mirror of
https://github.com/HackTricks-wiki/hacktricks.git
synced 2025-10-10 18:36:50 +00:00
111 lines
5.0 KiB
Markdown
111 lines
5.0 KiB
Markdown
# unlink
|
|
|
|
{% hint style="success" %}
|
|
Learn & practice AWS Hacking:<img src="../../../.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="../../../.gitbook/assets/arte.png" alt="" data-size="line">\
|
|
Learn & practice GCP Hacking: <img src="../../../.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="../../../.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
|
|
|
|
<details>
|
|
|
|
<summary>Support HackTricks</summary>
|
|
|
|
* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
|
|
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks_live)**.**
|
|
* **Share hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
|
|
|
|
</details>
|
|
{% endhint %}
|
|
|
|
### 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
|
|
|
|
{% hint style="success" %}
|
|
Learn & practice AWS Hacking:<img src="../../../.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="../../../.gitbook/assets/arte.png" alt="" data-size="line">\
|
|
Learn & practice GCP Hacking: <img src="../../../.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="../../../.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
|
|
|
|
<details>
|
|
|
|
<summary>Support HackTricks</summary>
|
|
|
|
* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
|
|
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks_live)**.**
|
|
* **Share hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
|
|
|
|
</details>
|
|
{% endhint %}
|
|
|