# 4.13 利用 _IO\_FILE 结构

- [FILE 结构](#file-结构)
- [FSOP](#fsop)
- [libc-2.24 防御机制](#libc-224-防御机制)
- [libc-2.24 利用技术](#libc-224-利用技术)
- [最新动态](#最新动态)
- [CTF 实例](#ctf-实例)
- [参考资料](#参考资料)


## FILE 结构
FILE 结构体的利用是一种通用的控制流劫持技术。攻击者可以覆盖堆上的 FILE 指针使其指向一个伪造的结构，利用结构中一个叫做 `vtable` 的指针，来执行任意代码。

我们知道 FILE 结构被一系列流操作函数（`fopen()`、`fread()`、`fclose()`等）所使用，大多数的 FILE 结构体保存在堆上（stdin、stdout、stderr除外，位于libc数据段），其指针动态创建并由 `fopen()` 返回。在 glibc（2.23） 中，这个结构体是 `_IO_FILE_plout`，包含了一个 `_IO_FILE` 结构体和一个指向 `_IO_jump_t` 结构体的指针：
```c
// libio/libioP.h

struct _IO_jump_t
{
    JUMP_FIELD(size_t, __dummy);
    JUMP_FIELD(size_t, __dummy2);
    JUMP_FIELD(_IO_finish_t, __finish);
    JUMP_FIELD(_IO_overflow_t, __overflow);
    JUMP_FIELD(_IO_underflow_t, __underflow);
    JUMP_FIELD(_IO_underflow_t, __uflow);
    JUMP_FIELD(_IO_pbackfail_t, __pbackfail);
    /* showmany */
    JUMP_FIELD(_IO_xsputn_t, __xsputn);
    JUMP_FIELD(_IO_xsgetn_t, __xsgetn);
    JUMP_FIELD(_IO_seekoff_t, __seekoff);
    JUMP_FIELD(_IO_seekpos_t, __seekpos);
    JUMP_FIELD(_IO_setbuf_t, __setbuf);
    JUMP_FIELD(_IO_sync_t, __sync);
    JUMP_FIELD(_IO_doallocate_t, __doallocate);
    JUMP_FIELD(_IO_read_t, __read);
    JUMP_FIELD(_IO_write_t, __write);
    JUMP_FIELD(_IO_seek_t, __seek);
    JUMP_FIELD(_IO_close_t, __close);
    JUMP_FIELD(_IO_stat_t, __stat);
    JUMP_FIELD(_IO_showmanyc_t, __showmanyc);
    JUMP_FIELD(_IO_imbue_t, __imbue);
#if 0
    get_column;
    set_column;
#endif
};

/* We always allocate an extra word following an _IO_FILE.
   This contains a pointer to the function jump table used.
   This is for compatibility with C++ streambuf; the word can
   be used to smash to a pointer to a virtual function table. */

struct _IO_FILE_plus
{
  _IO_FILE file;
  const struct _IO_jump_t *vtable;
};

extern struct _IO_FILE_plus *_IO_list_all;
```
`vtable` 指向的函数跳转表其实是一种兼容 C++ 虚函数的实现。当程序对某个流进行操作时，会调用该流对应的跳转表中的某个函数。
```c
// libio/libio.h

struct _IO_FILE {
  int _flags;		/* High-order word is _IO_MAGIC; rest is flags. */
#define _IO_file_flags _flags

  /* The following pointers correspond to the C++ streambuf protocol. */
  /* Note:  Tk uses the _IO_read_ptr and _IO_read_end fields directly. */
  char* _IO_read_ptr;	/* Current read pointer */
  char* _IO_read_end;	/* End of get area. */
  char* _IO_read_base;	/* Start of putback+get area. */
  char* _IO_write_base;	/* Start of put area. */
  char* _IO_write_ptr;	/* Current put pointer. */
  char* _IO_write_end;	/* End of put area. */
  char* _IO_buf_base;	/* Start of reserve area. */
  char* _IO_buf_end;	/* End of reserve area. */
  /* The following fields are used to support backing up and undo. */
  char *_IO_save_base; /* Pointer to start of non-current get area. */
  char *_IO_backup_base;  /* Pointer to first valid character of backup area */
  char *_IO_save_end; /* Pointer to end of non-current get area. */

  struct _IO_marker *_markers;

  struct _IO_FILE *_chain;

  int _fileno;
#if 0
  int _blksize;
#else
  int _flags2;
#endif
  _IO_off_t _old_offset; /* This used to be _offset but it's too small.  */

#define __HAVE_COLUMN /* temporary */
  /* 1+column number of pbase(); 0 is unknown. */
  unsigned short _cur_column;
  signed char _vtable_offset;
  char _shortbuf[1];

  /*  char* _save_gptr;  char* _save_egptr; */

  _IO_lock_t *_lock;
#ifdef _IO_USE_OLD_IO_FILE
};

struct _IO_FILE_complete
{
  struct _IO_FILE _file;
#endif
#if defined _G_IO_IO_FILE_VERSION && _G_IO_IO_FILE_VERSION == 0x20001
  _IO_off64_t _offset;
# if defined _LIBC || defined _GLIBCPP_USE_WCHAR_T
  /* Wide character stream stuff.  */
  struct _IO_codecvt *_codecvt;
  struct _IO_wide_data *_wide_data;
  struct _IO_FILE *_freeres_list;
  void *_freeres_buf;
# else
  void *__pad1;
  void *__pad2;
  void *__pad3;
  void *__pad4;
# endif
  size_t __pad5;
  int _mode;
  /* Make sure we don't get into trouble again.  */
  char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
#endif
};

extern struct _IO_FILE_plus _IO_2_1_stdin_;
extern struct _IO_FILE_plus _IO_2_1_stdout_;
extern struct _IO_FILE_plus _IO_2_1_stderr_;
```
进程中的 FILE 结构会通过 `_chain` 域构成一个链表，链表头部用全局变量 `_IO_list_all` 表示。

