Macos Ipc Analysis

macOS IPC Analysis

A skill for analyzing and working with macOS Mach Inter-Process Communication (IPC) mechanisms, including ports, rights, task ports, and related security implications.

When to Use This Skill

Use this skill when you need to:

• Understand Mach port concepts and IPC mechanisms on macOS
• Debug or analyze Mach message communications
• Investigate port rights and permissions
• Work with task ports, thread ports, or special ports
• Analyze code injection techniques via task ports
• Understand bootstrap server and service registration
• Investigate privilege escalation via IPC mechanisms
• Debug XPC or MIG-based communications

Core Concepts

Mach Ports Overview

Mach uses tasks as the smallest unit for sharing resources. Each task maps 1:1 to POSIX processes and can contain multiple threads.

Key concepts:

• Ports: Basic IPC elements acting as message queues managed by the kernel
• IPC Table: Each process has an IPC table containing its Mach ports
• Port Names: Numbers (pointers to kernel objects)
• Communication: One-way channels between ports

Port Rights

Port rights define what operations a task can perform:

Important:

• Receive rights are MPSC (multiple-producer, single-consumer)
• Send rights can be cloned and transferred to other tasks
• Port rights can be passed through Mach messages
• If the Receive right owner dies, Send rights become dead names

Bootstrap Server

The bootstrap server (launchd on macOS) enables initial communication:

1. Task A creates a port with RECEIVE right
2. Task A generates a SEND right for the port
3. Task A sends the SEND right to the bootstrap server
4. Task A registers the port with a name (e.g., com.apple.taska)
5. Task B looks up the service name via bootstrap server
6. Bootstrap server duplicates Task A's SEND right and sends to Task B
7. Task B can now send messages to Task A

Security note: The bootstrap server cannot authenticate service names claimed by tasks. System services are protected via SIP-protected directories (/System/Library/LaunchDaemons, /System/Library/LaunchAgents).

Port Enumeration and Analysis

Using lsmp

# List ports for a specific process
lsmp -p <pid>

# List ports for launchd (requires sudo)
sudo lsmp -p 1

Output columns:

• name: Default port name (increasing in first 3 bytes)
• ipc-object: Obfuscated unique identifier
• rights: Port rights (send, recv, send+recv)
• +: Indicates other tasks connected to the same port

Using procexp

# List all host special ports
procexp all ports | grep "HSP"

# List ports for a specific process (requires SIP disabled for some info)
procexp 1 ports

Task Ports

Task ports are critical for process control and memory manipulation.

Key Functions

Task Port Capabilities

With SEND right over a task port, you can:

• task_threads: Get SEND rights to all threads
• task_info: Get task information
• task_suspend/resume: Control task execution
• vm_read/vm_write: Read/write task memory
• thread_create: Create new threads
• task_get/set_state: Control task state

Security implications:

• SEND right over another task's port enables code injection
• task_for_pid requires specific entitlements or root
• com.apple.security.get-task-allow: Same-user access (debugging)
• com.apple.system-task-ports: Any process access (Apple binaries only)
• Root can access non-hardened runtime applications

Task Port Restrictions

Thread Ports

Threads have associated ports visible via task_threads.

Thread Port Capabilities

With SEND right over a thread port:

• thread_terminate: Terminate the thread
• thread_get/set_state: Control thread state
• thread_suspend/resume: Pause/resume execution
• thread_info: Get thread information

Getting Thread Port

mach_thread_self()  // Get thread port for current thread

Mach Message Structure

Message Header

typedef struct {
    mach_msg_bits_t               msgh_bits;      // Bitmap flags
    mach_msg_size_t               msgh_size;      // Total packet size
    mach_port_t                   msgh_remote_port;  // Destination port
    mach_port_t                   msgh_local_port;   // Reply port
    mach_port_name_t              msgh_voucher_port; // Voucher port
    mach_msg_id_t                 msgh_id;        // Message ID
} mach_msg_header_t;

msgh_bits Bitmap

Port Type Constants

#define MACH_MSG_TYPE_MOVE_RECEIVE      16
#define MACH_MSG_TYPE_MOVE_SEND         17
#define MACH_MSG_TYPE_MOVE_SEND_ONCE    18
#define MACH_MSG_TYPE_COPY_SEND         19
#define MACH_MSG_TYPE_MAKE_SEND         20
#define MACH_MSG_TYPE_MAKE_SEND_ONCE    21
#define MACH_MSG_TYPE_DISPOSE_RECEIVE   24
#define MACH_MSG_TYPE_DISPOSE_SEND      25
#define MACH_MSG_TYPE_DISPOSE_SEND_ONCE 26

