Apple macos 10.13.2 double mach_port_deallocate in kextd due to failure to comply with mig ownership rules Vulnerability / Exploit

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Exploits / Vulnerability Discovered : 2018-04-30 | Type : dos | Platform : macos
This exploit / vulnerability Apple macos 10.13.2 double mach_port_deallocate in kextd due to failure to comply with mig ownership rules is for educational purposes only and if it is used you will do on your own risk!

[+] Code ...

Here's a kextd method exposed via MIG (com.apple.KernelExtensionServer)

kern_return_t _kextmanager_unlock_kextload(
mach_port_t server,
mach_port_t client)
{
kern_return_t mig_result = KERN_FAILURE;

if (gClientUID != 0) {
OSKextLog(/* kext */ NULL,
kOSKextLogErrorLevel | kOSKextLogIPCFlag,
"Non-root kextutil doesn't need to lock/unlock.");
mig_result = KERN_SUCCESS;
goto finish;
}

if (client != (mach_port_t)dispatch_source_get_handle(_gKextutilLock)) {
OSKextLog(/* kext */ NULL,
kOSKextLogErrorLevel | kOSKextLogIPCFlag,
"%d not used to lock for kextutil.", client);
goto finish;
}

removeKextutilLock();

mig_result = KERN_SUCCESS;

finish:
// we don't need the extra send right added by MiG
mach_port_deallocate(mach_task_self(), client);

return mig_result;
}

If the client has UID 0 but passes an invalid client port this code will
drop a UREF on client port then return KERN_FAILURE.

Returning KERN_FAILURE in MIG means all resources will be released which will
cause client to be passed to mach_port_deallocate again, even though only
one UREF was taken.

You'll have to use a debugger attached to kextd to see this behaviour.

This class of bug is exploitable; please see the writeup for mach_portal from 2016
where I exploited a similar issue [https://bugs.chromium.org/p/project-zero/issues/detail?id=959]
The TL;DR is that an attacker can drop an extra UREF on any send rights in kextd for which the
attacker also has a send right; you could use this to cause a name for a privileged service
to be deallocated then cause the name to be reused to name a port you control.

Exploitation of this would be a privesc from unentitled root to root with
com.apple.rootless.kext-management and com.apple.rootless.storage.KernelExtensionManagement entitlements,
which at least last time I looked was equal to kernel code execution.

tested on MacOS 10.13.2

Proof of Concept:
https://github.com/offensive-security/exploitdb-bin-sploits/raw/master/bin-sploits/44561.zip