另外 `_IO_wide_data` 结构也是后面需要的：
```c
/* Extra data for wide character streams.  */
struct _IO_wide_data
{
  wchar_t *_IO_read_ptr;	/* Current read pointer */
  wchar_t *_IO_read_end;	/* End of get area. */
  wchar_t *_IO_read_base;	/* Start of putback+get area. */
  wchar_t *_IO_write_base;	/* Start of put area. */
  wchar_t *_IO_write_ptr;	/* Current put pointer. */
  wchar_t *_IO_write_end;	/* End of put area. */
  wchar_t *_IO_buf_base;	/* Start of reserve area. */
  wchar_t *_IO_buf_end;		/* End of reserve area. */
  /* The following fields are used to support backing up and undo. */
  wchar_t *_IO_save_base;	/* Pointer to start of non-current get area. */
  wchar_t *_IO_backup_base;	/* Pointer to first valid character of
				   backup area */
  wchar_t *_IO_save_end;	/* Pointer to end of non-current get area. */

  __mbstate_t _IO_state;
  __mbstate_t _IO_last_state;
  struct _IO_codecvt _codecvt;

  wchar_t _shortbuf[1];

  const struct _IO_jump_t *_wide_vtable;
};
```

下面我们来看几个函数的实现。
#### fopen
```c
// libio/iofopen.c

_IO_FILE *
__fopen_internal (const char *filename, const char *mode, int is32)
{
  struct locked_FILE
  {
    struct _IO_FILE_plus fp;
#ifdef _IO_MTSAFE_IO
    _IO_lock_t lock;
#endif
    struct _IO_wide_data wd;
  } *new_f = (struct locked_FILE *) malloc (sizeof (struct locked_FILE));   // 为 FILE 结构分配空间

  if (new_f == NULL)
    return NULL;
#ifdef _IO_MTSAFE_IO
  new_f->fp.file._lock = &new_f->lock;
#endif
#if defined _LIBC || defined _GLIBCPP_USE_WCHAR_T
  _IO_no_init (&new_f->fp.file, 0, 0, &new_f->wd, &_IO_wfile_jumps);
#else
  _IO_no_init (&new_f->fp.file, 1, 0, NULL, NULL);
#endif
  _IO_JUMPS (&new_f->fp) = &_IO_file_jumps;                                 // 设置 vtable = &_IO_file_jumps
  _IO_file_init (&new_f->fp);                                               // 调用 _IO_file_init 函数进行初始化
#if  !_IO_UNIFIED_JUMPTABLES
  new_f->fp.vtable = NULL;
#endif
  if (_IO_file_fopen ((_IO_FILE *) new_f, filename, mode, is32) != NULL)    // 打开目标文件
    return __fopen_maybe_mmap (&new_f->fp.file);

  _IO_un_link (&new_f->fp);
  free (new_f);
  return NULL;
}

_IO_FILE *
_IO_new_fopen (const char *filename, const char *mode)
{
  return __fopen_internal (filename, mode, 1);
}
```
```c
// libio/fileops.c

# define _IO_new_file_init _IO_file_init

void
_IO_new_file_init (struct _IO_FILE_plus *fp)
{
  /* POSIX.1 allows another file handle to be used to change the position
