unlink

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Code

// 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:

https://ctf-wiki.mahaloz.re/pwn/linux/glibc-heap/implementation/figure/unlink_smallbin_intro.png

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