esp-tls: Add support for the CERTIFICATE SELECTION HOOK. The hook has access to required information so that the application can make a more informed decision on which certificate to serve (such as alpn value, server certificate type, etc.)

Closes https://github.com/espressif/esp-idf/pull/9833

Signed-off-by: Aditya Patwardhan <aditya.patwardhan@espressif.com>

components/esp-tls/Kconfig

@@ -57,6 +57,14 @@ menu "ESP-TLS"
         help
             Sets the session ticket timeout used in the tls server.
 
+    config ESP_TLS_SERVER_CERT_SELECT_HOOK
+        bool "Certificate selection hook"
+        depends on ESP_TLS_USING_MBEDTLS
+        help
+            Ability to configure and use a certificate selection callback during server handshake,
+            to select a certificate to present to the client based on the TLS extensions supplied in
+            the client hello (alpn, sni, etc).
+
     config ESP_TLS_SERVER_MIN_AUTH_MODE_OPTIONAL
         bool "ESP-TLS Server: Set minimum Certificate Verification mode to Optional"
         depends on ESP_TLS_SERVER && ESP_TLS_USING_MBEDTLS

components/esp-tls/esp_tls.h

@@ -197,6 +197,20 @@ typedef struct esp_tls_server_session_ticket_ctx {
 } esp_tls_server_session_ticket_ctx_t;
 #endif
 
+
+#if defined(CONFIG_ESP_TLS_SERVER_CERT_SELECT_HOOK)
+/**
+ * @brief tls handshake callback
+ * Can be used to configure per-handshake attributes for the TLS connection.
+ * E.g. Client certificate / Key, Authmode, Client CA verification, etc.
+ *
+ * @param ssl mbedtls_ssl_context that can be used for changing settings
+ * @return The reutn value of the callback must be 0 if successful,
+ *         or a specific MBEDTLS_ERR_XXX code, which will cause the handhsake to abort
+ */
+typedef mbedtls_ssl_hs_cb_t esp_tls_handshake_callback;
+#endif
+
 typedef struct esp_tls_cfg_server {
     const char **alpn_protos;                   /*!< Application protocols required for HTTP2.
                                                      If HTTP2/ALPN support is required, a list
@@ -259,6 +273,14 @@ typedef struct esp_tls_cfg_server {
                                                     Call esp_tls_cfg_server_session_tickets_free
                                                     to free the data associated with this context. */
 #endif
+
+#if defined(CONFIG_ESP_TLS_SERVER_CERT_SELECT_HOOK)
+    esp_tls_handshake_callback cert_select_cb;  /*!< Certificate selection callback that gets called after ClientHello is processed.
+                                                     Can be used as an SNI callback, but also has access to other
+                                                     TLS extensions, such as ALPN and server_certificate_type . */
+#endif
+
+    void *userdata; /*!< User data to be add to the ssl context. Can be retrieved by callbacks */
 } esp_tls_cfg_server_t;
 
 /**

components/esp-tls/esp_tls_mbedtls.c

@@ -512,12 +512,21 @@ esp_err_t set_server_config(esp_tls_cfg_server_t *cfg, esp_tls_t *tls)
         return ESP_ERR_MBEDTLS_SSL_CONFIG_DEFAULTS_FAILED;
     }
 
+    mbedtls_ssl_conf_set_user_data_p(&tls->conf, cfg->userdata);
+
 #ifdef CONFIG_MBEDTLS_SSL_ALPN
     if (cfg->alpn_protos) {
         mbedtls_ssl_conf_alpn_protocols(&tls->conf, cfg->alpn_protos);
     }
 #endif
 
