caddytls: Make peer certificate verification pluggable (#4389)

* caddytls: Adding ClientCertValidator for custom client cert validations

* caddytls: Cleanups for ClientCertValidator changes

caddytls: Cleanups for ClientCertValidator changes

* Update modules/caddytls/connpolicy.go

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

* Update modules/caddytls/connpolicy.go

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

* Update modules/caddytls/connpolicy.go

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

* Update modules/caddytls/connpolicy.go

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

* Update modules/caddytls/connpolicy.go

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

* Update modules/caddytls/connpolicy.go

Co-authored-by: Matt Holt <mholt@users.noreply.github.com>

* Unexported field Validators, corrected renaming of LeafVerificationValidator to LeafCertClientAuth

* admin: Write proper status on invalid requests (#4569) (fix #4561)

* Apply suggestions from code review

* Register module; fix compilation

* Add log for deprecation notice

Co-authored-by: Roettges Florian <roettges.florian@scheidt-bachmann.de>
Co-authored-by: Francis Lavoie <lavofr@gmail.com>
Co-authored-by: Matt Holt <mholt@users.noreply.github.com>
Co-authored-by: Alok Naushad <alokme123@gmail.com>

modules/caddytls/connpolicy.go

@@ -18,6 +18,7 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/base64"
+	"encoding/json"
 	"fmt"
 	"os"
 	"strings"
@@ -26,6 +27,10 @@ import (
 	"github.com/mholt/acmez"
 )
 
+func init() {
+	caddy.RegisterModule(LeafCertClientAuth{})
+}
+
 // ConnectionPolicies govern the establishment of TLS connections. It is
 // an ordered group of connection policies; the first matching policy will
 // be used to configure TLS connections at handshake-time.
@@ -55,6 +60,16 @@ func (cp ConnectionPolicies) Provision(ctx caddy.Context) error {
 		if err != nil {
 			return fmt.Errorf("connection policy %d: building standard TLS config: %s", i, err)
 		}
+
+		if pol.ClientAuthentication != nil && len(pol.ClientAuthentication.VerifiersRaw) > 0 {
+			clientCertValidations, err := ctx.LoadModule(pol.ClientAuthentication, "VerifiersRaw")
+			if err != nil {
+				return fmt.Errorf("loading client cert verifiers: %v", err)
+			}
+			for _, validator := range clientCertValidations.([]interface{}) {
+				cp[i].ClientAuthentication.verifiers = append(cp[i].ClientAuthentication.verifiers, validator.(ClientCertificateVerifier))
+			}
+		}
 	}
 
 	return nil
@@ -62,7 +77,7 @@ func (cp ConnectionPolicies) Provision(ctx caddy.Context) error {
 
 // TLSConfig returns a standard-lib-compatible TLS configuration which
 // selects the first matching policy based on the ClientHello.
-func (cp ConnectionPolicies) TLSConfig(ctx caddy.Context) *tls.Config {
+func (cp ConnectionPolicies) TLSConfig(_ caddy.Context) *tls.Config {
 	// using ServerName to match policies is extremely common, especially in configs
 	// with lots and lots of different policies; we can fast-track those by indexing
 	// them by SNI, so we don't have to iterate potentially thousands of policies
@@ -293,11 +308,22 @@ type ClientAuthentication struct {
 	// these CA certificates will be rejected.
 	TrustedCACertPEMFiles []string `json:"trusted_ca_certs_pem_files,omitempty"`
 
+	// DEPRECATED: This field is deprecated and will be removed in
+	// a future version. Please use the `validators` field instead
+	// with the tls.client_auth.leaf module instead.
+	//
 	// A list of base64 DER-encoded client leaf certs
 	// to accept. If this list is not empty, client certs
 	// which are not in this list will be rejected.
 	TrustedLeafCerts []string `json:"trusted_leaf_certs,omitempty"`
 
+	// Client certificate verification modules. These can perform
+	// custom client authentication checks, such as ensuring the
+	// certificate is not revoked.
+	VerifiersRaw []json.RawMessage `json:"verifiers,omitempty" caddy:"namespace=tls.client_auth inline_key=verifier"`
+
+	verifiers []ClientCertificateVerifier
+
 	// The mode for authenticating the client. Allowed values are:
 	//
 	// Mode | Description
@@ -312,8 +338,6 @@ type ClientAuthentication struct {
 	// are provided; otherwise, the default mode is `require`.
 	Mode string `json:"mode,omitempty"`
 
-	// state established with the last call to ConfigureTLSConfig
-	trustedLeafCerts       []*x509.Certificate
 	existingVerifyPeerCert func([][]byte, [][]*x509.Certificate) error
 }
 
@@ -321,7 +345,8 @@ type ClientAuthentication struct {
 func (clientauth ClientAuthentication) Active() bool {
 	return len(clientauth.TrustedCACerts) > 0 ||
 		len(clientauth.TrustedCACertPEMFiles) > 0 ||
-		len(clientauth.TrustedLeafCerts) > 0 ||
+		len(clientauth.TrustedLeafCerts) > 0 || // TODO: DEPRECATED
+		len(clientauth.VerifiersRaw) > 0 ||
 		len(clientauth.Mode) > 0
 }
 