     of our file descriptor.  Hence we actually don't know the actual
     position before we do the first fseek (and until a following fflush). */
  fp->file._offset = _IO_pos_BAD;
  fp->file._IO_file_flags |= CLOSED_FILEBUF_FLAGS;

  _IO_link_in (fp);         // 调用 _IO_link_in 函数将 fp 放进链表
  fp->file._fileno = -1;
}
```
```c
// libio/genops.c

void
_IO_link_in (struct _IO_FILE_plus *fp)
{
  if ((fp->file._flags & _IO_LINKED) == 0)
    {
      fp->file._flags |= _IO_LINKED;
#ifdef _IO_MTSAFE_IO
      _IO_cleanup_region_start_noarg (flush_cleanup);
      _IO_lock_lock (list_all_lock);
      run_fp = (_IO_FILE *) fp;
      _IO_flockfile ((_IO_FILE *) fp);
#endif
      fp->file._chain = (_IO_FILE *) _IO_list_all;  // fp 放到链表头部
      _IO_list_all = fp;                            // 链表头 _IO_list_all 指向 fp
      ++_IO_list_all_stamp;
#ifdef _IO_MTSAFE_IO
      _IO_funlockfile ((_IO_FILE *) fp);
      run_fp = NULL;
      _IO_lock_unlock (list_all_lock);
      _IO_cleanup_region_end (0);
#endif
    }
}
```

#### fread
```c
// libio/iofread.c

_IO_size_t
_IO_fread (void *buf, _IO_size_t size, _IO_size_t count, _IO_FILE *fp)
{
  _IO_size_t bytes_requested = size * count;
  _IO_size_t bytes_read;
  CHECK_FILE (fp, 0);
  if (bytes_requested == 0)
    return 0;
  _IO_acquire_lock (fp);
  bytes_read = _IO_sgetn (fp, (char *) buf, bytes_requested);   // 调用 _IO_sgetn 函数
  _IO_release_lock (fp);
  return bytes_requested == bytes_read ? count : bytes_read / size;
}
```
```c
// libio/genops.c

_IO_size_t
_IO_sgetn (_IO_FILE *fp, void *data, _IO_size_t n)
{
  /* FIXME handle putback buffer here! */
  return _IO_XSGETN (fp, data, n);          // 调用宏 _IO_XSGETN
}
```
```c
// libio/libioP.h

#define _IO_JUMPS_FILE_plus(THIS) \
  _IO_CAST_FIELD_ACCESS ((THIS), struct _IO_FILE_plus, vtable)

#if _IO_JUMPS_OFFSET
# define _IO_JUMPS_FUNC(THIS) \
 (*(struct _IO_jump_t **) ((void *) &_IO_JUMPS_FILE_plus (THIS) \
			   + (THIS)->_vtable_offset))
# define _IO_vtable_offset(THIS) (THIS)->_vtable_offset
#else
# define _IO_JUMPS_FUNC(THIS) _IO_JUMPS_FILE_plus (THIS)
# define _IO_vtable_offset(THIS) 0
#endif

#define JUMP2(FUNC, THIS, X1, X2) (_IO_JUMPS_FUNC(THIS)->FUNC) (THIS, X1, X2)

#define _IO_XSGETN(FP, DATA, N) JUMP2 (__xsgetn, FP, DATA, N)
```
所以 `_IO_XSGETN` 宏最终会调用 `vtable` 中的函数，即：
```c
// libio/fileops.c

_IO_size_t
_IO_file_xsgetn (_IO_FILE *fp, void *data, _IO_size_t n)
{
```

