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https://github.com/carlospolop/hacktricks
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109 lines
5 KiB
Markdown
109 lines
5 KiB
Markdown
# unlink
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{% hint style="success" %}
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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">\
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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)
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<details>
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<summary>Support HackTricks</summary>
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* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
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* **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)**.**
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* **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.
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</details>
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{% endhint %}
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### Code
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```c
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// From https://github.com/bminor/glibc/blob/master/malloc/malloc.c
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/* Take a chunk off a bin list. */
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static void
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unlink_chunk (mstate av, mchunkptr p)
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{
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if (chunksize (p) != prev_size (next_chunk (p)))
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malloc_printerr ("corrupted size vs. prev_size");
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mchunkptr fd = p->fd;
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mchunkptr bk = p->bk;
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if (__builtin_expect (fd->bk != p || bk->fd != p, 0))
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malloc_printerr ("corrupted double-linked list");
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fd->bk = bk;
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bk->fd = fd;
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if (!in_smallbin_range (chunksize_nomask (p)) && p->fd_nextsize != NULL)
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{
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if (p->fd_nextsize->bk_nextsize != p
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|| p->bk_nextsize->fd_nextsize != p)
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malloc_printerr ("corrupted double-linked list (not small)");
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// Added: If the FD is not in the nextsize list
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if (fd->fd_nextsize == NULL)
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{
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if (p->fd_nextsize == p)
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fd->fd_nextsize = fd->bk_nextsize = fd;
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else
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// Link the nexsize list in when removing the new chunk
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{
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fd->fd_nextsize = p->fd_nextsize;
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fd->bk_nextsize = p->bk_nextsize;
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p->fd_nextsize->bk_nextsize = fd;
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p->bk_nextsize->fd_nextsize = fd;
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}
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}
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else
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{
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p->fd_nextsize->bk_nextsize = p->bk_nextsize;
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p->bk_nextsize->fd_nextsize = p->fd_nextsize;
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}
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}
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}
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```
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### Graphical Explanation
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Check this great graphical explanation of the unlink process:
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<figure><img src="../../../.gitbook/assets/image (3) (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>
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### Security Checks
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* Check if the indicated size of the chunk is the same as the prev\_size indicated in the next chunk
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* Check also that `P->fd->bk == P` and `P->bk->fw == P`
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* If the chunk is not small, check that `P->fd_nextsize->bk_nextsize == P` and `P->bk_nextsize->fd_nextsize == P`
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### Leaks
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An unlinked chunk is not cleaning the allocated addreses, so having access to rad it, it's possible to leak some interesting addresses:
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Libc Leaks:
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* If P is located in the head of the doubly linked list, `bk` will be pointing to `malloc_state` in libc
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* If P is located at the end of the doubly linked list, `fd` will be pointing to `malloc_state` in libc
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* 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`.
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Heap leaks:
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* If P is located in the head of the doubly linked list, `fd` will be pointing to an available chunk in the heap
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* If P is located at the end of the doubly linked list, `bk` will be pointing to an available chunk in the heap
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* 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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{% hint style="success" %}
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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">\
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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)
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<details>
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<summary>Support HackTricks</summary>
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* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
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* **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)**.**
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* **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.
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</details>
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{% endhint %}
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