driver-core: Fix use-after-free triggered by bus_unregister()

driver-core: Fix use-after-free triggered by bus_unregister()

Avoid that bus_unregister() triggers a use-after-free with
CONFIG_DEBUG_KOBJECT_RELEASE=y. This patch avoids that the
following sequence triggers a kernel crash with memory poisoning
enabled:
* bus_register()
* driver_register()
* driver_unregister()
* bus_unregister()

The above sequence causes the bus private data to be freed from
inside the bus_unregister() call although it is not guaranteed in
that function that the reference count on the bus private data has
dropped to zero. As an example, with CONFIG_DEBUG_KOBJECT_RELEASE=y
the ${bus}/drivers kobject is still holding a reference on
bus->p->subsys.kobj via its parent pointer at the time the bus
private data is freed. Fix this by deferring freeing the bus private
data until the last kobject_put() call on bus->p->subsys.kobj.

The kernel oops triggered by the above sequence and with memory
poisoning enabled and that is fixed by this patch is as follows:

general protection fault: 0000 [#1] PREEMPT SMP
CPU: 3 PID: 2711 Comm: kworker/3:32 Tainted: G        W  O 3.13.0-rc4-debug+ #1
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
Workqueue: events kobject_delayed_cleanup
task: ffff880037f866d0 ti: ffff88003b638000 task.ti: ffff88003b638000
Call Trace:
 [<ffffffff81263105>] ? kobject_get_path+0x25/0x100
 [<ffffffff81264354>] kobject_uevent_env+0x134/0x600
 [<ffffffff8126482b>] kobject_uevent+0xb/0x10
 [<ffffffff81262fa2>] kobject_delayed_cleanup+0xc2/0x1b0
 [<ffffffff8106c047>] process_one_work+0x217/0x700
 [<ffffffff8106bfdb>] ? process_one_work+0x1ab/0x700
 [<ffffffff8106c64b>] worker_thread+0x11b/0x3a0
 [<ffffffff8106c530>] ? process_one_work+0x700/0x700
 [<ffffffff81074b70>] kthread+0xf0/0x110
 [<ffffffff81074a80>] ? insert_kthread_work+0x80/0x80
 [<ffffffff815673bc>] ret_from_fork+0x7c/0xb0
 [<ffffffff81074a80>] ? insert_kthread_work+0x80/0x80
Code: 89 f8 48 89 e5 f6 82 c0 27 63 81 20 74 15 0f 1f 44 00 00 48 83 c0 01 0f b6 10 f6 82 c0 27 63 81 20 75 f0 5d c3 66 0f 1f 44 00 00 <80> 3f 00 55 48 89 e5 74 15 48 89 f8 0f 1f 40 00 48 83 c0 01 80
RIP  [<ffffffff81267ed0>] strlen+0x0/0x30
 RSP <ffff88003b639c70>
---[ end trace 210f883ef80376aa ]---

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>