Validate portions of mpy load to detect corruption

Fixes adafruit#1917

Author: tannewt

py/persistentcode.c

@@ -128,6 +128,12 @@ STATIC qstr load_qstr(mp_reader_t *reader) {
  size_t len = read_uint(reader);
  char str[len];
  read_bytes(reader, (byte*)str, len);
+ // Validate the QSTRs by ensuring they do not contain any null terminations. They are length encoded instead.
+ for (size_t i = 0; i < len; i++) {
+ if (str[i] == '\0') {
+ mp_raise_RuntimeError(translate("Corrupt .mpy file"));
+ }
+ }
  qstr qst = qstr_from_strn(str, len);
  return qst;
 }
@@ -145,11 +151,12 @@ STATIC mp_obj_t load_obj(mp_reader_t *reader) {
  return mp_obj_new_str_from_vstr(obj_type == 's' ? &mp_type_str : &mp_type_bytes, &vstr);
  } else if (obj_type == 'i') {
  return mp_parse_num_integer(vstr.buf, vstr.len, 10, NULL);
- } else {
- assert(obj_type == 'f' || obj_type == 'c');
+ } else if (obj_type == 'f' || obj_type == 'c') {
  return mp_parse_num_decimal(vstr.buf, vstr.len, obj_type == 'c', false, NULL);
  }
  }
+ mp_raise_RuntimeError(translate("Corrupt .mpy file"));
+ return MP_OBJ_FROM_PTR(&mp_const_none_obj);
 }
 
 STATIC void load_bytecode_qstrs(mp_reader_t *reader, byte *ip, byte *ip_top) {