+#if defined(CONFIG_ESP_TLS_SERVER_CERT_SELECT_HOOK)
+    if (cfg->cert_select_cb != NULL) {
+        ESP_LOGI(TAG, "Initializing server side certificate selection callback");
+        mbedtls_ssl_conf_cert_cb(&tls->conf, cfg->cert_select_cb);
+    }
+#endif
+
     if (cfg->cacert_buf != NULL) {
         esp_ret = set_ca_cert(tls, cfg->cacert_buf, cfg->cacert_bytes);
         if (esp_ret != ESP_OK) {
@@ -569,7 +578,16 @@ esp_err_t set_server_config(esp_tls_cfg_server_t *cfg, esp_tls_t *tls)
             return esp_ret;
         }
     } else {
+#if defined(CONFIG_ESP_TLS_SERVER_CERT_SELECT_HOOK)
+        if (cfg->cert_select_cb == NULL) {
+            ESP_LOGE(TAG, "Missing server certificate and/or key and no certificate selection callback is defined");
+        } else {
+            ESP_LOGD(TAG, "Missing server certificate and/or key, but certificate selection callback is defined. Callback MUST ALWAYS call mbedtls_ssl_set_hs_own_cert, or the handshake will abort!");
+            return ESP_OK;
+        }
+#else
         ESP_LOGE(TAG, "Missing server certificate and/or key");
+#endif
         return ESP_ERR_INVALID_STATE;
     }
 
@@ -790,6 +808,7 @@ int esp_mbedtls_server_session_create(esp_tls_cfg_server_t *cfg, int sockfd, esp
         tls->conn_state = ESP_TLS_FAIL;
         return -1;
     }
+
     tls->read = esp_mbedtls_read;
     tls->write = esp_mbedtls_write;
     int ret;

components/esp_https_server/include/esp_https_server.h

@@ -96,6 +96,11 @@ struct httpd_ssl_config {
 
     /** User callback for esp_https_server */
     esp_https_server_user_cb *user_cb;
+
+#if defined(CONFIG_ESP_TLS_SERVER_CERT_SELECT_HOOK)
+    esp_tls_handshake_callback cert_select_cb; /*!< Certificate selection callback to use */
+#endif
+    void *ssl_userdata; /*!< user data to add to the ssl context  */
 };
 
 typedef struct httpd_ssl_config httpd_ssl_config_t;

components/esp_https_server/src/https_server.c

@@ -200,65 +200,101 @@ static httpd_ssl_ctx_t *create_secure_context(const struct httpd_ssl_config *con
     }
     esp_tls_cfg_server_t *cfg = (esp_tls_cfg_server_t *)calloc(1, sizeof(esp_tls_cfg_server_t));
     if (!cfg) {
-        free(ssl_ctx);
+        goto free_ssl_ctx;
-        return NULL;
     }
 
     if (config->session_tickets) {
         if ( esp_tls_cfg_server_session_tickets_init(cfg) != ESP_OK ) {
             ESP_LOGE(TAG, "Failed to init session ticket support");
-            free(ssl_ctx);
+            goto free_cfg;
-            free(cfg);
-            return NULL;
         }
     }
 
+    cfg->userdata = config->ssl_userdata;
+
+#if defined(CONFIG_ESP_TLS_SERVER_CERT_SELECT_HOOK)
+    cfg->cert_select_cb = config->cert_select_cb;
+#endif
+
     ssl_ctx->tls_cfg = cfg;
     ssl_ctx->user_cb = config->user_cb;
 
 /* cacert = CA which signs client cert, or client cert itself */
-    if(config->cacert_pem != NULL) {
+    if (config->cacert_pem != NULL && config->cacert_len > 0) {
         cfg->cacert_buf = (unsigned char *)malloc(config->cacert_len);
-        if (!cfg->cacert_buf) {
+
-            ESP_LOGE(TAG, "Could not allocate memory");
+        if (cfg->cacert_buf) {
-            free(cfg);
+            memcpy((char *) cfg->cacert_buf, config->cacert_pem, config->cacert_len);
-            free(ssl_ctx);
+            cfg->cacert_bytes = config->cacert_len;
-            return NULL;
+        } else {
+            ESP_LOGE(TAG, "Could not allocate memory for client certificate authority");
+            goto free_cfg;
         }
-        memcpy((char *)cfg->cacert_buf, config->cacert_pem, config->cacert_len);
-        cfg->cacert_bytes = config->cacert_len;
     }
 
