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268 lines
13 KiB
Markdown
268 lines
13 KiB
Markdown
# WWW2Exec - atexit(), TLS Storage & Other mangled Pointers
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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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## **\_\_atexit Structures**
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{% hint style="danger" %}
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Nowadays is very **weird to exploit this!**
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{% endhint %}
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**`atexit()`** is a function to which **other functions are passed as parameters.** These **functions** will be **executed** when executing an **`exit()`** or the **return** of the **main**.\
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If you can **modify** the **address** of any of these **functions** to point to a shellcode for example, you will **gain control** of the **process**, but this is currently more complicated.\
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Currently the **addresses to the functions** to be executed are **hidden** behind several structures and finally the address to which it points are not the addresses of the functions, but are **encrypted with XOR** and displacements with a **random key**. So currently this attack vector is **not very useful at least on x86** and **x64\_86**.\
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The **encryption function** is **`PTR_MANGLE`**. **Other architectures** such as m68k, mips32, mips64, aarch64, arm, hppa... **do not implement the encryption** function because it **returns the same** as it received as input. So these architectures would be attackable by this vector.
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You can find an in depth explanation on how this works in [https://m101.github.io/binholic/2017/05/20/notes-on-abusing-exit-handlers.html](https://m101.github.io/binholic/2017/05/20/notes-on-abusing-exit-handlers.html)
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## link\_map
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As explained [**in this post**](https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md#2---targetting-ldso-link\_map-structure), If the program exits using `return` or `exit()` it'll run `__run_exit_handlers()` which will call registered destructors.
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{% hint style="danger" %}
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If the program exits via **`_exit()`** function, it'll call the **`exit` syscall** and the exit handlers will not be executed. So, to confirm `__run_exit_handlers()` is executed you can set a breakpoint on it.
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{% endhint %}
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The important code is ([source](https://elixir.bootlin.com/glibc/glibc-2.32/source/elf/dl-fini.c#L131)):
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```c
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ElfW(Dyn) *fini_array = map->l_info[DT_FINI_ARRAY];
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if (fini_array != NULL)
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{
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ElfW(Addr) *array = (ElfW(Addr) *) (map->l_addr + fini_array->d_un.d_ptr);
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size_t sz = (map->l_info[DT_FINI_ARRAYSZ]->d_un.d_val / sizeof (ElfW(Addr)));
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while (sz-- > 0)
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((fini_t) array[sz]) ();
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}
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[...]
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// This is the d_un structure
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ptype l->l_info[DT_FINI_ARRAY]->d_un
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type = union {
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Elf64_Xword d_val; // address of function that will be called, we put our onegadget here
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Elf64_Addr d_ptr; // offset from l->l_addr of our structure
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}
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```
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Note how `map -> l_addr + fini_array -> d_un.d_ptr` is used to **calculate** the position of the **array of functions to call**.
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There are a **couple of options**:
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* Overwrite the value of `map->l_addr` to make it point to a **fake `fini_array`** with instructions to execute arbitrary code
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* Overwrite `l_info[DT_FINI_ARRAY]` and `l_info[DT_FINI_ARRAYSZ]` entries (which are more or less consecutive in memory) , to make them **points to a forged `Elf64_Dyn`** structure that will make again **`array` points to a memory** zone the attacker controlled. 
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* [**This writeup**](https://github.com/nobodyisnobody/write-ups/tree/main/DanteCTF.2023/pwn/Sentence.To.Hell) overwrites `l_info[DT_FINI_ARRAY]` with the address of a controlled memory in `.bss` containing a fake `fini_array`. This fake array contains **first a** [**one gadget**](../rop-return-oriented-programing/ret2lib/one-gadget.md) **address** which will be executed and then the **difference** between in the address of this **fake array** and the v**alue of `map->l_addr`** so `*array` will point to the fake array.
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* According to main post of this technique and [**this writeup**](https://activities.tjhsst.edu/csc/writeups/angstromctf-2021-wallstreet) ld.so leave a pointer on the stack that points to the binary `link_map` in ld.so. With an arbitrary write it's possible to overwrite it and make it point to a fake `fini_array` controlled by the attacker with the address to a [**one gadget**](../rop-return-oriented-programing/ret2lib/one-gadget.md) for example.
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Following the previous code you can find another interesting section with the code:
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```c
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/* Next try the old-style destructor. */
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ElfW(Dyn) *fini = map->l_info[DT_FINI];
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if (fini != NULL)
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DL_CALL_DT_FINI (map, ((void *) map->l_addr + fini->d_un.d_ptr));
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}
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```
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In this case it would be possible to overwrite the value of `map->l_info[DT_FINI]` pointing to a forged `ElfW(Dyn)` structure. Find [**more information here**](https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md#2---targetting-ldso-link\_map-structure).
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## TLS-Storage dtor\_list overwrite in **`__run_exit_handlers`**
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As [**explained here**](https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md#5---code-execution-via-tls-storage-dtor\_list-overwrite), if a program exits via `return` or `exit()`, it'll execute **`__run_exit_handlers()`** which will call any destructors function registered.
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Code from `_run_exit_handlers()`:
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```c
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/* Call all functions registered with `atexit' and `on_exit',
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in the reverse of the order in which they were registered
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perform stdio cleanup, and terminate program execution with STATUS. */
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void
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attribute_hidden
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__run_exit_handlers (int status, struct exit_function_list **listp,
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bool run_list_atexit, bool run_dtors)
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{
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/* First, call the TLS destructors. */
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#ifndef SHARED
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if (&__call_tls_dtors != NULL)
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#endif
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if (run_dtors)
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__call_tls_dtors ();
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```
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Code from **`__call_tls_dtors()`**:
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```c
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typedef void (*dtor_func) (void *);
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struct dtor_list //struct added
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{
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dtor_func func;
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void *obj;
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struct link_map *map;
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struct dtor_list *next;
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};
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[...]
