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IOKit Reversing Tips

I’ve been reversing some IOKit drivers for iOS for a couple months now, and I have collected a bunch of tips that would have accelerated my progress if I had known them earlier, so I decided to share them with anyone who wants to get into iOS security research.

IOreg

My first tip when it comes to IOKit, is to get familiar with ioreg. It’s a handy tool to list the IOKit registry, and see what drivers are currently loaded. Use it with the right flags - I like to use it to find information about a specific loaded driver by looking up its name like this: ioreg -f -r -t -n name, or by looking up a specific class like this: ioreg -f -r -t -c IOClass. The combination of -r -t will print the full tree for your classes clients, and its providers. For example, to check out my Macs display, I can use ioreg -f -r -t -n display0 to get:

+-o Root  <class IORegistryEntry, id 0x100000100, retain 24>
  +-o MacBookAir9,1  <class IOPlatformExpertDevice, id 0x100000114, registered, matched, active, busy 0 (13676 ms), retain 67>
    +-o AppleACPIPlatformExpert  <class AppleACPIPlatformExpert, id 0x100000115, registered, matched, active, busy 0 (8818 ms), retain 52>
      +-o PCI0@0  <class IOACPIPlatformDevice, id 0x10000015d, registered, matched, active, busy 0 (4972 ms), retain 55>
        +-o AppleACPIPCI  <class AppleACPIPCI, id 0x1000001e8, registered, matched, active, busy 0 (4957 ms), retain 39>
          +-o IGPU@2  <class IOPCIDevice, id 0x1000001b3, registered, matched, active, busy 0 (1912 ms), retain 34>
            +-o AppleIntelFramebuffer@0  <class AppleIntelFramebuffer, id 0x10000046d, registered, matched, active, busy 0 (119 ms), retain 21>
              +-o display0  <class IODisplayConnect, id 0x1000005e4, registered, matched, active, busy 0 (1 ms), retain 6>
                | {
                | }
                |
                +-o AppleBacklightDisplay  <class AppleBacklightDisplay, id 0x1000005e5, registered, matched, active, busy 0 (0 ms), retain 9>

On MacOS, I recommend getting the IORegistryExplorer.app from Apples additional Developer tools, which provides the same functionality as ioreg, but in a more comfortable visual UI.

Get IDA Pro

I started reversing with Ghidra, simply because it’s free. After some time however I made the switch to IDA Pro, and the difference for iOS is immense. They did a really good job of integrating a lot of iOS specific functionality to IDA in newer versions, such as natively working with the kernelcache, or dyld_shared_cache. I am aware that it is expensive, and I was lucky to be able to use it through my university.

Get a symbolicated kernel cache

Normally, Apple removes symbols from the kernel cache, making reversing a bit more tedious. However sometimes symbols are not removed, e.g. in a research kernel. One of those can be found in this ipsw (Shoutout to tihmstar for posting about this apparently secret technique! I didn’t know about it before..)

Accelerate IDA Analysis

The following tweet is self-explanatory:

IDA Kernelcache Scripts

Brandon Azad released an amazing IDA Toolkit for iOS kernelcache analysis. However his version only worked for IDA 6 and up to iOS 12. Fortunately, there is a public fork by cellebrite-srl, porting it to IDA Pro 7.5 and iOS 14: https://github.com/cellebrite-srl/ida_kernelcache

This script will use the class metainformation found in any IOKit class to create class structures with vtables, making analysis much more comfortable. An example from bazads blogpost:

Before running ida_kernelcache:

__int64 __fastcall AppleKeyStoreUserClient::registerNotificationPort(__int64 a1, ipc_port *a2, __int64 a3, int a4)
{
    __int64 v4; // x0@2

    if ( *(_BYTE *)(a1 + 248) & 0x10 )
    {
        v4 = *(_QWORD *)(a1 + 216);
        if ( a4 == 43 )
        {
            *(_QWORD *)(v4 + 208) = a2;
            if ( *(_BYTE *)(v4 + 0xE0) )
                sub_FFFFFFF0069D0AF4(v4, 0, 0);
        }
        else
        {
            *(_QWORD *)(v4 + 0xC8) = a2;
        }
    }
    return 0LL;
}

After running ida_kernelcache:

IOReturn __fastcall AppleKeyStoreUserClient::registerNotificationPort(AppleKeyStoreUserClient *this, ipc_port *port, unsigned int type, unsigned int refcon)
{
    AppleKeyStore *provider; // x0@2

    if ( this->AppleKeyStoreUserClient.entitlements_flags & 0x10 )
    {
        provider = this->AppleKeyStoreUserClient.provider;
        if ( refcon == 43 )
        {
            provider->AppleKeyStore.system_keybag_update_port = port;
            if ( provider->AppleKeyStore.field_e0 )
                AppleSEPKeyStore::tickle_system_keybag_update_port(provider, 0, 0);
        }
        else
        {
            provider->AppleKeyStore.notification_port = port;
        }
    }
    return 0;
}

Siguzas IOKit tools

Next, I would like to mention Siguzas open source tools, that he released on github:

OS X and iOS Kernel Programming Book

This book helped me a lot with understanding the iOS kernel. There are also some other good resources, especially the official documentation

Check the sources

You probably already know that XNU is open source, but did you know that some IOKit Families, and tools like ioreg are also open source? Some Families even include open source sample driver implementations!

Know the tools

On MacOS, you will find some additional tools to interact with drivers. I don’t find them incredibly useful for reverse engineering, but it’s good to know about them nonetheless:


Thank you for reading, if you know of any tip that I missed, I would love to hear about it!

@r0bre, 13.01.2021