The OS X regex engine function tre_tnfa_run_parallel contains the following code:

  int tbytes;
...
  if (!match_tags)
    num_tags = 0;
  else
    num_tags = tnfa->num_tags;

...
  {
    int rbytes, pbytes, total_bytes;
    char *tmp_buf;
    /* Compute the length of the block we need. */
    tbytes = sizeof(*tmp_tags) * num_tags;
    rbytes = sizeof(*reach_next) * (tnfa->num_states + 1);
    pbytes = sizeof(*reach_pos) * tnfa->num_states;
    total_bytes =
      (sizeof(long) - 1) * 4 /* for alignment paddings */
      + (rbytes + tbytes * tnfa->num_states) * 2 + tbytes + pbytes;

    DPRINT(("tre_tnfa_run_parallel, allocate %d bytes\n", total_bytes));
    /* Allocate the memory. */
#ifdef TRE_USE_ALLOCA
    buf = alloca(total_bytes);
#else /* !TRE_USE_ALLOCA */
    buf = xmalloc((unsigned)total_bytes);  <-- malloc is called, not alloca
#endif /* !TRE_USE_ALLOCA */
    if (buf == NULL)
      return REG_ESPACE;
    memset(buf, 0, (size_t)total_bytes);


num_states and num_tags are computed based on the requirements of the regex and it's quite easy to make them each >64k with a relatively small regex. Note that total_bytes is an int and part of its calculation is the product of num_states and num_tags.

The types here are all over the place and there's conversion between int, unsigned's and size_t.

The attached PoC causes total_bytes to become negative leading to total_bytes being sign-extended in the memset call.

Severity medium because I haven't looked for exposed attack surface yet, but this doesn't require any non-standard flags (only REG_EXTENDED which is almost always used.)
 

tre_signedness_bad_bzero.c 1.1 KB Download