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/* Call the destructors. This is called either when a thread returns from the
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initial function or when the process exits via the exit function. */
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void
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__call_tls_dtors (void)
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{
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while (tls_dtor_list) // parse the dtor_list chained structures
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{
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struct dtor_list *cur = tls_dtor_list; // cur point to tls-storage dtor_list
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dtor_func func = cur->func;
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PTR_DEMANGLE (func); // demangle the function ptr
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tls_dtor_list = tls_dtor_list->next; // next dtor_list structure
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func (cur->obj);
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[...]
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}
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}
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```
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For each registered function in **`tls_dtor_list`**, it'll demangle the pointer from **`cur->func`** and call it with the argument **`cur->obj`**.
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Using the **`tls`** function from this [**fork of GEF**](https://github.com/bata24/gef), it's possible to see that actually the **`dtor_list`** is very **close** to the **stack canary** and **PTR\_MANGLE cookie**. So, with an overflow on it's it would be possible to **overwrite** the **cookie** and the **stack canary**.\
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Overwriting the PTR\_MANGLE cookie, it would be possible to **bypass the `PTR_DEMANLE` function** by setting it to 0x00, will mean that the **`xor`** used to get the real address is just the address configured. Then, by writing on the **`dtor_list`** it's possible **chain several functions** with the function **address** and it's **argument.**
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Finally notice that the stored pointer is not only going to be xored with the cookie but also rotated 17 bits:
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```armasm
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0x00007fc390444dd4 <+36>: mov rax,QWORD PTR [rbx] --> mangled ptr
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0x00007fc390444dd7 <+39>: ror rax,0x11 --> rotate of 17 bits
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0x00007fc390444ddb <+43>: xor rax,QWORD PTR fs:0x30 --> xor with PTR_MANGLE
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```
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So you need to take this into account before adding a new address.
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Find an example in the [**original post**](https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md#5---code-execution-via-tls-storage-dtor\_list-overwrite).
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## Other mangled pointers in **`__run_exit_handlers`**
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This technique is [**explained here**](https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md#5---code-execution-via-tls-storage-dtor\_list-overwrite) and depends again on the program **exiting calling `return` or `exit()`** so **`__run_exit_handlers()`** is called.
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Let's check more code of this function:
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```c
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while (true)
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{
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struct exit_function_list *cur;
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restart:
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cur = *listp;
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if (cur == NULL)
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{
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/* Exit processing complete. We will not allow any more
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atexit/on_exit registrations. */
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__exit_funcs_done = true;
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break;
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}
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while (cur->idx > 0)
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{
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struct exit_function *const f = &cur->fns[--cur->idx];
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const uint64_t new_exitfn_called = __new_exitfn_called;
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switch (f->flavor)
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{
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void (*atfct) (void);
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void (*onfct) (int status, void *arg);
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void (*cxafct) (void *arg, int status);
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void *arg;
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case ef_free:
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case ef_us:
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break;
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case ef_on:
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onfct = f->func.on.fn;
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arg = f->func.on.arg;
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PTR_DEMANGLE (onfct);
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/* Unlock the list while we call a foreign function. */
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__libc_lock_unlock (__exit_funcs_lock);
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onfct (status, arg);
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__libc_lock_lock (__exit_funcs_lock);
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break;
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case ef_at:
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atfct = f->func.at;
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PTR_DEMANGLE (atfct);
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/* Unlock the list while we call a foreign function. */
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__libc_lock_unlock (__exit_funcs_lock);
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atfct ();
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__libc_lock_lock (__exit_funcs_lock);
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break;
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case ef_cxa:
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/* To avoid dlclose/exit race calling cxafct twice (BZ 22180),
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we must mark this function as ef_free. */
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f->flavor = ef_free;
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cxafct = f->func.cxa.fn;
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arg = f->func.cxa.arg;
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PTR_DEMANGLE (cxafct);
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/* Unlock the list while we call a foreign function. */
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__libc_lock_unlock (__exit_funcs_lock);
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cxafct (arg, status);
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__libc_lock_lock (__exit_funcs_lock);
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break;
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}
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if (__glibc_unlikely (new_exitfn_called != __new_exitfn_called))
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/* The last exit function, or another thread, has registered
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more exit functions. Start the loop over. */
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goto restart;
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}
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*listp = cur->next;
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if (*listp != NULL)
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/* Don't free the last element in the chain, this is the statically
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allocate element. */
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free (cur);
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}
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__libc_lock_unlock (__exit_funcs_lock);
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```
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The variable `f` points to the **`initial`** structure and depending on the value of `f->flavor` different functions will be called.\
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Depending on the value, the address of the function to call will be in a different place, but it'll always be **demangled**.
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Moreover, in the options **`ef_on`** and **`ef_cxa`** it's also possible to control an **argument**.
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It's possible to check the **`initial` structure** in a debugging session with GEF running **`gef> p initial`**.
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To abuse this you need either to **leak or erase the `PTR_MANGLE`cookie** and then overwrite a `cxa` entry in initial with `system('/bin/sh')`.\
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You can find an example of this in the [**original blog post about the technique**](https://github.com/nobodyisnobody/docs/blob/main/code.execution.on.last.libc/README.md#6---code-execution-via-other-mangled-pointers-in-initial-structure).
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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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