#### fwrite
```c
// libio/iofwrite.c

_IO_size_t
_IO_fwrite (const void *buf, _IO_size_t size, _IO_size_t count, _IO_FILE *fp)
{
  _IO_size_t request = size * count;
  _IO_size_t written = 0;
  CHECK_FILE (fp, 0);
  if (request == 0)
    return 0;
  _IO_acquire_lock (fp);
  if (_IO_vtable_offset (fp) != 0 || _IO_fwide (fp, -1) == -1)
    written = _IO_sputn (fp, (const char *) buf, request);      // 调用 _IO_sputn 函数
  _IO_release_lock (fp);
  /* We have written all of the input in case the return value indicates
     this or EOF is returned.  The latter is a special case where we
     simply did not manage to flush the buffer.  But the data is in the
     buffer and therefore written as far as fwrite is concerned.  */
  if (written == request || written == EOF)
    return count;
  else
    return written / size;
}
```
```c
// libio/libioP.h

#define _IO_XSPUTN(FP, DATA, N) JUMP2 (__xsputn, FP, DATA, N)

#define _IO_sputn(__fp, __s, __n) _IO_XSPUTN (__fp, __s, __n)
```
`_IO_XSPUTN` 最终将调用下面的函数：
```c
// libio/fileops.c

_IO_size_t
_IO_new_file_xsputn (_IO_FILE *f, const void *data, _IO_size_t n)
{
```

#### fclose
```c
// libio/iofclose.c

int
_IO_new_fclose (_IO_FILE *fp)
{
  int status;

  CHECK_FILE(fp, EOF);

#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
  /* We desperately try to help programs which are using streams in a
     strange way and mix old and new functions.  Detect old streams
     here.  */
  if (_IO_vtable_offset (fp) != 0)
    return _IO_old_fclose (fp);
#endif

  /* First unlink the stream.  */
  if (fp->_IO_file_flags & _IO_IS_FILEBUF)
    _IO_un_link ((struct _IO_FILE_plus *) fp);  // 将 fp 从链表中取出

  _IO_acquire_lock (fp);
  if (fp->_IO_file_flags & _IO_IS_FILEBUF)
    status = _IO_file_close_it (fp);            // 关闭目标文件
  else
    status = fp->_flags & _IO_ERR_SEEN ? -1 : 0;
  _IO_release_lock (fp);
  _IO_FINISH (fp);
  if (fp->_mode > 0)
    {
#if _LIBC
      /* This stream has a wide orientation.  This means we have to free
	 the conversion functions.  */
      struct _IO_codecvt *cc = fp->_codecvt;

      __libc_lock_lock (__gconv_lock);
      __gconv_release_step (cc->__cd_in.__cd.__steps);
      __gconv_release_step (cc->__cd_out.__cd.__steps);
      __libc_lock_unlock (__gconv_lock);
#endif
    }
  else
    {
      if (_IO_have_backup (fp))
	_IO_free_backup_area (fp);
    }
  if (fp != _IO_stdin && fp != _IO_stdout && fp != _IO_stderr)
    {
      fp->_IO_file_flags = 0;
      free(fp);                                 // 释放 FILE 结构体
    }

  return status;
}
```


