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Rooting the KORG Kronos

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Donnerstag, 5. Mai 2016

Streamlining the Kronos boot process / what does loadmod.ko do?

I have already sched some light on the Kronos boot process in an older article (What does /sbin/loadoa do?). There you can see that a kernel module called "loadmod.ko" is loaded just before the encrypted loop files are mounted.

loadmod.ko plays a vital role in booting the Kronos and is the center of Korg's attempts to protect the Kronos from hacking.

So, what does loadmod.ko do?


  1. First, loadmod.ko performs a file system integrity check. It opens and reads a lot of files and hashes them using MD5. The hash is then compared to a constant value encoded in loadmod.ko. If the values do not match, loadmod.ko will bail out with error code 1.
  2. Next, the Linux kernel is checked. loadmod.ko calls the functions register_cdrom() and init_cdrom_command() with some magic values. A normal Linux kernel will just return error codes because the parameters make no sense. The Linux kernel delivered by Korg has modified versions of this functions that return other magic values to signal loadmod.ko that the kernel is "genuine".

    Korg has not published the source code for those modifications to the Linux kernel even though they are distributing a binary version of Linux with those modifications. This is a clear violation of the GPL.

    You can find a more detailed description of the kernel authentication mechanism in About the Linux kernel used in KORG Kronos.

    I provide a patch that reimplements the modified register_cdrom()/init_cdrom_command() functions here. With this patch you can compile your own kernel that will be accepted by loadmod.ko.

    If the kernel check fails, loadmod.ko will bail out with error code 3.
  3. After filesystem and kernel have been checked, loadmod.ko tries to read the authorization file called "pairFact". This file contains the encryption keys for the encrypted file systems in the Kronos (detailed explanation). If loadmod.ko cannot read that authorization data, it will bail
    out with error code 4.
  4. The contents of "pairFact" are encrypted with a key stored in the security IC. So, the next step for loadmod.ko is to talk to the security IC (the primitives for the communication are provided by OmapNKS4Module.ko. The Public ID of the Kronos is read and the key needed to decrypt the authorization data is read. If communication with the security IC fails, loadmod.ko will bail out with error code 5.
  5. If all checks were successful, a magic value (0x22FB39CC) is stored at a certain address in kernel memory. This magic value will be checked by the synthesizer module (OA.ko) later. If it is not there, OA.ko will skip some important initializations which will cause the sound output to be bad. So even if you do not need loadmod.ko anymore because you have copied the synthesizer files out of the encrypted file systems, you still need to run loadmod.ko (or at least write this magic value) to have useful sound output.
  6. The error code (or zero if there were no errors) from the checks above is written to "/tmp/stgStatus". This is used by /sbin/loadoa to display a more or less useful error message (or the reauthentication screen) if one of the integrity checks failed.
  7. Now, things are getting crazy: loadmod.ko replaces some of the kernel's syscalls by overwriting sys_call_table. The following syscalls are overwritten: sys_mount, sys_ioctl, sys_oldumount, sys_umount, sys_create_module, sys_init_module.

    The syscalls sys_mount and sys_oldumount are replaced with versions that "magically" mount a cryptoloop filesystem if one of the magic paths (/korg/Eva, /korg/Mod, /korg/WaveMotion) is mounted.

    The other syscall replacements don't do anything but call the original handlers. It looks like Korg wanted to prevent users from loading additional modules (maybe to prevent access to the kernel memory), but that mechanism was not finished.
  8. Finally, a kernel thread is started. This kernel thread will handle system updates by listening on /proc/.update and running /sbin/UpdateOS or /tmp/UpdateOS as a usermode helper if necessary.


So, why is that interesting?

From a users point of view, loadmod.ko doesn't do many useful things. The filesystem integrity checks take precious boot time and prevent you from changing your file system. The encrypted loop file systems also degrade system performance and you can easily copy their contents onto the unencrypted file system, 

However, if you decide to save boot time and trouble by getting rid of loadmod.ko, you still have to write the magic value or else the Kronos' sound will be crippled.

I have updated my kernel patches and my precompiled kernel to do that, Using this kernel, you do not need to load loadmod.ko. You can get rid of the filesystem/kernel integrity check and still OA.ko will work perfectly.



Kommentare:

  1. hi , great work!!! so this mods decrease booting time...am i right? thanks

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  2. Hi! I just went through some of my stuff, and found kronos CDs that contain among backup data etc also some source code (most interestingly something that looks like kernel source (2.6.32.11 says on the file) and other tools like gcc etc. Maybe they could be of use, or maybe not, but still interesting that they included them on the disks among drivers.

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    1. If this could be something of use, you can contact me on youtube (since I don't want to publish my email here):
      https://www.youtube.com/channel/UCjHQf71v04nix4ZHyr76GYg

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