Potential heap corruption via `write_escaped_data` in control.c

Here follows another bug report by guido from hackerone.

We think this is safe to list on trac, because the write_escaped_data function is only called from the control API, and we believe it's not possible to force the control API to write enough amounts of data to trigger this bug (since descriptors, dir documents, and cells are all capped in size, and this bug requires message lengths close to INT_MAX).

Bug report follows:

---

/** Given a <b>len</b>-character string in <b>data</b>, made of lines
 * terminated by CRLF, allocate a new string in *<b>out</b>, and copy the
 * contents of <b>data</b> into *<b>out</b>, adding a period before any period
 * that appears at the start of a line, and adding a period-CRLF line at
 * the end. Replace all LF characters sequences with CRLF.  Return the number
 * of bytes in *<b>out</b>.
 */
STATIC size_t
write_escaped_data(const char *data, size_t len, char **out)
{
  size_t sz_out = len+8;
  char *outp;
  const char *start = data, *end;
  int i;
  int start_of_line;
  for (i=0; i<(int)len; ++i) {
    if (data[i]== '\n')
      sz_out += 2; /* Maybe add a CR; maybe add a dot. */
  }
  *out = outp = tor_malloc(sz_out+1);
  end = data+len;
  start_of_line = 1;
  while (data < end) {
    if (*data == '\n') {
      if (data > start && data[-1] != '\r')
        *outp++ = '\r';
      start_of_line = 1;
    } else if (*data == '.') {
      if (start_of_line) {
        start_of_line = 0;
        *outp++ = '.';
      }
    } else {
      start_of_line = 0;
    }
    *outp++ = *data++;
  }
  if (outp < *out+2 || fast_memcmp(outp-2, "\r\n", 2)) {
    *outp++ = '\r';
    *outp++ = '\n';
  }
  *outp++ = '.';
  *outp++ = '\r';
  *outp++ = '\n';
  *outp = '\0'; /* NUL-terminate just in case. */
  tor_assert((outp - *out) <= (int)sz_out);
  return outp - *out;
}

There are two potential vulnerabilities lurking here:

1. If the input size (len) >= 0x80000000, then this loop will not execute at all:

  for (i=0; i<(int)len; ++i) {
    if (data[i]== '\n')
      sz_out += 2; /* Maybe add a CR; maybe add a dot. */
  }

Because the condition i<(int)len is effectively i<(negative number) and i is intialized to 0, this can never be true. As a result of this, the output buffer (whose size is based on sz_out) is too small to hold the result for an input buffer containing '\n' characters. Triggering this is typically only feasible on a 64-bit system, because if the input buffer is >= 0x80000000 bytes, then sz_out is set to 0x80000008 bytes, and allocating such an amount twice (one for the input buffer, and one for the output buffer) is not possible on a 32-bit system.

2. If the equation (number of '\n' characters in input buffer * 2 + size of input buffer) exceeds 0xFFFFFFFF, then this will cause heap corruption on a 32-bit system, because sz_out overflows.