## FSOP
FSOP（File Stream Oriented Programming）是一种劫持 `_IO_list_all`（libc.so中的全局变量） 来伪造链表的利用技术，通过调用 `_IO_flush_all_lockp()` 函数来触发，该函数会在下面三种情况下被调用：
- libc 检测到内存错误时
- 执行 exit 函数时
- main 函数返回时

当 glibc 检测到内存错误时，会依次调用这样的函数路径：`malloc_printerr -> __libc_message -> __GI_abort -> _IO_flush_all_lockp -> _IO_OVERFLOW`。
```c
// libio/genops.c

int
_IO_flush_all_lockp (int do_lock)
{
  int result = 0;
  struct _IO_FILE *fp;
  int last_stamp;

#ifdef _IO_MTSAFE_IO
  __libc_cleanup_region_start (do_lock, flush_cleanup, NULL);
  if (do_lock)
    _IO_lock_lock (list_all_lock);
#endif

  last_stamp = _IO_list_all_stamp;
  fp = (_IO_FILE *) _IO_list_all;   // 将其覆盖为伪造的链表
  while (fp != NULL)
    {
      run_fp = fp;
      if (do_lock)
	_IO_flockfile (fp);

      if (((fp->_mode <= 0 && fp->_IO_write_ptr > fp->_IO_write_base)   // 条件
#if defined _LIBC || defined _GLIBCPP_USE_WCHAR_T
	   || (_IO_vtable_offset (fp) == 0
	       && fp->_mode > 0 && (fp->_wide_data->_IO_write_ptr
				    > fp->_wide_data->_IO_write_base))
#endif
	   )
	  && _IO_OVERFLOW (fp, EOF) == EOF)     // fp 指向伪造的 vtable
	result = EOF;

      if (do_lock)
	_IO_funlockfile (fp);
      run_fp = NULL;

      if (last_stamp != _IO_list_all_stamp)
	{
	  /* Something was added to the list.  Start all over again.  */
	  fp = (_IO_FILE *) _IO_list_all;
	  last_stamp = _IO_list_all_stamp;
	}
      else
	fp = fp->_chain;    // 指向下一个 IO_FILE 对象
    }

#ifdef _IO_MTSAFE_IO
  if (do_lock)
    _IO_lock_unlock (list_all_lock);
  __libc_cleanup_region_end (0);
#endif

  return result;
}
```
```c
// libio/libioP.h

#define _IO_OVERFLOW(FP, CH) JUMP1 (__overflow, FP, CH)
#define _IO_WOVERFLOW(FP, CH) WJUMP1 (__overflow, FP, CH)
```
于是在 `_IO_OVERFLOW(fp, EOF)` 的执行过程中最终会调用 `system('/bin/sh')`。

还有一条 FSOP 的路径是在关闭 stream 的时候：
```c
// libio/iofclose.c

int
_IO_new_fclose (_IO_FILE *fp)
{
  int status;

  CHECK_FILE(fp, EOF);

#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_1)
  /* We desperately try to help programs which are using streams in a
     strange way and mix old and new functions.  Detect old streams
     here.  */
  if (_IO_vtable_offset (fp) != 0)
    return _IO_old_fclose (fp);
#endif

  /* First unlink the stream.  */
  if (fp->_IO_file_flags & _IO_IS_FILEBUF)
    _IO_un_link ((struct _IO_FILE_plus *) fp);

  _IO_acquire_lock (fp);
  if (fp->_IO_file_flags & _IO_IS_FILEBUF)
    status = _IO_file_close_it (fp);
  else
    status = fp->_flags & _IO_ERR_SEEN ? -1 : 0;
  _IO_release_lock (fp);
  _IO_FINISH (fp);                      // fp 指向伪造的 vtable
  if (fp->_mode > 0)
    {
#if _LIBC
      /* This stream has a wide orientation.  This means we have to free
	 the conversion functions.  */
      struct _IO_codecvt *cc = fp->_codecvt;

      __libc_lock_lock (__gconv_lock);
      __gconv_release_step (cc->__cd_in.__cd.__steps);
      __gconv_release_step (cc->__cd_out.__cd.__steps);
      __libc_lock_unlock (__gconv_lock);
#endif
    }
  else
    {
      if (_IO_have_backup (fp))
	_IO_free_backup_area (fp);
    }
  if (fp != _IO_stdin && fp != _IO_stdout && fp != _IO_stderr)
    {
      fp->_IO_file_flags = 0;
      free(fp);
    }

  return status;
}
```
```c
// libio/libioP.h

#define _IO_FINISH(FP) JUMP1 (__finish, FP, 0)
#define _IO_WFINISH(FP) WJUMP1 (__finish, FP, 0)
```
于是在 `_IO_FINISH (fp)` 的执行过程中最终会调用 `system('/bin/sh')`。


