# macOS MIG - Mach Interface Generator

<details>

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MIG was created to **simplify the process of Mach IPC** code creation. It basically **generates the needed code** for server and client to communicate with a given definition. Even if the generated code is ugly, a developer will just need to import it and his code will be much simpler than before.

### Example

Create a definition file, in this case with a very simple function:

{% code title="myipc.defs" %}
```cpp
subsystem myipc 500; // Arbitrary name and id
 
userprefix USERPREF;        // Prefix for created functions in the client
serverprefix SERVERPREF;    // Prefix for created functions in the server

#include <mach/mach_types.defs> 
#include <mach/std_types.defs>

simpleroutine Subtract(
    server_port :  mach_port_t;
    n1          :  uint32_t;
    n2          :  uint32_t);
```
{% endcode %}

Now use mig to generate the server and client code that will be able to comunicate within each other to call the Subtract function:

```bash
mig -header myipcUser.h -sheader myipcServer.h myipc.defs
```

Several new files will be created in the current directory.

In the files **`myipcServer.c`** and **`myipcServer.h`** you can find the declaration and definition of the struct **`SERVERPREFmyipc_subsystem`**, which basically defines the function to call based on the received message ID (we indicated a starting number of 500):

{% tabs %}
{% tab title="myipcServer.c" %}
```c
/* Description of this subsystem, for use in direct RPC */
const struct SERVERPREFmyipc_subsystem SERVERPREFmyipc_subsystem = {
	myipc_server_routine,
	500, // start ID
	501, // end ID
	(mach_msg_size_t)sizeof(union __ReplyUnion__SERVERPREFmyipc_subsystem),
	(vm_address_t)0,
	{
          { (mig_impl_routine_t) 0,
          // Function to call
          (mig_stub_routine_t) _XSubtract, 3, 0, (routine_arg_descriptor_t)0, (mach_msg_size_t)sizeof(__Reply__Subtract_t)},
	}
};
```
{% endtab %}

{% tab title="myipcServer.h" %}
```c
/* Description of this subsystem, for use in direct RPC */
extern const struct SERVERPREFmyipc_subsystem {
	mig_server_routine_t	server;	/* Server routine */
	mach_msg_id_t	start;	/* Min routine number */
	mach_msg_id_t	end;	/* Max routine number + 1 */
	unsigned int	maxsize;	/* Max msg size */
	vm_address_t	reserved;	/* Reserved */
	struct routine_descriptor	/* Array of routine descriptors */
		routine[1];
} SERVERPREFmyipc_subsystem;
```
{% endtab %}
{% endtabs %}

Based on the previous struct the function **`myipc_server_routine`** will get the **message ID** and return the proper function to call:

```c
mig_external mig_routine_t myipc_server_routine
	(mach_msg_header_t *InHeadP)
{
	int msgh_id;

	msgh_id = InHeadP->msgh_id - 500;

	if ((msgh_id > 0) || (msgh_id < 0))
		return 0;

	return SERVERPREFmyipc_subsystem.routine[msgh_id].stub_routine;
}
```

In this example we have only defined 1 function in the definitions, but if we would have defined more functions, they would have been inside the array of **`SERVERPREFmyipc_subsystem`** and the first one would have been assigned to the ID **500**, the second one to the ID **501**...

Actually it's possible to identify this relation in the struct **`subsystem_to_name_map_myipc`** from **`myipcServer.h`**:

```c
#ifndef subsystem_to_name_map_myipc
#define subsystem_to_name_map_myipc \
    { "Subtract", 500 }
#endif
```

Finally, another important function to make the server work will be **`myipc_server`**, which is the one that will actually **call the function** related to the received id:

<pre class="language-c"><code class="lang-c">mig_external boolean_t myipc_server
	(mach_msg_header_t *InHeadP, mach_msg_header_t *OutHeadP)
{
	/*
	 * typedef struct {
	 * 	mach_msg_header_t Head;
	 * 	NDR_record_t NDR;
	 * 	kern_return_t RetCode;
	 * } mig_reply_error_t;
	 */

	mig_routine_t routine;

	OutHeadP->msgh_bits = MACH_MSGH_BITS(MACH_MSGH_BITS_REPLY(InHeadP->msgh_bits), 0);
	OutHeadP->msgh_remote_port = InHeadP->msgh_reply_port;
	/* Minimal size: routine() will update it if different */
	OutHeadP->msgh_size = (mach_msg_size_t)sizeof(mig_reply_error_t);
	OutHeadP->msgh_local_port = MACH_PORT_NULL;
	OutHeadP->msgh_id = InHeadP->msgh_id + 100;
	OutHeadP->msgh_reserved = 0;

	if ((InHeadP->msgh_id > 500) || (InHeadP->msgh_id &#x3C; 500) ||
<strong>	    ((routine = SERVERPREFmyipc_subsystem.routine[InHeadP->msgh_id - 500].stub_routine) == 0)) {
</strong>		((mig_reply_error_t *)OutHeadP)->NDR = NDR_record;
		((mig_reply_error_t *)OutHeadP)->RetCode = MIG_BAD_ID;
		return FALSE;
	}
<strong>	(*routine) (InHeadP, OutHeadP);
</strong>	return TRUE;
}
</code></pre>

Check the previously highlighted lines accessing the function to call by ID.

