hacktricks/binary-exploitation/libc-heap/heap-memory-functions/unlink.md

# unlink

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### Code

```c
// From https://github.com/bminor/glibc/blob/master/malloc/malloc.c

/* Take a chunk off a bin list.  */
static void
unlink_chunk (mstate av, mchunkptr p)
{
  if (chunksize (p) != prev_size (next_chunk (p)))
    malloc_printerr ("corrupted size vs. prev_size");

  mchunkptr fd = p->fd;
  mchunkptr bk = p->bk;

  if (__builtin_expect (fd->bk != p || bk->fd != p, 0))
    malloc_printerr ("corrupted double-linked list");

  fd->bk = bk;
  bk->fd = fd;
  if (!in_smallbin_range (chunksize_nomask (p)) && p->fd_nextsize != NULL)
    {
      if (p->fd_nextsize->bk_nextsize != p
	  || p->bk_nextsize->fd_nextsize != p)
	malloc_printerr ("corrupted double-linked list (not small)");
      
      // Added: If the FD is not in the nextsize list
      if (fd->fd_nextsize == NULL)
	{

	  if (p->fd_nextsize == p)
	    fd->fd_nextsize = fd->bk_nextsize = fd;
	  else
	    // Link the nexsize list in when removing the new chunk
	    {
	      fd->fd_nextsize = p->fd_nextsize;
	      fd->bk_nextsize = p->bk_nextsize;
	      p->fd_nextsize->bk_nextsize = fd;
	      p->bk_nextsize->fd_nextsize = fd;
	    }
	}
      else
	{
	  p->fd_nextsize->bk_nextsize = p->bk_nextsize;
	  p->bk_nextsize->fd_nextsize = p->fd_nextsize;
	}
    }
}
```

### Graphical Explanation

Check this great graphical explanation of the unlink process:

<figure><img src="../../../.gitbook/assets/image (3) (1) (1) (1) (1) (1).png" alt=""><figcaption><p><a href="https://ctf-wiki.mahaloz.re/pwn/linux/glibc-heap/implementation/figure/unlink_smallbin_intro.png">https://ctf-wiki.mahaloz.re/pwn/linux/glibc-heap/implementation/figure/unlink_smallbin_intro.png</a></p></figcaption></figure>

### Security Checks

* Check if the indicated size of the chunk is the same as the prev\_size indicated in the next chunk
* Check also that `P->fd->bk == P` and `P->bk->fw == P`
* If the chunk is not small, check that `P->fd_nextsize->bk_nextsize == P` and `P->bk_nextsize->fd_nextsize == P`

### Leaks

An unlinked chunk is not cleaning the allocated addreses, so having access to rad it, it's possible to leak some interesting addresses:

Libc Leaks:

* If P is located in the head of the doubly linked list, `bk` will be pointing to `malloc_state` in libc
* If P is located at the end of the doubly linked list, `fd` will be pointing to `malloc_state` in libc
* When the doubly linked list contains only one free chunk, P is in the doubly linked list, and both `fd` and `bk` can leak the address inside `malloc_state`.

Heap leaks:

* If P is located in the head of the doubly linked list, `fd` will be pointing to an available chunk in the heap
* If P is located at the end of the doubly linked list, `bk` will be pointing to an available chunk in the heap
* If P is in the doubly linked list, both `fd` and `bk` will be pointing to an available chunk in the heap

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