## libc-2.24 防御机制
但是在 libc-2.24 中加入了对 vtable 指针的检查。这个 [commit](https://sourceware.org/git/gitweb.cgi?p=glibc.git;a=commitdiff;h=db3476aff19b75c4fdefbe65fcd5f0a90588ba51) 新增了两个函数：`IO_validate_vtable` 和 `_IO_vtable_check`。
```c
// 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;
}
```
```c
// 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。


## libc-2.24 利用技术
#### _IO_str_jumps
在防御机制下通过修改虚表的利用技术已经用不了了，但同时出现了新的利用技术。既然无法将 vtable 指针指向 `__libc_IO_vtables` 以外的地方，那么就在 `__libc_IO_vtables` 里面找些有用的东西。比如 `_IO_str_jumps`（该符号在strip后会丢失）：
```c
// libio/strops.c

const struct _IO_jump_t _IO_str_jumps libio_vtable =
{
  JUMP_INIT_DUMMY,
  JUMP_INIT(finish, _IO_str_finish),
  JUMP_INIT(overflow, _IO_str_overflow),
  JUMP_INIT(underflow, _IO_str_underflow),
  JUMP_INIT(uflow, _IO_default_uflow),
  JUMP_INIT(pbackfail, _IO_str_pbackfail),
  JUMP_INIT(xsputn, _IO_default_xsputn),
  JUMP_INIT(xsgetn, _IO_default_xsgetn),
  JUMP_INIT(seekoff, _IO_str_seekoff),
  JUMP_INIT(seekpos, _IO_default_seekpos),
  JUMP_INIT(setbuf, _IO_default_setbuf),
  JUMP_INIT(sync, _IO_default_sync),
  JUMP_INIT(doallocate, _IO_default_doallocate),
  JUMP_INIT(read, _IO_default_read),
  JUMP_INIT(write, _IO_default_write),
  JUMP_INIT(seek, _IO_default_seek),
  JUMP_INIT(close, _IO_default_close),
  JUMP_INIT(stat, _IO_default_stat),
  JUMP_INIT(showmanyc, _IO_default_showmanyc),
  JUMP_INIT(imbue, _IO_default_imbue)
};
```
```c
// libio/libioP.h

#define JUMP_INIT_DUMMY JUMP_INIT(dummy, 0), JUMP_INIT (dummy2, 0)
```
这个 vtable 中包含了一个叫做 `_IO_str_overflow` 的函数，该函数中存在相对地址的引用（可伪造）：
```c
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")
    [...]
```
```c
// 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;
```
所以可以像下面这样构造：
- fp->_flags = 0
- fp->_IO_buf_base = 0
- fp->_IO_buf_end = (bin_sh_addr - 100) / 2
- fp->_IO_write_ptr = 0xffffffff
- fp->_IO_write_base = 0
- fp->_mode = 0

有一点要注意的是，如果 bin_sh_addr 的地址以奇数结尾，为了避免除法向下取整的干扰，可以将该地址加 1。另外 system("/bin/sh") 是可以用 one_gadget 来代替的，这样似乎更加简单。

完整的调用过程：`malloc_printerr -> __libc_message -> __GI_abort -> _IO_flush_all_lockp -> __GI__IO_str_overflow`。

与传统的 house-of-orange 不同的是，这种利用方法不再需要知道 heap 的地址，因为 `_IO_str_jumps` vtable 是在 libc 上的，所以只要能泄露出 libc 的地址就可以了。

在这个 vtable 中，还有另一个函数 `_IO_str_finish`，它的检查条件比较简单：
```c
void
_IO_str_finish (_IO_FILE *fp, int dummy)
{
  if (fp->_IO_buf_base && !(fp->_flags & _IO_USER_BUF))             // 条件
    (((_IO_strfile *) fp)->_s._free_buffer) (fp->_IO_buf_base);     // 在这个相对地址放上 system 的地址
  fp->_IO_buf_base = NULL;

