ubus: add CORS header support

In order to support cross-domain AJAX requests to the /ubus endpoint
we need to implement the Cross-Origin Resource Sharing (CORS) spec
in the ubus plugin.

- Implement a new option "-X" to enable CORS support in ubus
- Implement rudimentary support for "OPTIONS" HTTP requests
- Implement essential CORS headers the ubus plugin

The current CORS response headers merely reflect the request headers
sent by the client, this way any requesting origin is automatically
allowed. Cross-domain cookies (Access-Control-Allow-Credentials) are
unconditionally enabled.

Restricting permitted origins and toggle the credential accepting can
be made configurable in a future commit to allow more fine grained
control over permitted AJAX clients.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

5 files changed, 78 insertions, 10 deletions

diff --git a/client.c b/client.c
index 3a6b09e..9b8fb07 100644
--- a/client.c
+++ b/client.c
@@ -39,6 +39,7 @@ const char * const http_methods[] = {
 	[UH_HTTP_MSG_GET] = "GET",
 	[UH_HTTP_MSG_POST] = "POST",
 	[UH_HTTP_MSG_HEAD] = "HEAD",
+	[UH_HTTP_MSG_OPTIONS] = "OPTIONS",
 };
 
 void uh_http_header(struct client *cl, int code, const char *summary)
diff --git a/main.c b/main.c
index e89a99a..44c3226 100644
--- a/main.c
+++ b/main.c
@@ -151,6 +151,7 @@ static int usage(const char *name)
 		"	-u string       URL prefix for UBUS via JSON-RPC handler\n"
 		"	-U file         Override ubus socket path\n"
 		"	-a              Do not authenticate JSON-RPC requests against UBUS session api\n"
+		"	-X		Enable CORS HTTP headers on JSON-RPC api\n"
 #endif
 		"	-x string       URL prefix for CGI handler, default is '/cgi-bin'\n"
 		"	-i .ext=path    Use interpreter at path for files with the given extension\n"
@@ -226,7 +227,7 @@ int main(int argc, char **argv)
 	init_defaults_pre();
 	signal(SIGPIPE, SIG_IGN);
 
-	while ((ch = getopt(argc, argv, "afSDRC:K:E:I:p:s:h:c:l:L:d:r:m:n:N:x:i:t:k:T:A:u:U:")) != -1) {
+	while ((ch = getopt(argc, argv, "afSDRXC:K:E:I:p:s:h:c:l:L:d:r:m:n:N:x:i:t:k:T:A:u:U:")) != -1) {
 		switch(ch) {
 #ifdef HAVE_TLS
 		case 'C':
@@ -402,10 +403,15 @@ int main(int argc, char **argv)
 		case 'U':
 			conf.ubus_socket = optarg;
 			break;
+
+		case 'X':
+			conf.ubus_cors = 1;
+			break;
 #else
 		case 'a':
 		case 'u':
 		case 'U':
+		case 'X':
 			fprintf(stderr, "uhttpd: UBUS support not compiled, "
 			                "ignoring -%c\n", opt);
 			break;
diff --git a/ubus.c b/ubus.c
index 39dca35..269f7cd 100644
--- a/ubus.c
+++ b/ubus.c
@@ -104,6 +104,13 @@ static const struct {
 	[ERROR_TIMEOUT] = { -32003, "ubus request timed out" },
 };
 
+enum cors_hdr {
+	HDR_ORIGIN,
+	HDR_ACCESS_CONTROL_REQUEST_METHOD,
+	HDR_ACCESS_CONTROL_REQUEST_HEADERS,
+	__HDR_MAX
+};
+
 static void __uh_ubus_next_batched_request(struct uloop_timeout *timeout);
 
 static void uh_ubus_next_batched_request(struct client *cl)
@@ -114,10 +121,52 @@ static void uh_ubus_next_batched_request(struct client *cl)
 	uloop_timeout_set(&du->timeout, 1);
 }
 