In the following is the code to create a simple **server** and **client** where the client can call the functions Subtract from the server:

{% tabs %}
{% tab title="myipc_server.c" %}
```c
// gcc myipc_server.c myipcServer.c -o myipc_server

#include <stdio.h>
#include <mach/mach.h>
#include <servers/bootstrap.h>
#include "myipcServer.h"

kern_return_t SERVERPREFSubtract(mach_port_t server_port, uint32_t n1, uint32_t n2)
{
    printf("Received: %d - %d = %d\n", n1, n2, n1 - n2);
    return KERN_SUCCESS;
}

int main() {

    mach_port_t port;
    kern_return_t kr;
    
    // Register the mach service
    kr = bootstrap_check_in(bootstrap_port, "xyz.hacktricks.mig", &port);
    if (kr != KERN_SUCCESS) {
        printf("bootstrap_check_in() failed with code 0x%x\n", kr);
        return 1;
    }
    
    // myipc_server is the function that handles incoming messages (check previous exlpanation)
    mach_msg_server(myipc_server, sizeof(union __RequestUnion__SERVERPREFmyipc_subsystem), port, MACH_MSG_TIMEOUT_NONE);
}
```
{% endtab %}

{% tab title="myipc_client.c" %}
```c
// gcc myipc_client.c myipcUser.c -o myipc_client

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

#include <mach/mach.h>
#include <servers/bootstrap.h>
#include "myipcUser.h"

int main() {

    // Lookup the receiver port using the bootstrap server.
    mach_port_t port;
    kern_return_t kr = bootstrap_look_up(bootstrap_port, "xyz.hacktricks.mig", &port);
    if (kr != KERN_SUCCESS) {
        printf("bootstrap_look_up() failed with code 0x%x\n", kr);
        return 1;
    }
    printf("Port right name %d\n", port);
    USERPREFSubtract(port, 40, 2);
}
```
{% endtab %}
{% endtabs %}

### Binary Analysis

As many binaries now use MIG to expose mach ports, it's interesting to know how to **identify that MIG was used** and the **functions that MIG executes** with each message ID.

[**jtool2**](../../macos-apps-inspecting-debugging-and-fuzzing/#jtool2) can parse MIG information from a Mach-O binary indicating the message ID and identifying the function to execute:

```bash
jtool2 -d __DATA.__const myipc_server | grep MIG
```

It was previously mentioned that the function that will take care of **calling the correct function depending on the received message ID** was `myipc_server`. However, you usually won't have the symbols of the binary (no functions names), so it's interesting to **check how it looks like decompiled** as it will always be very similar (the code of this function is independent from the functions exposed):

{% tabs %}
{% tab title="myipc_server decompiled 1" %}
<pre class="language-c"><code class="lang-c">int _myipc_server(int arg0, int arg1) {
    var_10 = arg0;
    var_18 = arg1;
    // Initial instructions to find the proper function ponters
    *(int32_t *)var_18 = *(int32_t *)var_10 &#x26; 0x1f;
    *(int32_t *)(var_18 + 0x8) = *(int32_t *)(var_10 + 0x8);
    *(int32_t *)(var_18 + 0x4) = 0x24;
    *(int32_t *)(var_18 + 0xc) = 0x0;
    *(int32_t *)(var_18 + 0x14) = *(int32_t *)(var_10 + 0x14) + 0x64;
    *(int32_t *)(var_18 + 0x10) = 0x0;
    if (*(int32_t *)(var_10 + 0x14) &#x3C;= 0x1f4 &#x26;&#x26; *(int32_t *)(var_10 + 0x14) >= 0x1f4) {
            rax = *(int32_t *)(var_10 + 0x14);
            // Call to sign_extend_64 that can help to identifyf this function
            // This stores in rax the pointer to the call that needs to be called
            // Check the used of the address 0x100004040 (functions addresses array)
            // 0x1f4 = 500 (the strating ID)
<strong>            rax = *(sign_extend_64(rax - 0x1f4) * 0x28 + 0x100004040);
</strong>            var_20 = rax;
            // If - else, the if returns false, while the else call the correct function and returns true
<strong>            if (rax == 0x0) {
</strong>                    *(var_18 + 0x18) = **_NDR_record;
                    *(int32_t *)(var_18 + 0x20) = 0xfffffffffffffed1;
                    var_4 = 0x0;
            }
            else {
                    // Calculated address that calls the proper function with 2 arguments
<strong>                    (var_20)(var_10, var_18);
</strong>                    var_4 = 0x1;
            }
    }
    else {
            *(var_18 + 0x18) = **_NDR_record;
            *(int32_t *)(var_18 + 0x20) = 0xfffffffffffffed1;
            var_4 = 0x0;
    }
    rax = var_4;
    return rax;
}
</code></pre>
{% endtab %}