  _IO_default_finish (fp, 0);
}
```
只要在 `fp->_IO_buf_base` 放上 "/bin/sh" 的地址，然后设置 `fp->_flags = 0` 就可以了绕过函数里的条件。

那么怎样让程序进入 `_IO_str_finish` 执行呢，`fclose(fp)` 是一条路，但似乎有局限。还是回到异常处理上来，在 `_IO_flush_all_lockp` 函数中是通过 `_IO_OVERFLOW` 执行的 `__GI__IO_str_overflow`，而 `_IO_OVERFLOW` 是根据 `__overflow` 相对于 `_IO_str_jumps` vtable 的偏移找到具体函数的。所以如果我们伪造传递给 `_IO_OVERFLOW(fp)` 的 fp 是 vtable 的地址减去 0x8，那么根据偏移，程序将找到 `_IO_str_finish` 并执行。

所以可以像下面这样构造：
- fp->_mode = 0
- fp->_IO_write_ptr = 0xffffffff
- fp->_IO_write_base = 0
- fp->_wide_data->_IO_buf_base = bin_sh_addr （也就是 fp->_IO_write_end）
- fp->_flags2 = 0
- fp->_mode = 0

完整的调用过程：`malloc_printerr -> __libc_message -> __GI_abort -> _IO_flush_all_lockp -> __GI__IO_str_finish`。

#### _IO_wstr_jumps
`_IO_wstr_jumps` 也是一个符合条件的 vtable，总体上和上面讲的 `_IO_str_jumps` 差不多：
```c
// libio/wstrops.c

const struct _IO_jump_t _IO_wstr_jumps libio_vtable =
{
  JUMP_INIT_DUMMY,
  JUMP_INIT(finish, _IO_wstr_finish),
  JUMP_INIT(overflow, (_IO_overflow_t) _IO_wstr_overflow),
  JUMP_INIT(underflow, (_IO_underflow_t) _IO_wstr_underflow),
  JUMP_INIT(uflow, (_IO_underflow_t) _IO_wdefault_uflow),
  JUMP_INIT(pbackfail, (_IO_pbackfail_t) _IO_wstr_pbackfail),
  JUMP_INIT(xsputn, _IO_wdefault_xsputn),
  JUMP_INIT(xsgetn, _IO_wdefault_xsgetn),
  JUMP_INIT(seekoff, _IO_wstr_seekoff),
  JUMP_INIT(seekpos, _IO_default_seekpos),
  JUMP_INIT(setbuf, _IO_default_setbuf),
  JUMP_INIT(sync, _IO_default_sync),
  JUMP_INIT(doallocate, _IO_wdefault_doallocate),
  JUMP_INIT(read, _IO_default_read),
  JUMP_INIT(write, _IO_default_write),
  JUMP_INIT(seek, _IO_default_seek),
  JUMP_INIT(close, _IO_default_close),
  JUMP_INIT(stat, _IO_default_stat),
  JUMP_INIT(showmanyc, _IO_default_showmanyc),
  JUMP_INIT(imbue, _IO_default_imbue)
};
```

利用函数 `_IO_wstr_overflow`：
```c
_IO_wint_t
_IO_wstr_overflow (_IO_FILE *fp, _IO_wint_t c)
{
  int flush_only = c == WEOF;
  _IO_size_t pos;
  if (fp->_flags & _IO_NO_WRITES)
      return flush_only ? 0 : WEOF;
  if ((fp->_flags & _IO_TIED_PUT_GET) && !(fp->_flags & _IO_CURRENTLY_PUTTING))
    {
      fp->_flags |= _IO_CURRENTLY_PUTTING;
      fp->_wide_data->_IO_write_ptr = fp->_wide_data->_IO_read_ptr;
      fp->_wide_data->_IO_read_ptr = fp->_wide_data->_IO_read_end;
    }
  pos = fp->_wide_data->_IO_write_ptr - fp->_wide_data->_IO_write_base;
  if (pos >= (_IO_size_t) (_IO_wblen (fp) + flush_only))    // 条件 #define _IO_wblen(fp) ((fp)->_wide_data->_IO_buf_end - (fp)->_wide_data->_IO_buf_base)
    {
      if (fp->_flags2 & _IO_FLAGS2_USER_WBUF) /* not allowed to enlarge */
	return WEOF;
      else
	{
	  wchar_t *new_buf;
	  wchar_t *old_buf = fp->_wide_data->_IO_buf_base;
	  size_t old_wblen = _IO_wblen (fp);
	  _IO_size_t new_size = 2 * old_wblen + 100;              // 使 new_size * sizeof(wchar_t) 为 "/bin/sh" 的地址

