net: sockets: tls: Add new option to register certificate verify cb

Add new TLS socket option, TLS_CERT_VERIFY_CALLBACK, which allows to
register an application callback to verify certificates obtained during
the TLS handshake.

Signed-off-by: Robert Lubos <[email protected]>

Author: rlubos

include/zephyr/net/socket.h

@@ -247,6 +247,19 @@ extern "C" {
  *  section of x509.h header.
  */
 #define TLS_CERT_VERIFY_RESULT 19
+/** Write-only socket option to configure a certificate verification callback for
+ *  the socket. The option accepts a pointer to a @ref tls_cert_verify_cb
+ *  structure, which contains pointers to the actual callback function and
+ *  application-defined context.
+ *
+ *  If set, the certificate verification is delegated to the registered callback.
+ *  In such case it's the application's responsibility to verify the provided
+ *  certificates and decide whether to proceed or abort the handshake.
+ *
+ *  The option is only available if CONFIG_NET_SOCKETS_TLS_CERT_VERIFY_CALLBACK
+ *  Kconfig option is enabled.
+ */
+#define TLS_CERT_VERIFY_CALLBACK 20
 
 /* Valid values for @ref TLS_PEER_VERIFY option */
 #define TLS_PEER_VERIFY_NONE 0     /**< Peer verification disabled. */
@@ -275,6 +288,19 @@ extern "C" {
 #define TLS_DTLS_CID_STATUS_DOWNLINK		1 /**< CID is in use by us */
 #define TLS_DTLS_CID_STATUS_UPLINK		2 /**< CID is in use by peer */
 #define TLS_DTLS_CID_STATUS_BIDIRECTIONAL	3 /**< CID is in use by us and peer */
+
+/** Data structure for @ref TLS_CERT_VERIFY_CALLBACK socket option. */
+struct tls_cert_verify_cb {
+	/** A pointer to the certificate verification callback function.
+	 *
+	 *  The actual callback function type is defined by mbed TLS, see
+	 *  documentation of mbedtls_x509_crt_verify() function.
+	 */
+	void *cb;
+
+	/** A pointer to an opaque context passed to the callback. */
+	void *ctx;
+};
 /** @} */ /* for @name */
 /** @} */ /* for @defgroup */
 

subsys/net/lib/sockets/Kconfig

@@ -243,6 +243,14 @@ config NET_SOCKETS_TLS_MAX_CLIENT_SESSION_COUNT
 	    This variable specifies maximum number of stored TLS/DTLS sessions,
 	    used for TLS/DTLS session resumption.
 
+config NET_SOCKETS_TLS_CERT_VERIFY_CALLBACK
+	bool "TLS certificate verification callback support"
+	depends on NET_SOCKETS_SOCKOPT_TLS
+	help
+	  This option controls whether TLS_CERT_VERIFY_CALLBACK TLS socket option
+	  is available to use. It allows to register a certificate verification
+	  callback, which is called by the TLS backend during the TLS handshake.
+
 config NET_SOCKETS_OFFLOAD
 	bool "Offload Socket APIs"
 	help

subsys/net/lib/sockets/sockets_tls.c

@@ -207,6 +207,10 @@ __net_socket struct tls_context {
 
 		bool dtls_handshake_on_connect;
 #endif /* CONFIG_NET_SOCKETS_ENABLE_DTLS */
+
+#if defined(CONFIG_NET_SOCKETS_TLS_CERT_VERIFY_CALLBACK)
+		struct tls_cert_verify_cb cert_verify;
+#endif /* CONFIG_NET_SOCKETS_TLS_CERT_VERIFY_CALLBACK */
 	} options;
 
 #if defined(CONFIG_NET_SOCKETS_ENABLE_DTLS)
@@ -1425,6 +1429,14 @@ static int tls_mbedtls_init(struct tls_context *context, bool is_server)
 	mbedtls_ssl_conf_early_data(&context->config, MBEDTLS_SSL_EARLY_DATA_ENABLED);
 #endif
 
+#if defined(CONFIG_NET_SOCKETS_TLS_CERT_VERIFY_CALLBACK)
+	if (context->options.cert_verify.cb != NULL) {
+		mbedtls_ssl_conf_verify(&context->config,
+					context->options.cert_verify.cb,
+					context->options.cert_verify.ctx);
+	}
+#endif /* CONFIG_NET_SOCKETS_TLS_CERT_VERIFY_CALLBACK */
+
 	ret = mbedtls_ssl_setup(&context->ssl,
 				&context->config);
 	if (ret != 0) {
@@ -2044,6 +2056,42 @@ static int tls_opt_dtls_role_set(struct tls_context *context,
 	return 0;
 }
 
+#if defined(CONFIG_NET_SOCKETS_TLS_CERT_VERIFY_CALLBACK)
+static int tls_opt_cert_verify_callback_set(struct tls_context *context,
+					    const void *optval,
+					    socklen_t optlen)
+{
+	struct tls_cert_verify_cb *cert_verify;
+
+	if (!optval) {
+		return -EINVAL;
+	}
+
+	if (optlen != sizeof(struct tls_cert_verify_cb)) {
+		return -EINVAL;
+	}
+
+	cert_verify = (struct tls_cert_verify_cb *)optval;
+	if (cert_verify->cb == NULL) {
+		return -EINVAL;
+	}
+
+	context->options.cert_verify = *cert_verify;
+
+	return 0;
+}
+#else /* CONFIG_NET_SOCKETS_TLS_CERT_VERIFY_CALLBACK */
+static int tls_opt_cert_verify_callback_set(struct tls_context *context,
+					    const void *optval,
+					    socklen_t optlen)
+{
+	NET_ERR("TLS_CERT_VERIFY_CALLBACK option requires "
+		"CONFIG_NET_SOCKETS_TLS_CERT_VERIFY_CALLBACK enabled");
+
+	return -ENOPROTOOPT;
+}
+#endif /* CONFIG_NET_SOCKETS_TLS_CERT_VERIFY_CALLBACK */
+
 static int protocol_check(int family, int type, int *proto)
 {
 	if (family != AF_INET && family != AF_INET6) {
@@ -3631,6 +3679,10 @@ int ztls_setsockopt_ctx(struct tls_context *ctx, int level, int optname,
 		err = tls_opt_session_cache_purge_set(ctx, optval, optlen);
 		break;
 
+	case TLS_CERT_VERIFY_CALLBACK:
+		err = tls_opt_cert_verify_callback_set(ctx, optval, optlen);
+		break;
+
 #if defined(CONFIG_NET_SOCKETS_ENABLE_DTLS)
 	case TLS_DTLS_HANDSHAKE_TIMEOUT_MIN:
 		err = tls_opt_dtls_handshake_timeout_set(ctx, optval,