+
+	existingSkp := &types.AsymmetricKeyPair{
+		PrivateKey: string(existingKeyPEM),
+		Cert:       string(existingCertPEM),
+	}
+
+	existingConnectorName := "existing"
+	existingConnectors := mockSAMLGetter{
+		existingConnectorName: &types.SAMLConnectorV2{
+			Spec: types.SAMLConnectorSpecV2{
+				SigningKeyPair: existingSkp,
+			},
+		},
+	}
+
+	_, unrelatedCertPEM, err := utils.GenerateSelfSignedSigningCert(pkix.Name{
+		Organization: []string{"Teleport OSS"},
+		CommonName:   "teleport.localhost.localdomain",
+	}, nil, 10*365*24*time.Hour)
+	require.NoError(t, err)
+
+	// Test setup: define test cases
+	testCases := []struct {
+		name          string
+		connectorName string
+		connectorSpec types.SAMLConnectorSpecV2
+		assertErr     require.ErrorAssertionFunc
+		assertResult  require.ValueAssertionFunc
+	}{
+		{
+			name:          "should read singing key from existing connector with matching cert",
+			connectorName: existingConnectorName,
+			connectorSpec: types.SAMLConnectorSpecV2{
+				SigningKeyPair: &types.AsymmetricKeyPair{
+					PrivateKey: "",
+					Cert:       string(existingCertPEM),
+				},
+			},
+			assertErr: require.NoError,
+			assertResult: func(t require.TestingT, value interface{}, args ...interface{}) {
+				require.Implements(t, (*types.SAMLConnector)(nil), value)
+				connector := value.(types.SAMLConnector)
+				skp := connector.GetSigningKeyPair()
+				require.Equal(t, existingSkp, skp)
+			},
+		},
+		{
+			name:          "should error when there's no existing connector",
+			connectorName: "non-existing",
+			connectorSpec: types.SAMLConnectorSpecV2{
+				SigningKeyPair: &types.AsymmetricKeyPair{
+					PrivateKey: "",
+					Cert:       string(unrelatedCertPEM),
+				},
+			},
+			assertErr: require.Error,
+		},
+		{
+			name:          "should error when existing connector cert is not matching",
+			connectorName: existingConnectorName,
+			connectorSpec: types.SAMLConnectorSpecV2{
+				SigningKeyPair: &types.AsymmetricKeyPair{
+					PrivateKey: "",
+					Cert:       string(unrelatedCertPEM),
+				},
+			},
+			assertErr: require.Error,
+		},
+	}
+
+	// Test execution
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			ctx := context.Background()
+			connector := &types.SAMLConnectorV2{
+				Metadata: types.Metadata{
+					Name: tc.connectorName,
+				},
+				Spec: tc.connectorSpec,
+			}
+
+			err := FillSAMLSigningKeyFromExisting(ctx, connector, existingConnectors)
+			tc.assertErr(t, err)
+			if tc.assertResult != nil {
+				tc.assertResult(t, connector)
+			}
+		})
+	}
+}