struct _IO_FILE{  int _flags;                /* High-order word is _IO_MAGIC; rest is flags. */  /* The following pointers correspond to the C++ streambuf protocol. */  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;  int _flags2;  __off_t _old_offset; /* This used to be _offset but it's too small.  */  /* 1+column number of pbase(); 0 is unknown. */  unsigned short _cur_column;  signed char _vtable_offset;  char _shortbuf[1];  _IO_lock_t *_lock;#ifdef _IO_USE_OLD_IO_FILE};

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

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);};

struct _IO_str_fields{  /* These members are preserved for ABI compatibility.  The glibc     implementation always calls malloc/free for user buffers if     _IO_USER_BUF or _IO_FLAGS2_USER_WBUF are not set.  */  _IO_alloc_type _allocate_buffer_unused;  _IO_free_type _free_buffer_unused;};/* This is needed for the Irix6 N32 ABI, which has a 64 bit off_t type,   but a 32 bit pointer type.  In this case, we get 4 bytes of padding   after the vtable pointer.  Putting them in a structure together solves   this problem.  */struct _IO_streambuf{  FILE _f;  const struct _IO_jump_t *vtable;};typedef struct _IO_strfile_{  struct _IO_streambuf _sbf;  struct _IO_str_fields _s;} _IO_strfile;

#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

FSOP

FSOP的核心思想就是劫持 _IO_list_all 的值来伪造链表和其中的 _IO_FILE 项，但是单纯的伪造只是构造了数据还需要某种方法进行触发。
FSOP选择的触发方法是调用 _IO_flush_all_lockp，
这个函数会刷新 _IO_list_all 链表中所有项的文件流，相当于对每个 FILE 调用 fflush，也对应着会调用 _IO_FILE_plus.vtable 中的 _IO_overflow

_IO_flush_all_lockp (int do_lock){  int result = 0;  FILE *fp;  // ...  for (fp = (FILE *) _IO_list_all; fp != NULL; fp = fp->_chain)    {      run_fp = fp;      if (do_lock)        _IO_flockfile (fp);      if (((fp->_mode <= 0 && fp->_IO_write_ptr > fp->_IO_write_base)           || (_IO_vtable_offset (fp) == 0               && fp->_mode > 0 && (fp->_wide_data->_IO_write_ptr                                    > fp->_wide_data->_IO_write_base))           )          && _IO_OVERFLOW (fp, EOF) == EOF)        result = EOF;      //...    }  // ...  return result;}

_IO_flush_all_lockp 的触发方式:

1. 当执行 exit 函数时
2. 当执行流从 main 函数返回时
3. 当 libc 执行 abort 流程时 [glibc <= 2.23]

trace:

[#0] _IO_flush_all_lockp(do_lock=0x0)[#1] _IO_cleanup()[#2] __run_exit_handlers(...)[#3] __GI_exit(status=<optimized out>)[#4] __libc_start_main(...)[#5] _start()

glibc < 2.24

由于位于 libc 数据段的 vtable 是不可以进行写入的，所以我们只能伪造 vtable

伪造 vtable 劫持程序流程的中心思想就是针对 _IO_FILE_plus 的 vtable 动手脚，通过把 vtable 指向我们控制的内存，并在其中布置函数指针来实现。

2.24 <= glibc <= 2.27

_IO_OVERFLOW 中加入 IO_validate_vtable 对 vtable 做了检测，
vtable 必须要满足在 __stop___IO_vtables 和 __start___libc_IO_vtables 之间，而我们伪造的vtable通常不满足这个条件。

但是我们找到一条出路，那就是 _IO_str_jumps 与 _IO_wstr_jumps 就位于二者之间，
所以我们是可以利用他们来通过 IO_validate_vtable 的检测的，只需要将 vtable 填成_IO_str_jumps 或 _IO_wstr_jumps 就行。

