feat: provide dynamic client CA matching

Use a "hack" with cloned tls.Config to allow dynamic reloading of client
CA certs.

Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>

tls/provider.go

@@ -19,9 +19,12 @@ import (
 
 // CertificateProvider describes an interface by which TLS certificates may be managed.
 type CertificateProvider interface {
-	// GetCA returns the active root CA.
+	// GetCA returns the active root CA certificate(s).
 	GetCA() ([]byte, error)
 
+	// GetCACertPool returns the active root CA certificates as a certificate pool.
+	GetCACertPool() (*x509.CertPool, error)
+
 	// GetCertificate returns the current certificate matching the given client request.
 	GetCertificate(*tls.ClientHelloInfo) (*tls.Certificate, error)
 
@@ -40,8 +43,9 @@ type certificateProvider struct {
 
 	generator Generator
 
-	ca  []byte
-	crt *tls.Certificate
+	ca         []byte
+	caCertPool *x509.CertPool
+	crt        *tls.Certificate
 
 	csrOptions []talosx509.Option
 }
@@ -98,6 +102,17 @@ func (p *certificateProvider) GetCA() ([]byte, error) {
 	return p.ca, nil
 }
 
+func (p *certificateProvider) GetCACertPool() (*x509.CertPool, error) {
+	if p == nil {
+		return nil, errors.New("no provider")
+	}
+
+	p.rw.RLock()
+	defer p.rw.RUnlock()
+
+	return p.caCertPool, nil
+}
+
 func (p *certificateProvider) GetCertificate(*tls.ClientHelloInfo) (*tls.Certificate, error) {
 	if p == nil {
 		return nil, errors.New("no provider")
@@ -119,6 +134,9 @@ func (p *certificateProvider) updateCertificates(ca []byte, cert *tls.Certificat
 
 	p.ca = ca
 	p.crt = cert
+
+	p.caCertPool = x509.NewCertPool()
+	p.caCertPool.AppendCertsFromPEM(ca)
 }
 
 func (p *certificateProvider) manageUpdates(ctx context.Context) error {

tls/tls.go

@@ -108,6 +108,29 @@ func WithCACertPEM(ca []byte) func(*tls.Config) error {
 	}
 }
 
+// WithDynamicClientCA declares a dynamic client CA which will be updated on each client request.
+//
+// This methods is for server-side TLS.
+// For client TLS, build the new `tls.Config` on each request.
+func WithDynamicClientCA(p CertificateProvider) func(*tls.Config) error {
+	return func(cfg *tls.Config) error {
+		cfg.GetConfigForClient = func(*tls.ClientHelloInfo) (*tls.Config, error) {
+			newCfg := cfg.Clone()
+
+			var err error
+
+			newCfg.ClientCAs, err = p.GetCACertPool()
+			if err != nil {
+				return nil, err
+			}
+
+			return newCfg, nil
+		}
+
+		return nil
+	}
+}
+
 func defaultConfig() *tls.Config {
 	return &tls.Config{
 		RootCAs:   x509.NewCertPool(),