+static void uh_ubus_add_cors_headers(struct client *cl)
+{
+	struct blob_attr *tb[__HDR_MAX];
+	static const struct blobmsg_policy hdr_policy[__HDR_MAX] = {
+		[HDR_ORIGIN] = { "origin", BLOBMSG_TYPE_STRING },
+		[HDR_ACCESS_CONTROL_REQUEST_METHOD] = { "access-control-request-method", BLOBMSG_TYPE_STRING },
+		[HDR_ACCESS_CONTROL_REQUEST_HEADERS] = { "access-control-request-headers", BLOBMSG_TYPE_STRING },
+	};
+
+	blobmsg_parse(hdr_policy, __HDR_MAX, tb, blob_data(cl->hdr.head), blob_len(cl->hdr.head));
+
+	if (!tb[HDR_ORIGIN])
+		return;
+
+	if (tb[HDR_ACCESS_CONTROL_REQUEST_METHOD])
+	{
+		char *hdr = (char *) blobmsg_data(tb[HDR_ACCESS_CONTROL_REQUEST_METHOD]);
+
+		if (strcmp(hdr, "POST") && strcmp(hdr, "OPTIONS"))
+			return;
+	}
+
+	ustream_printf(cl->us, "Access-Control-Allow-Origin: %s\r\n",
+	               blobmsg_data(tb[HDR_ORIGIN]));
+
+	if (tb[HDR_ACCESS_CONTROL_REQUEST_HEADERS])
+		ustream_printf(cl->us, "Access-Control-Allow-Headers: %s\r\n",
+		               blobmsg_data(tb[HDR_ACCESS_CONTROL_REQUEST_HEADERS]));
+
+	ustream_printf(cl->us, "Access-Control-Allow-Methods: POST, OPTIONS\r\n");
+	ustream_printf(cl->us, "Access-Control-Allow-Credentials: true\r\n");
+}
+
 static void uh_ubus_send_header(struct client *cl)
 {
 	ops->http_header(cl, 200, "OK");
-	ustream_printf(cl->us, "Content-Type: application/json\r\n\r\n");
+
+	if (conf.ubus_cors)
+		uh_ubus_add_cors_headers(cl);
+
+	ustream_printf(cl->us, "Content-Type: application/json\r\n");
+
+	if (cl->request.method == UH_HTTP_MSG_OPTIONS)
+		ustream_printf(cl->us, "Content-Length: 0\r\n");
+
+	ustream_printf(cl->us, "\r\n");
 }
 
 static void uh_ubus_send_response(struct client *cl)
@@ -571,14 +620,24 @@ static void uh_ubus_handle_request(struct client *cl, char *url, struct path_inf
 
 	blob_buf_init(&buf, 0);
 
-	if (cl->request.method != UH_HTTP_MSG_POST)
-		return ops->client_error(cl, 400, "Bad Request", "Invalid Request");
+	switch (cl->request.method)
+	{
+	case UH_HTTP_MSG_POST:
+		d->data_send = uh_ubus_data_send;
+		d->data_done = uh_ubus_data_done;
+		d->close_fds = uh_ubus_close_fds;
+		d->free = uh_ubus_request_free;
+		d->ubus.jstok = json_tokener_new();
+		break;
+
+	case UH_HTTP_MSG_OPTIONS:
+		uh_ubus_send_header(cl);
+		ops->request_done(cl);
+		break;
 
-	d->close_fds = uh_ubus_close_fds;
-	d->free = uh_ubus_request_free;
-	d->data_send = uh_ubus_data_send;
-	d->data_done = uh_ubus_data_done;
-	d->ubus.jstok = json_tokener_new();
+	default:
+		ops->client_error(cl, 400, "Bad Request", "Invalid Request");
+	}
 }
 
 static bool
diff --git a/uhttpd.h b/uhttpd.h
index daf68ce..6ef28d9 100644
--- a/uhttpd.h
+++ b/uhttpd.h
@@ -68,6 +68,7 @@ struct config {
 	int http_keepalive;
 	int script_timeout;
 	int ubus_noauth;
+	int ubus_cors;
 };
 
 struct auth_realm {
@@ -81,6 +82,7 @@ enum http_method {
 	UH_HTTP_MSG_GET,
 	UH_HTTP_MSG_POST,
 	UH_HTTP_MSG_HEAD,
+	UH_HTTP_MSG_OPTIONS,
 };
 
 enum http_version {
diff --git a/utils.c b/utils.c
index b1d7d37..1c21fc5 100644
--- a/utils.c
+++ b/utils.c
@@ -25,7 +25,7 @@ bool uh_use_chunked(struct client *cl)
 	if (cl->request.version != UH_HTTP_VER_1_1)
 		return false;
 
-	if (cl->request.method == UH_HTTP_MSG_HEAD)
+	if (cl->request.method == UH_HTTP_MSG_HEAD || cl->request.method == UH_HTTP_MSG_OPTIONS)
 		return false;
 
 	return true;