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IO_FILE_leak

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811 3 分钟

IO_FILE_leak是一种利用IO_FILE来达到leak效果的手法

利用方式：

针对_IO_2_1_stdout_结构体

_flags改为0xfbad1800

_IO_read_ptr = 0

_IO_read_end = 0

_IO_read_base = 0

_IO_write_base末字节改为0x58

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payload = p64(0xfbad1800)+p64(0)*3+b"\x58"

当程序调用_IO_2_1_stdout的时候就会输出_IO_write_base到_IO_write_ptr之间的内容

另外一种利用手法

如用下面payload，则会泄露_IO_2_1_stdin_(对于puts函数→不同函数输出不同)

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payload = p64(0xfbad3887)+p64(0)*3+p8(0)

原理

对于输出函数来说，比如puts

其调用流为

_IO_puts→_IO_sputn→_IO_XSPUTN(vtable)→_IO_OVERFLOW(当缓冲区还有东西的时候)→_IO_do_write→_IO_new_do_write

而当_IO_write_end > _IO_write_ptr时，会调用memcpy拷贝数据到缓冲区，之后判断是否有剩余，也就是拷贝了就会调用_IO_overflow

缓冲区机制：输出数据会先放到缓冲区，然后缓冲区再发送给用户，比如往缓冲区推了0x50个数据，但是只write了0x40个数据，这个时候缓冲区就会有剩余，在下一次输出的时候，将会输出完剩余的内容，而不是这次要输出的内容。

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int _IO_new_file_overflow (_IO_FILE *f, int ch)
{
  if (f->_flags & _IO_NO_WRITES) /* SET ERROR */
    {
      f->_flags |= _IO_ERR_SEEN;
      __set_errno (EBADF);
      return EOF;
    }
  /* If currently reading or no buffer allocated. */
  if ((f->_flags & _IO_CURRENTLY_PUTTING) == 0 || f->_IO_write_base == NULL)
    {
      /* Allocate a buffer if needed. */
      if (f->_IO_write_base == NULL)
    {
      _IO_doallocbuf (f);
      _IO_setg (f, f->_IO_buf_base, f->_IO_buf_base, f->_IO_buf_base);
    }
      /* Otherwise must be currently reading.
     If _IO_read_ptr (and hence also _IO_read_end) is at the buffer end,
     logically slide the buffer forwards one block (by setting the
     read pointers to all point at the beginning of the block).  This
     makes room for subsequent output.
     Otherwise, set the read pointers to _IO_read_end (leaving that
     alone, so it can continue to correspond to the external position). */
      if (__glibc_unlikely (_IO_in_backup (f)))
    {
      size_t nbackup = f->_IO_read_end - f->_IO_read_ptr;
      _IO_free_backup_area (f);
      f->_IO_read_base -= MIN (nbackup,
                   f->_IO_read_base - f->_IO_buf_base);
      f->_IO_read_ptr = f->_IO_read_base;
    }

      if (f->_IO_read_ptr == f->_IO_buf_end)
    f->_IO_read_end = f->_IO_read_ptr = f->_IO_buf_base;
      f->_IO_write_ptr = f->_IO_read_ptr;
      f->_IO_write_base = f->_IO_write_ptr;
      f->_IO_write_end = f->_IO_buf_end;
      f->_IO_read_base = f->_IO_read_ptr = f->_IO_read_end;

      f->_flags |= _IO_CURRENTLY_PUTTING;
      if (f->_mode <= 0 && f->_flags & (_IO_LINE_BUF | _IO_UNBUFFERED))
    f->_IO_write_end = f->_IO_write_ptr;
    }
  if (ch == EOF)
    return _IO_do_write (f, f->_IO_write_base,
             f->_IO_write_ptr - f->_IO_write_base);
  if (f->_IO_write_ptr == f->_IO_buf_end ) /* Buffer is really full */
............

我们的目标是调用到_IO_do_write将数据打印出来

需要绕过的检查：

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f->_flags & _IO_NO_WRITES == False
((f->_flags & _IO_CURRENTLY_PUTTING) == 0 || f->_IO_write_base == NULL) == False

满足上述条件 _flags & 8 == 0 , _flags & 0x800 == 1，且 _flags 魔数的常量为 0xfbad0000。 那么此时 _flags == 0xfbad0800

一些宏定义：

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#define _IO_MAGIC 0xFBAD0000 /* Magic number */
#define _OLD_STDIO_MAGIC 0xFABC0000 /* Emulate old stdio. */
#define _IO_MAGIC_MASK 0xFFFF0000
#define _IO_USER_BUF 1 /* User owns buffer; don't delete it on close. */
#define _IO_UNBUFFERED 2
#define _IO_NO_READS 4 /* Reading not allowed */
#define _IO_NO_WRITES 8 /* Writing not allowd */
#define _IO_EOF_SEEN 0x10
#define _IO_ERR_SEEN 0x20
#define _IO_DELETE_DONT_CLOSE 0x40 /* Don't call close(_fileno) on cleanup. */
#define _IO_LINKED 0x80 /* Set if linked (using _chain) to streambuf::_list_all.*/
#define _IO_IN_BACKUP 0x100
#define _IO_LINE_BUF 0x200
#define _IO_TIED_PUT_GET 0x400 /* Set if put and get pointer logicly tied. */
#define _IO_CURRENTLY_PUTTING 0x800
#define _IO_IS_APPENDING 0x1000
#define _IO_IS_FILEBUF 0x2000
#define _IO_BAD_SEEN 0x4000
#define _IO_USER_LOCK 0x8000

接着进入_IO_new_do_write

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static
_IO_size_t
new_do_write (_IO_FILE *fp, const char *data, _IO_size_t to_do)
{
  _IO_size_t count;
  if (fp->_flags & _IO_IS_APPENDING)
    /* On a system without a proper O_APPEND implementation,
       you would need to sys_seek(0, SEEK_END) here, but is
       not needed nor desirable for Unix- or Posix-like systems.
       Instead, just indicate that offset (before and after) is
       unpredictable. */
    fp->_offset = _IO_pos_BAD;
  else if (fp->_IO_read_end != fp->_IO_write_base)
    {
      _IO_off64_t new_pos
    = _IO_SYSSEEK (fp, fp->_IO_write_base - fp->_IO_read_end, 1);
      if (new_pos == _IO_pos_BAD)
    return 0;
      fp->_offset = new_pos;
    }
  count = _IO_SYSWRITE (fp, data, to_do);
  if (fp->_cur_column && count)
    fp->_cur_column = _IO_adjust_column (fp->_cur_column - 1, data, count) + 1;
  _IO_setg (fp, fp->_IO_buf_base, fp->_IO_buf_base, fp->_IO_buf_base);
  fp->_IO_write_base = fp->_IO_write_ptr = fp->_IO_buf_base;
  fp->_IO_write_end = (fp->_mode <= 0
               && (fp->_flags & (_IO_LINE_BUF | _IO_UNBUFFERED))
               ? fp->_IO_buf_base : fp->_IO_buf_end);
  return count;
}

目标是调用到_IO_SYSWRITE

校验：

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fp->_flags & _IO_IS_APPENDING == FALSE
fp->_IO_read_end != fp->_IO_write_base == FALSE

所以_flags位设为0xfbad1800即可