Don't let HTTP API pass through untrusted function

This has been a problem since the first day, oops.

builtin/game/misc.lua

@@ -250,7 +250,7 @@ end
 
 -- HTTP callback interface
 
-function core.http_add_fetch(httpenv)
+core.set_http_api_lua(function(httpenv)
 	httpenv.fetch = function(req, callback)
 		local handle = httpenv.fetch_async(req)
 
@@ -266,7 +266,8 @@ function core.http_add_fetch(httpenv)
 	end
 
 	return httpenv
-end
+end)
+core.set_http_api_lua = nil
 
 
 function core.close_formspec(player_name, formname)

src/script/common/c_internal.h

@@ -54,6 +54,8 @@ extern "C" {
 #define CUSTOM_RIDX_GLOBALS_BACKUP      (CUSTOM_RIDX_BASE + 1)
 #define CUSTOM_RIDX_CURRENT_MOD_NAME    (CUSTOM_RIDX_BASE + 2)
 #define CUSTOM_RIDX_BACKTRACE           (CUSTOM_RIDX_BASE + 3)
+#define CUSTOM_RIDX_HTTP_API_LUA        (CUSTOM_RIDX_BASE + 4)
+
 
 // Determine if CUSTOM_RIDX_SCRIPTAPI will hold a light or full userdata
 #if defined(__aarch64__) && USE_LUAJIT

src/script/lua_api/l_http.cpp

@@ -163,6 +163,20 @@ int ModApiHttp::l_http_fetch_async_get(lua_State *L)
 	return 1;
 }
 
+int ModApiHttp::l_set_http_api_lua(lua_State *L)
+{
+	NO_MAP_LOCK_REQUIRED;
+
+	// This is called by builtin to give us a function that will later
+	// populate the http_api table with additional method(s).
+	// We need this because access to the HTTP api is security-relevant and
+	// any mod could just mess with a global variable.
+	luaL_checktype(L, 1, LUA_TFUNCTION);
+	lua_rawseti(L, LUA_REGISTRYINDEX, CUSTOM_RIDX_HTTP_API_LUA);
+
+	return 0;
+}
+
 int ModApiHttp::l_request_http_api(lua_State *L)
 {
 	NO_MAP_LOCK_REQUIRED;
@@ -205,16 +219,16 @@ int ModApiHttp::l_request_http_api(lua_State *L)
 		return 1;
 	}
 
-	lua_getglobal(L, "core");
-	lua_getfield(L, -1, "http_add_fetch");
+	lua_rawgeti(L, LUA_REGISTRYINDEX, CUSTOM_RIDX_HTTP_API_LUA);
+	assert(lua_isfunction(L, -1));
 
 	lua_newtable(L);
 	HTTP_API(fetch_async);
 	HTTP_API(fetch_async_get);
 
 	// Stack now looks like this:
-	// <core.http_add_fetch> <table with fetch_async, fetch_async_get>
-	// Now call core.http_add_fetch to append .fetch(request, callback) to table
+	// <function> <table with fetch_async, fetch_async_get>
+	// Now call it to append .fetch(request, callback) to table
 	lua_call(L, 1, 1);
 
 	return 1;
@@ -247,6 +261,7 @@ void ModApiHttp::Initialize(lua_State *L, int top)
 		API_FCT(get_http_api);
 	} else {
 		API_FCT(request_http_api);
+		API_FCT(set_http_api_lua);
 	}
 
 #endif

src/script/lua_api/l_http.h

@@ -41,6 +41,9 @@ private:
 	// http_fetch_async_get(handle)
 	static int l_http_fetch_async_get(lua_State *L);
 
+	// set_http_api_lua() [internal]
+	static int l_set_http_api_lua(lua_State *L);
+
 	// request_http_api()
 	static int l_request_http_api(lua_State *L);