powerd (running as root) hosts the com.apple.PowerManagement.control mach service.

It checks in with launchd to get a server port and then wraps that in a CFPort:

  pmServerMachPort = _SC_CFMachPortCreateWithPort(
                          "PowerManagement",
                          serverPort, 
                          mig_server_callback, 
                          &context);

It also asks to receive dead name notifications for other ports on that same server port:

  mach_port_request_notification(
              mach_task_self(),           // task
              notify_port_in,                 // port that will die
              MACH_NOTIFY_DEAD_NAME,      // msgid
              1,                          // make-send count
              CFMachPortGetPort(pmServerMachPort),        // notify port
              MACH_MSG_TYPE_MAKE_SEND_ONCE,               // notifyPoly
              &oldNotify);                                // previous

mig_server_callback is called off of the mach port run loop source to handle new messages on pmServerMachPort:

  static void
  mig_server_callback(CFMachPortRef port, void *msg, CFIndex size, void *info)
  {
      mig_reply_error_t * bufRequest = msg;
      mig_reply_error_t * bufReply = CFAllocatorAllocate(
          NULL, _powermanagement_subsystem.maxsize, 0);
      mach_msg_return_t   mr;
      int                 options;

      __MACH_PORT_DEBUG(true, "mig_server_callback", serverPort);
      
      /* we have a request message */
      (void) pm_mig_demux(&bufRequest->Head, &bufReply->Head);

This passes the raw message to pm_mig_demux:

  static boolean_t 
  pm_mig_demux(
      mach_msg_header_t * request,
      mach_msg_header_t * reply)
  {
      mach_dead_name_notification_t *deadRequest = 
                      (mach_dead_name_notification_t *)request;
      boolean_t processed = FALSE;

      processed = powermanagement_server(request, reply);

      if (processed) 
          return true;
      
      if (MACH_NOTIFY_DEAD_NAME == request->msgh_id) 
      {
          __MACH_PORT_DEBUG(true, "pm_mig_demux: Dead name port should have 1+ send right(s)", deadRequest->not_port);

          PMConnectionHandleDeadName(deadRequest->not_port);

          __MACH_PORT_DEBUG(true, "pm_mig_demux: Deallocating dead name port", deadRequest->not_port);
          mach_port_deallocate(mach_task_self(), deadRequest->not_port);
          
          reply->msgh_bits            = 0;
          reply->msgh_remote_port     = MACH_PORT_NULL;

          return TRUE;
      }

This passes the message to the MIG-generated code for the powermanagement subsystem, if that fails (because the msgh_id doesn't
match the subsystem for example) then this compares the message's msgh_id field to MACH_NOTIFY_DEAD_NAME.

deadRequest is the message cast to a mach_dead_name_notification_t which is defined like this in mach/notify.h:

  typedef struct {
      mach_msg_header_t   not_header;
      NDR_record_t        NDR;
      mach_port_name_t not_port;/* MACH_MSG_TYPE_PORT_NAME */
      mach_msg_format_0_trailer_t trailer;
  } mach_dead_name_notification_t;

This is a simple message, not a complex one. not_port is just a completely controlled integer which in this case will get passed directly to
mach_port_deallocate.

The powerd code expects that only the kernel will send a MACH_NOTIFY_DEAD_NAME message but actually anyone can send this and force the privileged process
to drop a reference on a controlled mach port name :)

Multiplexing these two things (notifications and a mach service) onto the same port isn't possible to do safely as the kernel doesn't prevent
user->user spoofing of notification messages - usually this wouldn't be a problem as attackers shouldn't have access to the notification port.

You could use this bug to replace a mach port name in powerd (eg the bootstrap port, an IOService port etc) with a one for which the attacker holds the receieve right.

Since there's still no KDK for 10.12.1 you can test this by attaching to powerd in userspace and setting a breakpoint in pm_mig_demux at the
mach_port_deallocate call and you'll see the controlled value in rsi.

Tested on MacBookAir5,2 MacOS Sierra 10.12.1 (16B2555)
 

power_notify.c 5.6 KB View Download