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Issue metadata

Status: Fixed
Closed: Dec 2016

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MacOS kernel code execution due to writable privileged IOKit registry properties

Project Member Reported by, Oct 26 2016

Issue description

There are two ways for IOServices to define their IOUserClient classes: they can 
override IOService::newUserClient and allocate the correct type themselves
or they can set the IOUserClientClass key in their registry entry.

The default implementation of IOService::newUserClient does this:

  IOReturn IOService::newUserClient( task_t owningTask, void * securityID,
                                    UInt32 type,  OSDictionary * properties,
                                    IOUserClient ** handler )
      const OSSymbol *userClientClass = 0;
      IOUserClient *client;
      OSObject *temp;
      if (kIOReturnSuccess == newUserClient( owningTask, securityID, type, handler ))
          return kIOReturnSuccess;
      // First try my own properties for a user client class name
      temp = getProperty(gIOUserClientClassKey);
      if (temp) {
          if (OSDynamicCast(OSSymbol, temp))
              userClientClass = (const OSSymbol *) temp;
          else if (OSDynamicCast(OSString, temp)) {
              userClientClass = OSSymbol::withString((OSString *) temp);
              if (userClientClass)
                              (OSObject *) userClientClass);
      // Didn't find one so lets just bomb out now without further ado.
      if (!userClientClass)
          return kIOReturnUnsupported;
      // This reference is consumed by the IOServiceOpen call
      temp = OSMetaClass::allocClassWithName(userClientClass);
      if (!temp)
          return kIOReturnNoMemory;
      if (OSDynamicCast(IOUserClient, temp))
          client = (IOUserClient *) temp;
      else {
          return kIOReturnUnsupported;
      if ( !client->initWithTask(owningTask, securityID, type, properties) ) {

  ... continue on and call client->start(this) to connect the client to the service

This reads the "IOUserClientClass" entry in the services registry entry and uses the IOKit
reflection API to allocate it.

If an IOService doesn't want to have any IOUserClients then it has two options, either override
newUserClient to return kIOReturnUnsupported or make sure that there is no IOUserClientClass
entry in the service's registry entry.

AppleBroadcomBluetoothHostController takes the second approach but inherits from IOBluetoothHostController
which overrides ::setProperties to allow an unprivileged user to set *all* registry entry properties,
including IOUserClientClass.

This leads to a very exploitable type confusion issue as plenty of IOUserClient subclasses don't expect
to be connected to a different IOService provider. In this PoC I connect an IGAccelSharedUserClient to
a AppleBroadcomBluetoothHostController which leads immediately to an invalid virtual call. With more
investigation I'm sure you could build some very nice exploitation primitives with this bug.

Tested on MacBookAir5,2 MacOS Sierra 10.12.1 (16B2555)
4.1 KB View Download
Project Member

Comment 1 by, Oct 26 2016

Labels: Id-651167731 Reported-2016-Oct-26
Project Member

Comment 2 by, Dec 22 2016

Labels: -Restrict-View-Commit Fixed-2016-Dec-13 CVE-2016-7617
Status: Fixed (was: New)
Fixed in MacOS 10.12.2:

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