下面列出几种可行方法，但不限于这几种：

• 利用 _IO_str_jumps 中的 _IO_str_finish 函数

  1. 为绕过 _IO_flush_all_lockp 中的检查进入到 _IO_OVERFLOW，需构造
     • _mode <= 0
     • _IO_write_ptr > _IO_write_base
  2. 构造 vtable = _IO_str_jumps - 0x8, 使 _IO_str_finish 函数替代 _IO_OVERFLOW 函数，（因为 _IO_str_finish 在 _IO_str_jumps 中的偏移为 0x10，而_IO_OVERFLOW 在原 vtable 中的偏移为 0x18）
  3. 构造 fp -> _IO_buf_base 作为参数
  4. 构造 fp->flags & _IO_USER_BUF == 0
  5. 构造 fp->_s._free_buffer = &system (_free_buffer = fp->vtable + 0x10)
  6. 调用 _IO_flush_all_lockp 函数，触发 _IO_str_finish，从而达到调用
     (((_IO_strfile *) fp)->_s._free_buffer) (fp->_IO_buf_base)

• 利用 _IO_str_jumps 中的 _IO_str_overflow 函数

  1. 为绕过 _IO_flush_all_lockp 中的检查进入到 _IO_OVERFLOW，需构造
     • _mode <= 0
     • _IO_write_ptr > _IO_write_base
  2. 构造 vtable = _IO_str_jumps，使 _IO_str_overflow 函数替代 _IO_OVERFLOW 函数，（因为 _IO_str_finish 在 _IO_str_jumps 中的偏移为 0x18，而 _IO_OVERFLOW 在原vtable 中的偏移为 0x18）
  3. 构造 fp->flags & _IO_USER_BUF == 0 && fp->flags & _IO_NO_WRITES == 0
  4. 构造 new_size = (fp->_IO_buf_end - fp->_IO_buf_base) * 2 + 100 = binsh_addr
  5. 构造 fp->_s._allocate_buffer = &system (fp->_s._allocate_buffer = fp->vtable + 0x8)
  6. 调用 _IO_flush_all_lockp 函数，触发 _IO_str_finish，从而达到调用
     (*((_IO_strfile *) fp)->_s._allocate_buffer) (new_size)

2.28 <= glibc

由于指针函数被替换为库函数：

• _IO_str_finish 中 (((_IO_strfile *) fp)->_s._free_buffer) (fp->_IO_buf_base) 被替换为 free (fp->_IO_buf_base)
• _IO_str_overflow 中 (*((_IO_strfile *) fp)->_s._allocate_buffer) (new_size) 被替换为 malloc (new_size)

因此，上述两种方法不能再可行，但可以利用 _IO_str_overflow 中的 malloc 任意大小和 free 任意地址的 chunk。

Problem:

• https://www.root-me.org/en/Challenges/App-System/ELF-x64-FILE-structure-hijacking

Ref:

• https://www.anquanke.com/post/id/216290

利用 IO_write_base 实现 leak

修改 libc.sym["_IO_2_1_stdout_"] - 0x43 处的 chunk 为

payload = b''.join([    b'\0' * 0x33,    p64(0xfbad1887),                # _flags    p64(0),                         # _IO_read_ptr    p64(0),                         # _IO_read_end    p64(0),                         # _IO_read_base    p64(libc.sym['__curbrk']),      # _IO_write_base    p64(libc.sym['__curbrk'] + 8)   # _IO_write_ptr])

从而利用 _IO_new_file_xsputn 中的

/* Next flush the (full) buffer. */if (_IO_OVERFLOW (f, EOF) == EOF)// [#0] write// [#1] _IO_new_file_write// [#2] new_do_write// [#3] _IO_new_do_write// [#4] _IO_new_file_overflow// [#5] _IO_new_file_xsputn// [#6] _IO_puts

进行泄漏，泄漏后

count = new_do_write (f, s, do_write);// [#0] write// [#1] _IO_new_file_write// [#2] new_do_write// [#3] _IO_new_do_write// [#4] _IO_new_file_xsputn// [#5] _IO_puts

打印后续数据。

Refs

• https://ray-cp.github.io/archivers/IO_FILE_arbitrary_read_write
• https://jcxp.github.io/2019/09/19/利用-IO-2-1-stdout-泄露信息/
• https://xz.aliyun.com/t/2411