WWW2Exec - .dtors & .fini_array

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Nowadays is very weird to find a binary with a .dtors section!

The destructors are functions that are executed before program finishes (after the main function returns). The addresses to these functions are stored inside the .dtors section of the binary and therefore, if you manage to write the address to a shellcode in __DTOR_END__ , that will be executed before the programs ends.

Get the address of this section with:

objdump -s -j .dtors /exec
rabin -s /exec | grep “__DTOR”

Usually you will find the DTOR markers between the values ffffffff and 00000000. So if you just see those values, it means that there isn't any function registered. So overwrite the 00000000 with the address to the shellcode to execute it.

Ofc, you first need to find a place to store the shellcode in order to later call it.


Essentially this is a structure with functions that will be called before the program finishes, like .dtors. This is interesting if you can call your shellcode just jumping to an address, or in cases where you need to go back to main again to exploit the vulnerability a second time.

objdump -s -j .fini_array ./greeting

./greeting:     file format elf32-i386

Contents of section .fini_array:
 8049934 a0850408

#Put your address in 0x8049934

Note that when a function from the .fini_array is executed it moves to the next one, so it won't be executed several time (preventing eternal loops), but also it'll only give you 1 execution of the function placed here.

Note that entries in .fini_array are called in reverse order, so you probably wants to start writing from the last one.

Eternal loop

In order to abuse .fini_array to get an eternal loop you can check what was done here: If you have at least 2 entries in .fini_array, you can:

  • Use your first write to call the vulnerable arbitrary write function again

  • Then, calculate the return address in the stack stored by __libc_csu_fini (the function that it calling all the .fini_array functions) and put there the address of __libc_csu_fini

    • This will make __libc_csu_fini call himself again executing the .fini_array functions again which will call the vulnerable WWW function 2 times: one for arbitrary write and another one to overwrite again the return address of __libc_csu_fini on the stack to call itself again.

Note that with Full RELRO, the section .fini_array is made read-only.

As explained in this post, If the program exist using return or exit() it'll run __run_exit_handlers() which will call registered destructors.

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.

The important code is (source):

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.

    • This writeup 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 address which will be executed and then the difference between in the address of this fake array and the value of map->l_addr so *array will point to the fake array.

    • According to main post of this technique and this writeup 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 for example.

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

/* 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.

TLS-Storage dtor_list overwrite in __run_exit_handlers

As explained here, 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():

/* 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.  */
__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)
    if (run_dtors)
      __call_tls_dtors ();

Code from __call_tls_dtors():

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.  */
__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, 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 as setting it to 0x00, will mean that the xor used to get the real address is just the address configured. Then, 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:

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.

Other mangled pointers in __run_exit_handlers

This technique is explained here 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:

  while (true)
      struct exit_function_list *cur;

      cur = *listp;

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

      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:
	    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);
	    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);
	    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);

	  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_MANGLEcookie 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.

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