Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=882

mach_ports_register is a kernel task port MIG method.

It's defined in MIG like this:

  routine mach_ports_register(
      target_task : task_t;
      init_port_set : mach_port_array_t =
            ^array[] of mach_port_t);

Looking at the generated code for this we notice something kinda weird; here's the mach message structure
which actually gets sent:

  typedef struct {
    mach_msg_header_t Head;
    // start of the kernel processed data
    mach_msg_body_t msgh_body;
    mach_msg_ool_ports_descriptor_t init_port_set;
    // end of the kernel processed data
    NDR_record_t NDR;
    mach_msg_type_number_t init_port_setCnt;
  } Request __attribute__((unused));

The message contains an OOL ports descriptor, which is expected, but also contains a separate init_port_setCnt value
even though the ool_ports_descriptor_t already has the correct length of the descriptor.

When the kernel process this ool ports descriptor in ipc_kmsg_copyin_ool_ports_descriptor it will kalloc a buffer large enough
for all the ports and then copyin and convert them all. It does this using the init_port_set.count value, not init_port_setCnt.

The generated MIG code however calls mach_ports_register like this:

  OutP->RetCode = mach_ports_register(target_task, (mach_port_array_t)(In0P->init_port_set.address), In0P->init_port_setCnt);

without verifying that In0P->init_port_setCnt is equal to init_port_set.count.

This means that when we reach mach_ports_register lots of stuff goes wrong:

  kern_return_t
  mach_ports_register(
    task_t      task,
    mach_port_array_t memory,                       <-- points to kalloc'ed buffer
    mach_msg_type_number_t  portsCnt)               <-- completely controlled, not related to size of kalloc'ed buffer
  {
    ipc_port_t ports[TASK_PORT_REGISTER_MAX];
    unsigned int i;

    if ((task == TASK_NULL) ||
        (portsCnt > TASK_PORT_REGISTER_MAX) ||
        (portsCnt && memory == NULL))
      return KERN_INVALID_ARGUMENT;                 <-- portsCnt must be >=1 && <= 3

    for (i = 0; i < portsCnt; i++)
      ports[i] = memory[i];                         <-- if we only sent one OOL port but set portsCnt >1 this will read a mach_port_t (a pointer) out of bounds
    for (; i < TASK_PORT_REGISTER_MAX; i++)
      ports[i] = IP_NULL;

    itk_lock(task);
    if (task->itk_self == IP_NULL) {
      itk_unlock(task);
      return KERN_INVALID_ARGUMENT;
    }

    for (i = 0; i < TASK_PORT_REGISTER_MAX; i++) {
      ipc_port_t old;

      old = task->itk_registered[i];
      task->itk_registered[i] = ports[i];
      ports[i] = old;
    }

    itk_unlock(task);

    for (i = 0; i < TASK_PORT_REGISTER_MAX; i++)
      if (IP_VALID(ports[i]))
        ipc_port_release_send(ports[i]);           <-- this can decrement the ref on a pointer which was read out of bounds if we call this function multiple times

    if (portsCnt != 0)
      kfree(memory,
            (vm_size_t) (portsCnt * sizeof(mach_port_t)));   <-- this can call kfree with the wrong size

    return KERN_SUCCESS;
  }

For this PoC I've patched the MIG generated code to always only send one OOL mach port but still set init_port_setCnt to a controlled value - you should see a kernel
panic decrementing an invalid reference or something like that.

This bug however could be exploited quite nicely to cause a mach_port_t UaF which could have all kinds of fun consequences (getting another task's task port for example!)

tested on OS X 10.11.6 (15G31) on MacBookPro10,1


Proof of Concept:
https://github.com/offensive-security/exploitdb-bin-sploits/raw/master/bin-sploits/40654.zip