rfkill: Fix use-after-free in rfkill_resume()

If a device is getting removed or reprobed during resume, use-after-free
might happen. For example, h5_btrtl_resume() schedules a work queue for
device reprobing, which of course requires removal first.

If the removal happens in parallel with the device_resume() and wins the
race to acquire device_lock(), removal may remove the device from the PM
lists and all, but device_resume() is already running and will continue
when the lock can be acquired, thus calling rfkill_resume().

During this, if rfkill_set_block() is then called after the corresponding
*_unregister() and kfree() are called, there will be an use-after-free
in hci_rfkill_set_block():

BUG: KASAN: use-after-free in hci_rfkill_set_block+0x58/0xc0 [bluetooth]
...
Call trace:
  dump_backtrace+0x0/0x154
  show_stack+0x20/0x2c
  dump_stack+0xbc/0x12c
  print_address_description+0x88/0x4b0
  __kasan_report+0x144/0x168
  kasan_report+0x10/0x18
  check_memory_region+0x19c/0x1ac
  __kasan_check_write+0x18/0x24
  hci_rfkill_set_block+0x58/0xc0 [bluetooth]
  rfkill_set_block+0x9c/0x120
  rfkill_resume+0x34/0x70
  dpm_run_callback+0xf0/0x1f4
  device_resume+0x210/0x22c

Fix this by checking rfkill->registered in rfkill_resume(). device_del()
in rfkill_unregister() requires device_lock() and the whole rfkill_resume()
is also protected by the same lock via device_resume(), we can make sure
either the rfkill->registered is false before rfkill_resume() starts or the
rfkill device won't be unregistered before rfkill_resume() returns.

As async_resume() holds a reference to the device, at this level there can
be no use-after-free; only in the user that doesn't expect this scenario.

Fixes: 8589086f ("Bluetooth: hci_h5: Turn off RTL8723BS on suspend, reprobe on resume")
Signed-off-by: Claire Chang <tientzu@chromium.org>
Link: https://lore.kernel.org/r/20201110084908.219088-1-tientzu@chromium.org
[edit commit message for clarity and add more info provided later]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/rfkill/core.c

@@ -876,6 +876,9 @@ static int rfkill_resume(struct device *dev)
 
 	rfkill->suspended = false;
 
+	if (!rfkill->registered)
+		return 0;
+
 	if (!rfkill->persistent) {
 		cur = !!(rfkill->state & RFKILL_BLOCK_SW);
 		rfkill_set_block(rfkill, cur);