Mac OS Architecture
Kernel
XNU
The heart of Mac OS X is the XNU kernel. XNU is basically composed of a Mach core (covered in the next section) with supplementary features provided by Berkeley Software Distribution (BSD). Additionally, XNU is responsible for providing an environment for kernel drivers called the I/O Kit. XNU is a Darwin package, so all of the source code is freely available.
From a security researcher’s perspective, Mac OS X feels just like a FreeBSD box with a pretty windowing system and a large number of custom applications. For the most part, applications written for BSD will compile and run without modification on Mac OS X. All the tools you are accustomed to using in BSD are available in Mac OS X. Nevertheless, the fact that the XNU kernel contains all the Mach code means that some day, when you have to dig deeper, you’ll find many differences that may cause you problems and some you may be able to leverage for your own purposes.
Mach
Mach was originated as a UNIX-compatible operating system back in 1984. One of its primary design goals was to be a microkernel; that is, to minimize the amount of code running in the kernel and allow many typical kernel functions, such as file system, networking, and I/O, to run as user-level Mach tasks.
In XNU, Mach is responsible for many of the low-level operations you expect from a kernel, such as processor scheduling and multitasking and virtual- memory management.
BSD
The kernel also involves a large chunk of code derived from the FreeBSD code base. This code runs as part of the kernel along with Mach and uses the same address space. The FreeBSD code within XNU may differ significantly from the original FreeBSD code, as changes had to be made for it to coexist with Mach. FreeBSD provides many of the remaining operations the kernel needs, including:
Processes
Signals
Basic security, such as users and groups
System call infrastructure
TCP/IP stack and sockets
Firewall and packet filtering
To get an idea of just how complicated the interaction between these two sets of code can be, consider the idea of the fundamental executing unit. In BSD the fundamental unit is the process. In Mach it is a Mach thread. The disparity is settled by each BSD-style process being associated with a Mach task consisting of exactly one Mach thread. When the BSD fork() system call is made, the BSD code in the kernel uses Mach calls to create a task and thread structure. Also, it is important to note that both the Mach and BSD layers have different security models. The Mach security model is based on port rights, and the BSD model is based on process ownership. Disparities between these two models have resulted in a number of local privilege-escalation vulnerabilities. Additionally, besides typical system cells, there are Mach traps that allow user-space programs to communicate with the kernel.
I/O Kit - Drivers
I/O Kit is the open-source, object-oriented, device-driver framework in the XNU kernel and is responsible for the addition and management of dynamically loaded device drivers. These drivers allow for modular code to be added to the kernel dynamically for use with different hardware, for example. They are located in:
/System/Library/ExtensionsKEXT files built into the OS X operating system.
/Library/ExtensionsKEXT files installed by 3rd party software
Until the number 9 the listed drivers are loaded in the address 0. This means that those aren't real drivers but part of the kernel and they cannot be unloaded.
In order to find specific extensions you can use:
To load and unload kernel extensions do:
Applications
A kernel without applications isn’t very useful. Darwin is the non-Aqua, open-source core of Mac OS X. Basically it is all the parts of Mac OS X for which the source code is available. The code is made available in the form of a package that is easy to install. There are hundreds of available Darwin packages, such as X11, GCC, and other GNU tools. Darwin provides many of the applications you may already use in BSD or Linux for Mac OS X. Apple has spent significant time integrating these packages into their operating system so that everything behaves nicely and has a consistent look and feel when possible.
On the other hand, many familiar pieces of Mac OS X are not open source. The main missing piece to someone running just the Darwin code will be Aqua, the Mac OS X windowing and graphical-interface environment. Additionally, most of the common high-level applications, such as Safari, Mail, QuickTime, iChat, etc., are not open source (although some of their components are open source). Interestingly, these closed-source applications often rely on open- source software, for example, Safari relies on the WebKit project for HTML and JavaScript rendering. For perhaps this reason, you also typically have many more symbols in these applications when debugging than you would in a Windows environment.
Universal binaries
Mac OS binaries usually are compiled as universal binaries. **A universal binary can support multiple architectures in the same file**.
In the following example, a universal binary for the x86 and PowerPC architectures is created:
As you may be thinking usually a universal binary compiled for 2 architectures doubles the size of one compiled for just 1 arch.