 /* servercert = cert of server itself */
-    cfg->servercert_buf = (unsigned char *)malloc(config->servercert_len);
+    if (config->servercert != NULL && config->servercert_len > 0) {
-    if (!cfg->servercert_buf) {
+        cfg->servercert_buf = (unsigned char *)malloc(config->servercert_len);
-        ESP_LOGE(TAG, "Could not allocate memory");
+
-        free((void *)cfg->cacert_buf);
+        if (cfg->servercert_buf) {
-        free(cfg);
+            memcpy((char *) cfg->servercert_buf, config->servercert, config->servercert_len);
-        free(ssl_ctx);
+            cfg->servercert_bytes = config->servercert_len;
-        return NULL;
+        } else {
+            ESP_LOGE(TAG, "Could not allocate memory for server certificate");
+            goto free_cacert;
+        }
+    } else {
+#if defined(CONFIG_ESP_TLS_SERVER_CERT_SELECT_HOOK)
+        if (config->cert_select_cb == NULL) {
+#endif
+        ESP_LOGE(TAG, "No Server certificate supplied");
+        goto free_cacert;
+#if defined(CONFIG_ESP_TLS_SERVER_CERT_SELECT_HOOK)
+        } else {
+            ESP_LOGW(TAG, "Server certificate not supplied, make sure to supply it in the certificate selection hook!");
+        }
+#endif
     }
-    memcpy((char *)cfg->servercert_buf, config->servercert, config->servercert_len);
-    cfg->servercert_bytes = config->servercert_len;
 
     /* Pass on secure element boolean */
     cfg->use_secure_element = config->use_secure_element;
     if (!cfg->use_secure_element) {
-        cfg->serverkey_buf = (unsigned char *)malloc(config->prvtkey_len);
+        if (config->prvtkey_pem != NULL && config->prvtkey_len > 0) {
-        if (!cfg->serverkey_buf) {
+            cfg->serverkey_buf = (unsigned char *) malloc(config->prvtkey_len);
-            ESP_LOGE(TAG, "Could not allocate memory");
+
-            free((void *)cfg->servercert_buf);
+            if (cfg->serverkey_buf) {
-            free((void *)cfg->cacert_buf);
+                memcpy((char *) cfg->serverkey_buf, config->prvtkey_pem, config->prvtkey_len);
-            free(cfg);
+                cfg->serverkey_bytes = config->prvtkey_len;
-            free(ssl_ctx);
+            } else {
-            return NULL;
+                ESP_LOGE(TAG, "Could not allocate memory for server key");
+                goto free_servercert;
+            }
+        } else {
+#if defined(CONFIG_ESP_TLS_SERVER_CERT_SELECT_HOOK)
+            if (config->cert_select_cb == NULL) {
+                ESP_LOGE(TAG, "No Server key supplied and no certificate selection hook is present");
+                goto free_servercert;
+            } else {
+                ESP_LOGW(TAG, "Server key not supplied, make sure to supply it in the certificate selection hook");
+            }
+#else
+            ESP_LOGE(TAG, "No Server key supplied");
+            goto free_servercert;
+#endif
         }
     }
 
-    memcpy((char *)cfg->serverkey_buf, config->prvtkey_pem, config->prvtkey_len);
-    cfg->serverkey_bytes = config->prvtkey_len;
-
     return ssl_ctx;
+
+free_servercert:
+    free((void *) cfg->servercert_buf);
+free_cacert:
+    free((void *) cfg->cacert_buf);
+free_cfg:
+    free(cfg);
+free_ssl_ctx:
+    free(ssl_ctx);
+    return NULL;
 }
 
 /** Start the server */