libc-2.24新增了对vtable的校验

使得vtable的地址要在一个指定范围内

导致不可随意伪造vtable

源码如下

// libio/libioP.h
/* Perform vtable pointer validation.  If validation fails, terminate
   the process.  */
static inline const struct _IO_jump_t *
IO_validate_vtable (const struct _IO_jump_t *vtable)
{
  /* Fast path: The vtable pointer is within the __libc_IO_vtables
     section.  */
  uintptr_t section_length = __stop___libc_IO_vtables - __start___libc_IO_vtables;
  const char *ptr = (const char *) vtable;
  uintptr_t offset = ptr - __start___libc_IO_vtables;
  if (__glibc_unlikely (offset >= section_length))
    /* The vtable pointer is not in the expected section.  Use the
       slow path, which will terminate the process if necessary.  */
    _IO_vtable_check ();
  return vtable;
}

// libio/vtables.c
void attribute_hidden
_IO_vtable_check (void)
{
#ifdef SHARED
  /* Honor the compatibility flag.  */
  void (*flag) (void) = atomic_load_relaxed (&IO_accept_foreign_vtables);
#ifdef PTR_DEMANGLE
  PTR_DEMANGLE (flag);
#endif
  if (flag == &_IO_vtable_check)
    return;
  /* In case this libc copy is in a non-default namespace, we always
     need to accept foreign vtables because there is always a
     possibility that FILE * objects are passed across the linking
     boundary.  */
  {
    Dl_info di;
    struct link_map *l;
    if (_dl_open_hook != NULL
        || (_dl_addr (_IO_vtable_check, &di, &l, NULL) != 0
            && l->l_ns != LM_ID_BASE))
      return;
  }
#else /* !SHARED */
  /* We cannot perform vtable validation in the static dlopen case
     because FILE * handles might be passed back and forth across the
     boundary.  Therefore, we disable checking in this case.  */
  if (__dlopen != NULL)
    return;
#endif
  __libc_fatal ("Fatal error: glibc detected an invalid stdio handle\\n");
}

所有的 libio vtables 被放进了专用的只读的 __libc_IO_vtables 段，以使它们在内存中连续。在任何间接跳转之前，vtable 指针将根据段边界进行检查，如果指针不在这个段，则调用函数 _IO_vtable_check() 做进一步的检查，并且在必要时终止进程。

此时的利用思路就变成了利用vtables里面的函数

比如

其中有一个_IO_str_overflow

int
_IO_str_overflow (_IO_FILE *fp, int c)
{
  int flush_only = c == EOF;
  _IO_size_t pos;
  if (fp->_flags & _IO_NO_WRITES)
      return flush_only ? 0 : EOF;
  if ((fp->_flags & _IO_TIED_PUT_GET) && !(fp->_flags & _IO_CURRENTLY_PUTTING))
    {
      fp->_flags |= _IO_CURRENTLY_PUTTING;
      fp->_IO_write_ptr = fp->_IO_read_ptr;
      fp->_IO_read_ptr = fp->_IO_read_end;
    }
  pos = fp->_IO_write_ptr - fp->_IO_write_base;
  if (pos >= (_IO_size_t) (_IO_blen (fp) + flush_only))                       // 条件 #define _IO_blen(fp) ((fp)->_IO_buf_end - (fp)->_IO_buf_base)
    {
      if (fp->_flags & _IO_USER_BUF) /* not allowed to enlarge */
    return EOF;
      else
    {
      char *new_buf;
      char *old_buf = fp->_IO_buf_base;
      size_t old_blen = _IO_blen (fp);
      _IO_size_t new_size = 2 * old_blen + 100;                                 // 通过计算 new_size 为 "/bin/sh\\x00" 的地址
      if (new_size < old_blen)
        return EOF;
      new_buf
        = (char *) (*((_IO_strfile *) fp)->_s._allocate_buffer) (new_size);     // 在这个相对地址放上 system 的地址，即 system("/bin/sh")
    [...]

//可以看个好看点的版本
int _IO_str_overflow (FILE *fp, int c)
{
        // ......
      char *new_buf;
      char *old_buf = fp->_IO_buf_base; // 赋值为old_buf
      size_t old_blen = _IO_blen (fp);  #define _IO_blen(fp) ((fp)->_IO_buf_end - (fp)->_IO_buf_base)
      size_t new_size = 2 * old_blen + 100;
      if (new_size < old_blen)
        return EOF;
      new_buf = malloc (new_size); // 这里任意地址分配
      if (new_buf == NULL)
        {
          /*      __ferror(fp) = 1; */
          return EOF;
        }
      if (old_buf)
        {
          memcpy (new_buf, old_buf, old_blen); // 劫持_IO_buf_base后即可任意地址写任意值
          free (old_buf);
      // .......
  }

// libio/strfile.h
struct _IO_str_fields
{
  _IO_alloc_type _allocate_buffer;
  _IO_free_type _free_buffer;
};
struct _IO_streambuf
{
  struct _IO_FILE _f;
  const struct _IO_jump_t *vtable;
};
typedef struct _IO_strfile_
{
  struct _IO_streambuf _sbf;
  struct _IO_str_fields _s;
} _IO_strfile;