{% tab title="myipc_server decompiled 2" %}
This is the same function decompiled in a difefrent Hopper free version:

<pre class="language-c"><code class="lang-c">int _myipc_server(int arg0, int arg1) {
    r31 = r31 - 0x40;
    saved_fp = r29;
    stack[-8] = r30;
    var_10 = arg0;
    var_18 = arg1;
    // Initial instructions to find the proper function ponters
    *(int32_t *)var_18 = *(int32_t *)var_10 &#x26; 0x1f | 0x0;
    *(int32_t *)(var_18 + 0x8) = *(int32_t *)(var_10 + 0x8);
    *(int32_t *)(var_18 + 0x4) = 0x24;
    *(int32_t *)(var_18 + 0xc) = 0x0;
    *(int32_t *)(var_18 + 0x14) = *(int32_t *)(var_10 + 0x14) + 0x64;
    *(int32_t *)(var_18 + 0x10) = 0x0;
    r8 = *(int32_t *)(var_10 + 0x14);
    r8 = r8 - 0x1f4;
    if (r8 > 0x0) {
            if (CPU_FLAGS &#x26; G) {
                    r8 = 0x1;
            }
    }
    if ((r8 &#x26; 0x1) == 0x0) {
            r8 = *(int32_t *)(var_10 + 0x14);
            r8 = r8 - 0x1f4;
            if (r8 &#x3C; 0x0) {
                    if (CPU_FLAGS &#x26; L) {
                            r8 = 0x1;
                    }
            }
            if ((r8 &#x26; 0x1) == 0x0) {
                    r8 = *(int32_t *)(var_10 + 0x14);
                    // 0x1f4 = 500 (the strating ID)
<strong>                    r8 = r8 - 0x1f4;
</strong>                    asm { smaddl     x8, w8, w9, x10 };
                    r8 = *(r8 + 0x8);
                    var_20 = r8;
                    r8 = r8 - 0x0;
                    if (r8 != 0x0) {
                            if (CPU_FLAGS &#x26; NE) {
                                    r8 = 0x1;
                            }
                    }
                    // Same if else as in the previous version
                    // Check the used of the address 0x100004040 (functions addresses array)
<strong>                    if ((r8 &#x26; 0x1) == 0x0) {
</strong><strong>                            *(var_18 + 0x18) = **0x100004000;
</strong>                            *(int32_t *)(var_18 + 0x20) = 0xfffffed1;
                            var_4 = 0x0;
                    }
                    else {
                            // Call to the calculated address where the function should be
<strong>                            (var_20)(var_10, var_18);
</strong>                            var_4 = 0x1;
                    }
            }
            else {
                    *(var_18 + 0x18) = **0x100004000;
                    *(int32_t *)(var_18 + 0x20) = 0xfffffed1;
                    var_4 = 0x0;
            }
    }
    else {
            *(var_18 + 0x18) = **0x100004000;
            *(int32_t *)(var_18 + 0x20) = 0xfffffed1;
            var_4 = 0x0;
    }
    r0 = var_4;
    return r0;
}

</code></pre>
{% endtab %}
{% endtabs %}

Actually if you go to the function **`0x100004000`** you will find the array of **`routine_descriptor`** structs. The first element of the struct is the **address** where the **function** is implemented, and the **struct takes 0x28 bytes**, so each 0x28 bytes (starting from byte 0) you can get 8 bytes and that will be the **address of the function** that will be called:

<figure><img src="../../../../.gitbook/assets/image (1) (1) (1) (1).png" alt=""><figcaption></figcaption></figure>

<figure><img src="../../../../.gitbook/assets/image (1) (1) (1) (1) (1).png" alt=""><figcaption></figcaption></figure>

This data can be extracted [**using this Hopper script**](https://github.com/knightsc/hopper/blob/master/scripts/MIG%20Detect.py).

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