Isolate library tables between sandbox and insecure env

src/script/cpp_api/s_security.cpp

@@ -45,6 +45,21 @@ static inline void copy_safe(lua_State *L, const char *list[], unsigned len, int
 	}
 }
 
+static void shallow_copy_table(lua_State *L, int from=-2, int to=-1)
+{
+	if (from < 0) from = lua_gettop(L) + from + 1;
+	if (to   < 0) to   = lua_gettop(L) + to   + 1;
+	lua_pushnil(L);
+	while (lua_next(L, from) != 0) {
+		assert(lua_type(L, -1) != LUA_TTABLE);
+		// duplicate key and value for lua_rawset
+		lua_pushvalue(L, -2);
+		lua_pushvalue(L, -2);
+		lua_rawset(L, to);
+		lua_pop(L, 1);
+	}
+}
+
 // Pushes the original version of a library function on the stack, from the old version
 static inline void push_original(lua_State *L, const char *lib, const char *func)
 {
@@ -83,7 +98,10 @@ void ScriptApiSecurity::initializeSecurity()
 		"unpack",
 		"_VERSION",
 		"xpcall",
-		// Completely safe libraries
+	};
+	static const char *whitelist_tables[] = {
+		// These libraries are completely safe BUT we need to duplicate their table
+		// to ensure the sandbox can't affect the insecure env
 		"coroutine",
 		"string",
 		"table",
@@ -167,6 +185,17 @@ void ScriptApiSecurity::initializeSecurity()
 	lua_pop(L, 1);
 
 
+	// Copy safe libraries
+	for (const char *libname : whitelist_tables) {
+		lua_getfield(L, old_globals, libname);
+		lua_newtable(L);
+		shallow_copy_table(L);
+
+		lua_setglobal(L, libname);
+		lua_pop(L, 1);
+	}
+
+
 	// Copy safe IO functions
 	lua_getfield(L, old_globals, "io");
 	lua_newtable(L);
@@ -222,6 +251,19 @@ void ScriptApiSecurity::initializeSecurity()
 #endif
 
 	lua_pop(L, 1); // Pop globals_backup
+
+
+	/*
+	 * In addition to copying the tables in whitelist_tables, we also need to
+	 * replace the string metatable. Otherwise old_globals.string would
+	 * be accessible via getmetatable("").__index from inside the sandbox.
+	 */
+	lua_pushliteral(L, "");
+	lua_newtable(L);
+	lua_getglobal(L, "string");
+	lua_setfield(L, -2, "__index");
+	lua_setmetatable(L, -2);
+	lua_pop(L, 1); // Pop empty string
 }
 
 void ScriptApiSecurity::initializeSecurityClient()