hacktricks/binary-exploitation/arbitrary-write-2-exec/www2exec-atexit.md

# WWW2Exec - atexit(), TLS Storage & Other mangled Pointers

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## **\_\_atexit Structures**

{% hint style="danger" %}
Nowadays is very **weird to exploit this!**
{% endhint %}

**`atexit()`** is a function to which **other functions are passed as parameters.** These **functions** will be **executed** when executing an **`exit()`** or the **return** of the **main**.\
If you can **modify** the **address** of any of these **functions** to point to a shellcode for example, you will **gain control** of the **process**, but this is currently more complicated.\
Currently the **addresses to the functions** to be executed are **hidden** behind several structures and finally the address to which it points are not the addresses of the functions, but are **encrypted with XOR** and displacements with a **random key**. So currently this attack vector is **not very useful at least on x86** and **x64\_86**.\
The **encryption function** is **`PTR_MANGLE`**. **Other architectures** such as m68k, mips32, mips64, aarch64, arm, hppa... **do not implement the encryption** function because it **returns the same** as it received as input. So these architectures would be attackable by this vector.

You can find an in depth explanation on how this works in [https://m101.github.io/binholic/2017/05/20/notes-on-abusing-exit-handlers.html](https://m101.github.io/binholic/2017/05/20/notes-on-abusing-exit-handlers.html)

## link\_map

As explained [**in this post**](https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md#2---targetting-ldso-link\_map-structure), If the program exits using `return` or `exit()` it'll run `__run_exit_handlers()` which will call registered destructors.

{% hint style="danger" %}
If the program exits via **`_exit()`** function, it'll call the **`exit` syscall** and the exit handlers will not be executed. So, to confirm `__run_exit_handlers()` is executed you can set a breakpoint on it.
{% endhint %}

The important code is ([source](https://elixir.bootlin.com/glibc/glibc-2.32/source/elf/dl-fini.c#L131)):

```c
ElfW(Dyn) *fini_array = map->l_info[DT_FINI_ARRAY];
if (fini_array != NULL)
  {
    ElfW(Addr) *array = (ElfW(Addr) *) (map->l_addr + fini_array->d_un.d_ptr);
    size_t sz = (map->l_info[DT_FINI_ARRAYSZ]->d_un.d_val / sizeof (ElfW(Addr)));

    while (sz-- > 0)
      ((fini_t) array[sz]) ();
  }
  [...]




// This is the d_un structure
ptype l->l_info[DT_FINI_ARRAY]->d_un
type = union {
    Elf64_Xword d_val;	// address of function that will be called, we put our onegadget here
    Elf64_Addr d_ptr;	// offset from l->l_addr of our structure
}
```

Note how `map -> l_addr + fini_array -> d_un.d_ptr` is used to **calculate** the position of the **array of functions to call**.

There are a **couple of options**:

* Overwrite the value of `map->l_addr` to make it point to a **fake `fini_array`** with instructions to execute arbitrary code
* Overwrite `l_info[DT_FINI_ARRAY]` and `l_info[DT_FINI_ARRAYSZ]` entries (which are more or less consecutive in memory) , to make them **points to a forged `Elf64_Dyn`** structure that will make again **`array` points to a memory** zone the attacker controlled.&#x20;
  * [**This writeup**](https://github.com/nobodyisnobody/write-ups/tree/main/DanteCTF.2023/pwn/Sentence.To.Hell) overwrites `l_info[DT_FINI_ARRAY]` with the address of a controlled memory in `.bss` containing a fake `fini_array`. This fake array contains **first a** [**one gadget**](../rop-return-oriented-programing/ret2lib/one-gadget.md) **address** which will be executed and then the **difference** between in the address of this **fake array** and the v**alue of `map->l_addr`** so `*array` will point to the fake array.
  * According to main post of this technique and [**this writeup**](https://activities.tjhsst.edu/csc/writeups/angstromctf-2021-wallstreet) ld.so leave a pointer on the stack that points to the binary `link_map` in ld.so. With an arbitrary write it's possible to overwrite it and make it point to a fake `fini_array` controlled by the attacker with the address to a [**one gadget**](../rop-return-oriented-programing/ret2lib/one-gadget.md) for example.

Following the previous code you can find another interesting section with the code:

```c
/* Next try the old-style destructor.  */
ElfW(Dyn) *fini = map->l_info[DT_FINI];
if (fini != NULL)
  DL_CALL_DT_FINI (map, ((void *) map->l_addr + fini->d_un.d_ptr));
}
```

In this case it would be possible to overwrite the value of `map->l_info[DT_FINI]` pointing to a forged `ElfW(Dyn)` structure. Find [**more information here**](https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md#2---targetting-ldso-link\_map-structure).

## TLS-Storage dtor\_list overwrite in **`__run_exit_handlers`**

As [**explained here**](https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md#5---code-execution-via-tls-storage-dtor\_list-overwrite), if a program exits via `return` or `exit()`, it'll execute **`__run_exit_handlers()`** which will call any destructors function registered.