	  if (__glibc_unlikely (new_size < old_wblen)
	      || __glibc_unlikely (new_size > SIZE_MAX / sizeof (wchar_t)))
	    return EOF;

	  new_buf
	    = (wchar_t *) (*((_IO_strfile *) fp)->_s._allocate_buffer) (new_size
									* sizeof (wchar_t));                      // 在这个相对地址放上 system 的地址
    [...]
```

利用函数 `_IO_wstr_finish`：
```c
void
_IO_wstr_finish (_IO_FILE *fp, int dummy)
{
  if (fp->_wide_data->_IO_buf_base && !(fp->_flags2 & _IO_FLAGS2_USER_WBUF))    // 条件
    (((_IO_strfile *) fp)->_s._free_buffer) (fp->_wide_data->_IO_buf_base);     // 在这个相对地址放上 system 的地址
  fp->_wide_data->_IO_buf_base = NULL;

  _IO_wdefault_finish (fp, 0);
}
```


## 最新动态
来自 glibc 的 master 分支上的一次 [commit](https://sourceware.org/git/?p=glibc.git;a=commit;h=4e8a6346cd3da2d88bbad745a1769260d36f2783)，不出意外应该会出现在 libc-2.28 中。

该方法简单粗暴，用操作堆的 malloc 和 free 替换掉原来在 `_IO_str_fields` 里的 `_allocate_buffer` 和 `_free_buffer`。由于不再使用偏移，就不能再利用 `__libc_IO_vtables` 上的 vtable 绕过检查，于是上面的利用技术就都失效了。：（


## CTF 实例
请查看章节  6.1.24、6.1.25 和 6.1.26。另外在章节 3.1.8 中也有相关内容。

附上偏移，构造时候方便一点：
```
0x0   _flags
0x8   _IO_read_ptr
0x10  _IO_read_end
0x18  _IO_read_base
0x20  _IO_write_base
0x28  _IO_write_ptr
0x30  _IO_write_end
0x38  _IO_buf_base
0x40  _IO_buf_end
0x48  _IO_save_base
0x50  _IO_backup_base
0x58  _IO_save_end
0x60  _markers
0x68  _chain
0x70  _fileno
0x74  _flags2
0x78  _old_offset
0x80  _cur_column
0x82  _vtable_offset
0x83  _shortbuf
0x88  _lock
0x90  _offset
0x98  _codecvt
0xa0  _wide_data
0xa8  _freeres_list
0xb0  _freeres_buf
0xb8  __pad5
0xc0  _mode
0xc4  _unused2
0xd8  vtable
```


## 参考资料
- [abusing the FILE structure](https://outflux.net/blog/archives/2011/12/22/abusing-the-file-structure/)
- [Play with FILE Structure - Yet Another Binary Exploit Technique](https://www.slideshare.net/AngelBoy1/play-with-file-structure-yet-another-binary-exploit-technique)