Debugging Mach Messages

Setting Breakpoints

# In lldb
(lldb) b mach_msg
(lldb) r

Inspecting Arguments

# Read registers (ARM64 calling convention)
(lldb) reg read $x0 $x1 $x2 $x3 $x4 $x5 $x6

# Inspect message header
(lldb) x/6w $x0

Register mapping:

• $x0: mach_msg_header_t *msg
• $x1: mach_msg_option_t option
• $x2: mach_msg_size_t send_size
• $x3: mach_msg_size_t rcv_size
• $x4: mach_port_name_t rcv_name
• $x5: mach_msg_timeout_t timeout
• $x6: mach_port_name_t notify

Special Ports

Host Special Ports

Access:

• host_get_special_port: Get SEND rights (requires host_priv)
• host_set_special_port: Set RECEIVE rights (requires host_priv)

Task Special Ports

#define TASK_KERNEL_PORT        1   // Task control port
#define TASK_HOST_PORT          2   // Host port for task
#define TASK_BOOTSTRAP_PORT     4   // Bootstrap environment
#define TASK_WIRED_LEDGER_PORT  5   // Wired memory ledger
#define TASK_PAGED_LEDGER_PORT  6   // Paged memory ledger

Code Injection Techniques

Shellcode Injection via Task Port

Requirements:

• SEND right to target task port (via task_for_pid)
• Target process with com.apple.security.get-task-allow entitlement (same user)
• Or root access to non-hardened runtime apps

Steps:

1. Get task port via task_for_pid(mach_task_self(), pid, &remoteTask)
2. Allocate memory: mach_vm_allocate(remoteTask, &address, size, VM_FLAGS_ANYWHERE)
3. Write shellcode: mach_vm_write(remoteTask, address, shellcode, length)
4. Set permissions: vm_protect(remoteTask, address, size, FALSE, VM_PROT_READ | VM_PROT_EXECUTE)
5. Create thread: thread_create_running(remoteTask, ARM_THREAD_STATE64, &state, count, &thread)

Dylib Injection via Task Port

For POSIX-compliant injections:

1. Inject shellcode that calls pthread_create_from_mach_thread
2. The new pthread can call dlopen to load custom libraries
3. This enables more complex injections using system APIs

Thread Hijacking

Instead of creating new threads, hijack existing threads:

1. Get task port and enumerate threads via task_threads
2. Suspend target thread: thread_suspend(thread_port)
3. Modify thread state (PC, SP) to point to injected code
4. Resume thread: thread_resume(thread_port)

Detection and Evasion

Task Port Injection Detection

The kernel increments a counter in the task struct when:

• task_for_pid is called
• thread_create_* is called

Detection:

task_info(task, TASK_EXTMOD_INFO, ...)

Exception Ports

Exception triage flow:

1. Thread exception port
2. Task exception ports
3. Host port (launchd)
4. ReportCrash daemon

Note: Crash reporting tools like PLCrashReporter handle exceptions within the same task.

Common APIs Reference

Port Management

Message Operations

Complex Messages

For messages with port rights or shared memory:

• Set MSB of msgh_bits
• Use descriptors: MACH_MSG_PORT_DESCRIPTOR, MACH_MSG_OOL_DESCRIPTOR, etc.
• Kernel copies descriptors to kernel memory first (Feng Shui technique)

Security Considerations

Privilege Escalation Vectors

1. Task port access: SEND right enables full process control
2. Exception port hijacking: Handle exceptions to gain control
3. Bootstrap impersonation: Register fake service names (non-system tasks)
4. Port right transfer: Send rights to vulnerable processes

Mitigations

• SIP (System Integrity Protection): Protects system directories and ports
• Hardened Runtime: Restricts task port access
• Entitlements: Control access to sensitive APIs
• Code Signing: Validates binary authenticity

Practical Examples

Basic Port Communication

See the scripts/ directory for working examples:

• receiver.c: Register a port and receive messages
• sender.c: Look up a port and send messages

Enumerating System Ports

# List all ports for launchd
sudo lsmp -p 1

# Find host special ports
procexp all ports | grep "HSP"

# Check specific process ports
lsmp -p <pid>

Debugging IPC

# Start lldb with target application
lldb /path/to/app

# Set breakpoint on mach_msg
(lldb) b mach_msg

# Run and inspect
(lldb) r
(lldb) reg read $x0 $x1 $x2 $x3 $x4 $x5 $x6
(lldb) x/6w $x0

Related Topics

• XPC: Higher-level IPC framework (see macos-xpc skill)
• MIG: Mach Interface Generator for RPC code generation
• Sandbox: macOS sandboxing and entitlements
• Code Signing: Binary signing and verification

References

• Darling Mach Ports Documentation
• Apple XNU Source - mach/message.h
• Apple XNU Source - host_special_ports.h
• [OS Internals, Volume I, User Mode by Jonathan Levin
• Project Zero - Sound Barrier 2
• Sector7 - XPC Audit Token Spoofing