Code from `_run_exit_handlers()`:

```c
/* Call all functions registered with `atexit' and `on_exit',
   in the reverse of the order in which they were registered
   perform stdio cleanup, and terminate program execution with STATUS.  */
void
attribute_hidden
__run_exit_handlers (int status, struct exit_function_list **listp,
                     bool run_list_atexit, bool run_dtors)
{
  /* First, call the TLS destructors.  */
#ifndef SHARED
  if (&__call_tls_dtors != NULL)
#endif
    if (run_dtors)
      __call_tls_dtors ();
```

Code from **`__call_tls_dtors()`**:

```c
typedef void (*dtor_func) (void *);
struct dtor_list //struct added
{
  dtor_func func;
  void *obj;
  struct link_map *map;
  struct dtor_list *next;
};

[...]
/* Call the destructors.  This is called either when a thread returns from the
   initial function or when the process exits via the exit function.  */
void
__call_tls_dtors (void)
{
  while (tls_dtor_list)		// parse the dtor_list chained structures
    {
      struct dtor_list *cur = tls_dtor_list;		// cur point to tls-storage dtor_list
      dtor_func func = cur->func;
      PTR_DEMANGLE (func);						// demangle the function ptr

      tls_dtor_list = tls_dtor_list->next;		// next dtor_list structure
      func (cur->obj);					
      [...]
    }
}
```

For each registered function in **`tls_dtor_list`**, it'll demangle the pointer from **`cur->func`** and call it with the argument **`cur->obj`**.

Using the **`tls`** function from this [**fork of GEF**](https://github.com/bata24/gef), it's possible to see that actually the **`dtor_list`** is very **close** to the **stack canary** and **PTR\_MANGLE cookie**. So, with an overflow on it's it would be possible to **overwrite** the **cookie** and the **stack canary**.\
Overwriting the PTR\_MANGLE cookie, it would be possible to **bypass the `PTR_DEMANLE` function** by setting it to 0x00, will mean that the **`xor`** used to get the real address is just the address configured. Then, by writing on the **`dtor_list`** it's possible **chain several functions** with the function **address** and it's **argument.**

Finally notice that the stored pointer is not only going to be xored with the cookie but also rotated 17 bits:

```armasm
0x00007fc390444dd4 <+36>:	mov    rax,QWORD PTR [rbx]      --> mangled ptr
0x00007fc390444dd7 <+39>:	ror    rax,0x11		        --> rotate of 17 bits
0x00007fc390444ddb <+43>:	xor    rax,QWORD PTR fs:0x30	--> xor with PTR_MANGLE
```

So you need to take this into account before adding a new address.

Find an example in the [**original post**](https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md#5---code-execution-via-tls-storage-dtor\_list-overwrite).

## Other mangled pointers in **`__run_exit_handlers`**

This technique is [**explained here**](https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md#5---code-execution-via-tls-storage-dtor\_list-overwrite) and depends again on the program **exiting calling `return` or `exit()`** so **`__run_exit_handlers()`** is called.

Let's check more code of this function:



```c
  while (true)
    {
      struct exit_function_list *cur;

    restart:
      cur = *listp;

      if (cur == NULL)
	{
	  /* Exit processing complete.  We will not allow any more
	     atexit/on_exit registrations.  */
	  __exit_funcs_done = true;
	  break;
	}

      while (cur->idx > 0)
	{
	  struct exit_function *const f = &cur->fns[--cur->idx];
	  const uint64_t new_exitfn_called = __new_exitfn_called;

	  switch (f->flavor)
	    {
	      void (*atfct) (void);
	      void (*onfct) (int status, void *arg);
	      void (*cxafct) (void *arg, int status);
	      void *arg;

	    case ef_free:
	    case ef_us:
	      break;
	    case ef_on:
	      onfct = f->func.on.fn;
	      arg = f->func.on.arg;
	      PTR_DEMANGLE (onfct);

	      /* Unlock the list while we call a foreign function.  */
	      __libc_lock_unlock (__exit_funcs_lock);
	      onfct (status, arg);
	      __libc_lock_lock (__exit_funcs_lock);
	      break;
	    case ef_at:
	      atfct = f->func.at;
	      PTR_DEMANGLE (atfct);

	      /* Unlock the list while we call a foreign function.  */
	      __libc_lock_unlock (__exit_funcs_lock);
	      atfct ();
	      __libc_lock_lock (__exit_funcs_lock);
	      break;
	    case ef_cxa:
	      /* To avoid dlclose/exit race calling cxafct twice (BZ 22180),
		 we must mark this function as ef_free.  */
	      f->flavor = ef_free;
	      cxafct = f->func.cxa.fn;
	      arg = f->func.cxa.arg;
	      PTR_DEMANGLE (cxafct);

	      /* Unlock the list while we call a foreign function.  */
	      __libc_lock_unlock (__exit_funcs_lock);
	      cxafct (arg, status);
	      __libc_lock_lock (__exit_funcs_lock);
	      break;
	    }

	  if (__glibc_unlikely (new_exitfn_called != __new_exitfn_called))
	    /* The last exit function, or another thread, has registered
	       more exit functions.  Start the loop over.  */
	    goto restart;
	}

      *listp = cur->next;
      if (*listp != NULL)
	/* Don't free the last element in the chain, this is the statically
	   allocate element.  */
	free (cur);
    }

  __libc_lock_unlock (__exit_funcs_lock);
```

The variable `f` points to the **`initial`** structure and depending on the value of `f->flavor` different functions will be called.\
Depending on the value, the address of the function to call will be in a different place, but it'll always be **demangled**.

Moreover, in the options **`ef_on`** and **`ef_cxa`** it's also possible to control an **argument**.

It's possible to check the **`initial` structure** in a debugging session with GEF running **`gef> p initial`**.

To abuse this you need either to **leak or erase the `PTR_MANGLE`cookie** and then overwrite a `cxa` entry in initial with `system('/bin/sh')`.\
You can find an example of this in the [**original blog post about the technique**](https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md#6---code-execution-via-other-mangled-pointers-in-initial-structure).

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</details>
{% endhint %}