@@ -378,28 +403,31 @@ func (clientauth *ClientAuthentication) ConfigureTLSConfig(cfg *tls.Config) erro
 		cfg.ClientCAs = caPool
 	}
 
-	// enforce leaf verification by writing our own verify function
+	// TODO: DEPRECATED: Only here for backwards compatibility.
+	// If leaf cert is specified, enforce by adding a client auth module
 	if len(clientauth.TrustedLeafCerts) > 0 {
-		clientauth.trustedLeafCerts = []*x509.Certificate{}
+		caddy.Log().Named("tls.connection_policy").Warn("trusted_leaf_certs is deprecated; use leaf verifier module instead")
+		var trustedLeafCerts []*x509.Certificate
 		for _, clientCertString := range clientauth.TrustedLeafCerts {
 			clientCert, err := decodeBase64DERCert(clientCertString)
 			if err != nil {
 				return fmt.Errorf("parsing certificate: %v", err)
 			}
-			clientauth.trustedLeafCerts = append(clientauth.trustedLeafCerts, clientCert)
+			trustedLeafCerts = append(trustedLeafCerts, clientCert)
 		}
+		clientauth.verifiers = append(clientauth.verifiers, LeafCertClientAuth{TrustedLeafCerts: trustedLeafCerts})
+	}
+
 	// if a custom verification function already exists, wrap it
 	clientauth.existingVerifyPeerCert = cfg.VerifyPeerCertificate
 	cfg.VerifyPeerCertificate = clientauth.verifyPeerCertificate
-	}
-
 	return nil
 }
 
 // verifyPeerCertificate is for use as a tls.Config.VerifyPeerCertificate
 // callback to do custom client certificate verification. It is intended
 // for installation only by clientauth.ConfigureTLSConfig().
-func (clientauth ClientAuthentication) verifyPeerCertificate(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error {
+func (clientauth *ClientAuthentication) verifyPeerCertificate(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error {
 	// first use any pre-existing custom verification function
 	if clientauth.existingVerifyPeerCert != nil {
 		err := clientauth.existingVerifyPeerCert(rawCerts, verifiedChains)
@@ -407,23 +435,13 @@ func (clientauth ClientAuthentication) verifyPeerCertificate(rawCerts [][]byte,
 			return err
 		}
 	}
-
-	if len(rawCerts) == 0 {
-		return fmt.Errorf("no client certificate provided")
-	}
-
-	remoteLeafCert, err := x509.ParseCertificate(rawCerts[0])
+	for _, verifier := range clientauth.verifiers {
+		err := verifier.VerifyClientCertificate(rawCerts, verifiedChains)
 		if err != nil {
-		return fmt.Errorf("can't parse the given certificate: %s", err.Error())
+			return err
+		}
 	}
-
-	for _, trustedLeafCert := range clientauth.trustedLeafCerts {
-		if remoteLeafCert.Equal(trustedLeafCert) {
 	return nil
-		}
-	}
-
-	return fmt.Errorf("client leaf certificate failed validation")
 }
 
 // decodeBase64DERCert base64-decodes, then DER-decodes, certStr.
@@ -461,6 +479,38 @@ func setDefaultTLSParams(cfg *tls.Config) {
 	cfg.PreferServerCipherSuites = true
 }
 
+// LeafCertClientAuth verifies the client's leaf certificate.
+type LeafCertClientAuth struct {
+	TrustedLeafCerts []*x509.Certificate
+}
+
+// CaddyModule returns the Caddy module information.
+func (LeafCertClientAuth) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "tls.client_auth.leaf",
+		New: func() caddy.Module { return new(LeafCertClientAuth) },
+	}
+}
+
+func (l LeafCertClientAuth) VerifyClientCertificate(rawCerts [][]byte, _ [][]*x509.Certificate) error {
+	if len(rawCerts) == 0 {
+		return fmt.Errorf("no client certificate provided")
+	}
+
+	remoteLeafCert, err := x509.ParseCertificate(rawCerts[0])
+	if err != nil {
+		return fmt.Errorf("can't parse the given certificate: %s", err.Error())
+	}
+
+	for _, trustedLeafCert := range l.TrustedLeafCerts {
+		if remoteLeafCert.Equal(trustedLeafCert) {
+			return nil
+		}
+	}
+
+	return fmt.Errorf("client leaf certificate failed validation")
+}
+
 // PublicKeyAlgorithm is a JSON-unmarshalable wrapper type.
 type PublicKeyAlgorithm x509.PublicKeyAlgorithm
 
@@ -481,4 +531,10 @@ type ConnectionMatcher interface {
 	Match(*tls.ClientHelloInfo) bool
 }
 
+// ClientCertificateVerifier is a type which verifies client certificates.
+// It is called during verifyPeerCertificate in the TLS handshake.
+type ClientCertificateVerifier interface {
+	VerifyClientCertificate(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error
+}
+
 var defaultALPN = []string{"h2", "http/1.1"}