Mach-o Format
Header
The header contains basic information about the file, such as magic bytes to identify it as a Mach-O file and information about the target architecture. You can find it in: mdfind loader.h | grep -i mach-o | grep -E "loader.h$"
Filetypes:
MH_EXECUTE (0x2): Standard Mach-O executable
MH_DYLIB (0x6): A Mach-O dynamic linked library (i.e. .dylib)
MH_BUNDLE (0x8): A Mach-O bundle (i.e. .bundle)
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Load commands
This specifies the layout of the file in memory. It contains the location of the symbol table, the main thread context at the beginning of execution, and which shared libraries are required. The commands basically instruct the dynamic loader (dyld) how to load the binary in memory.
Load commands all begin with a load_command structure, defined in mach-o/loader.h:
A common type of load command is LC_SEGMENT/LC_SEGMENT_64, which describes a segment: A segment defines a range of bytes in a Mach-O file and the addresses and memory protection attributes at which those bytes are mapped into virtual memory when the dynamic linker loads the application.
Common segments:
__TEXT: Contains executable code and data that is read-only. Common sections of this segment:__text: **Compiled binary code__const: Constant data__cstring: String constants
__DATA: Contains data that is writable.__data: Global variables (that have been initialized)__bss: Static variables (that have not been initialized)__objc_*(__objc_classlist, __objc_protolist, etc): Information used by the Objective-C runtime
__LINKEDIT: Contains information for the linker (dyld) such as, "symbol, string, and relocation table entries."__OBJC: Contains information used by the Objective-C runtime. Though this information might also be found in the __DATA segment, within various in __objc_* sections.LC_MAIN: Contains the entrypoint in the entryoff attribute. At load time, dyld simply adds this value to the (in-memory) base of the binary, then jumps to this instruction to kickoff execution of the binary’s code.LC_LOAD_DYLIB: **This load command describes a dynamic library dependency which instructs the loader (dyld) to load and link said library. There is a LC_LOAD_DYLIB load command for each library** that the Mach-O binary requires.This load command is a structure of type
dylib_command(which contains a struct dylib, describing the actual dependent dynamic library):
Some potential malware related libraries are:
DiskArbitration: Monitoring USB drives
AVFoundation: Capture audio and video
CoreWLAN: Wifi scans.
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Data
The heart of the file is the final region, the data, which consists of a number of segments as laid out in the load-commands region. Each segment can contain a number of data sections. Each of these sections contains code or data of one particular type.
Get the info
Bundles
Basically, a bundle is a directory structure within the file system. Interestingly, by default this directory looks like a single object in Finder. The types of resources contained within a bundle may consist of applications, libraries, images, documentation, header files, etc. All these files are inside <application>.app/Contents/
Contents/_CodeSignatureContains code-signing information about the application (i.e., hashes, etc.).
Contents/MacOSContains the application’s binary (which is executed when the user double-clicks the application icon in the UI).
Contents/ResourcesContains UI elements of the application, such as images, documents, and nib/xib files (that describe various user interfaces).
Contents/Info.plist**The application’s main “configuration file.**” Apple notes that “the system relies on the presence of this file to identify relevant information about [the] application and any related files”.Pairs that may be of interest when analyzing an application include:
CFBundleExecutable
Contains the name of the application’s binary (found in Contents/MacOS).
CFBundleIdentifier
Contains the application’s bundle identifier (often used by the system to globally identify the application).
LSMinimumSystemVersion
Contains the oldest version of macOS that the application is compatible with.
Objective-C
Programs written in Objective-C retain their class declarations when compiled into (Mach-O) binaries. Such class declarations include the name and type of:
The class
The class methods
The class instance variables
Note that this names can be obfuscated to make the reversing of the binary more difficult.
Native Packages
There are some projects that allow to generate a binary executable by MacOS containing script code which will be executed. Some examples are:
Platypus: Generate MacOS binary executing **shell scripts, Python, Perl, Ruby, PHP, Swift, Expect, Tcl, AWK, JavaScript, AppleScript or any other user-specified interpreter.
It saves the script in
Contents/Resources/script. So finding this script is a good indicator that Platypus was used.
PyInstaller: Python
Ways to detect this is the use of the embedded **string “Py_SetPythonHome” or a a call into a function named
pyi_main.**
Electron: JavaScript, HTML, and CSS.
These binaries will use Electron Framework.framework. Moreover, the non-binary components (e.g. JavaScript files) maybe found in the application’s
Contents/Resources/directory, achieved in.asarfiles. These binaries will use Electron Framework.framework. Moreover, the non-binary components (e.g. JavaScript files) maybe found in the application’sContents/Resources/directory, achieved in.asarfiles. It's possible unpack such archives via the asar node module, or the npx utility:npx asar extract StrongBox.app/Contents/Resources/app.asar appUnpacked
References
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