Controlling execution with snippets of code
Gadgets are small snippets of code followed by a ret instruction, e.g. pop rdi; ret. We can manipulate the ret of these gadgets in such a way as to string together a large chain of them to do what we want.


Let's for a minute pretend the stack looks like this during the execution of a pop rdi; ret gadget.
What happens is fairly obvious - 0x10 gets popped into rdi as it is at the top of the stack during the pop rdi. Once the pop occurs, rsp moves:
And since ret is equivalent to pop rip, 0x5655576724 gets moved into rip. Note how the stack is laid out for this.

Utilising Gadgets

When we overwrite the return pointer, we overwrite the value pointed at by rsp. Once that value is popped, it points at the next value at the stack - but wait. We can overwrite the next value in the stack.
Let's say that we want to exploit a binary to jump to a pop rdi; ret gadget, pop 0x100 into rdi then jump to flag(). Let's step-by-step the execution.
On the original ret, which we overwrite the return pointer for, we pop the gadget address in. Now rip moves to point to the gadget, and rsp moves to the next memory address.
rsp moves to the 0x100; rip to the pop rdi. Now when we pop, 0x100 gets moved into rdi.
RSP moves onto the next items on the stack, the address of flag(). The ret is executed and flag() is called.


Essentially, if the gadget pops values from the stack, simply place those values afterwards (including the pop rip in ret). If we want to pop 0x10 into rdi and then jump to 0x16, our payload would look like this:
Note if you have multiple pop instructions, you can just add more values.
We use rdi as an example because, if you remember, that's the register for the first parameter in 64-bit. This means control of this register using this gadget is important.

Finding Gadgets

We can use the tool ROPgadget to find possible gadgets.
$ ROPgadget --binary vuln-64
Gadgets information
0x0000000000401069 : add ah, dh ; nop dword ptr [rax + rax] ; ret
0x000000000040109b : add bh, bh ; loopne 0x40110a ; nop ; ret
0x0000000000401037 : add byte ptr [rax], al ; add byte ptr [rax], al ; jmp 0x401024
Combine it with grep to look for specific registers.
$ ROPgadget --binary vuln-64 | grep rdi
0x0000000000401096 : or dword ptr [rdi + 0x404030], edi ; jmp rax
0x00000000004011db : pop rdi ; ret