nfqws2: improve ctx magic protection

nfq2/desync.c

@@ -710,7 +710,7 @@ static uint8_t desync(
 	struct func_list *func;
 	int ref_arg = LUA_NOREF, status;
 	bool b, b_cutoff_all, b_unwanted_payload;
-	t_lua_desync_context ctx = { .magic = MAGIC_CTX, .dp = dp, .ctrack = ctrack, .dis = dis, .cancel = false, .incoming = bIncoming };
+	t_lua_desync_context ctx = { .magic = 0, .dp = dp, .ctrack = ctrack, .dis = dis, .cancel = false, .incoming = bIncoming };
 	const char *sDirection = bIncoming ? "in" : "out";
 	struct packet_range *range;
 	size_t l;
@@ -860,8 +860,14 @@ static uint8_t desync(
 							lua_pushf_str("func", func->func);
 							lua_pushf_int("func_n", ctx.func_n);
 							lua_pushf_str("func_instance", instance);
-							int initial_stack_top = lua_gettop(params.L);
+
+							// lua should not store and access ctx outside of this call
+							// if this happens make our best to prevent access to bad memory
+							// this is not crash-proof but better than nothing
+							ctx.magic = MAGIC_CTX; // mark struct as valid
 							status = lua_pcall(params.L, 2, LUA_MULTRET, 0);
+							ctx.magic = 0; // mark struct as invalid
+
 							if (status)
 							{
 								lua_dlog_error();