Nocturnal Call Mac OS
The next step in my playing with chroot escapes is crafting some shellcode. Recently my main dev machine is a MacBook running OS X, so it felt reasonable to fiddle with making system calls of that platform.
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By the way, a system call is a function of the kernel invoked by a userspace program and it can be something like writing to a file descriptor, or even exiting. Usually, these are wrapped by C functions in the standard library.
The system calls
First, we need to know what system call we want to make, and what arguments it pretends.
A full list is hosted by Apple here. The header also hints at the fact that they are inherited from BSD. Yeah, that makes sense.
So, to write our proverbial Hello world we will pick the syscall 4
32-bit
Let’s start easy. A cute 32-bit program, written in NASM assembler language. Compile with nasm
or yasm
, output format MachO
, and link with ld
.
I’m on a Intel machine, so what we are looking for is the x86 syscall calling conventions for the OS X or BSD platform. They are pretty simple:
- arguments passed on the stack, pushed right-to-left
- stack 16-bytes aligned
- syscall number in the
eax
register - call by interrupt
0x80
So what we have to do to print a “Hello world” is:
- push the length of the string (
int
) to the stack - push a pointer to the string to the stack
- push the stdout file descriptor (1) to the stack
- align the stack by moving the stack pointer 4 more bytes (16 - 4 * 3)
- set the
eax
register to thewrite
syscall number (4) - interrupt
0x80
64-bit
Nocturnal Call Mac Os Update
64-bit is a bit cleaner, but completely different: OS X (and GNU/Linux and everyone except Windows) on 64 architectures adopt the System V AMD64 ABI reference. Jump to section A.2.1 for the syscall calling convention.
- arguments are passed on the registers
rdi
,rsi
,rdx
,r10
,r8
andr9
- syscall number in the
rax
register - the call is done via the
syscall
instruction - what OS X contributes to the mix is that you have to add
0x20000000
to the syscall number (still have to figure out why)
Nocturnal Call Mac Os Download
So, here is the (IMHO) much more clean 64-bit “Hello world”. Ah, if you want to do this at home and have it actually run, generate a macho64
object with a new version of NASM or with YASM, and link with ld
as usual.