Using RSP
Source
You can ignore most of it as it's mostly there to accomodate the existence of jmp rsp
- we don't actually want it called, so there's a negative if
statement.
The chance of jmp esp
gadgets existing in the binary are incredible low, but what you often do instead is find a sequence of bytes that code for jmp rsp and jump there - jmp rsp
is \xff\xe4
in shellcode, so if there's is any part of the executable section with bytes in this order, they can be used as if they are a jmp rsp
.
Exploitation
Try to do this yourself first, using the explanation on the previous page. Remember, RSP points at the thing after the return pointer once ret
has occured, so your shellcode goes after it.
Solution
Limited Space
You won't always have enough overflow - perhaps you'll only have 7 or 8 bytes. What you can do in this scenario is make the shellcode after the RIP equivalent to something like
Where 0x20
is the offset between the current value of RSP and the start of the buffer. In the buffer itself, we put the main shellcode. Let's try that!
The 10
is just a placeholder. Once we hit the pause()
, we attach with radare2 and set a breakpoint on the ret
, then continue. Once we hit it, we find the beginning of the A
string and work out the offset between that and the current value of RSP - it's 128
!
Solution
We successfully pivoted back to our shellcode - and because all our addresses are relative, it's completely reliable! ASLR beaten with pure shellcode.
This is harder with PIE as the location of jmp rsp
will change, so you might have to leak PIE base!
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