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 addresses, 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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