ALSA: usb-audio: Fix use-after-free of usb_device at disconnect

The usb-audio driver implements the deferred device disconnection for
the device in use.  In this mode, the disconnection callback returns
immediately while the actual ALSA card object removal happens later
when all files get closed.  As Shuah reported, this code flow,
however, leads to a use-after-free, detected by KASAN:

 BUG: KASAN: use-after-free in snd_usb_audio_free+0x134/0x160 [snd_usb_audio] at addr ffff8801c863ce10
 Write of size 8 by task pulseaudio/2244
 Call Trace:
  [<ffffffff81b31473>] dump_stack+0x67/0x94
  [<ffffffff81564ef1>] kasan_object_err+0x21/0x70
  [<ffffffff8156518a>] kasan_report_error+0x1fa/0x4e0
  [<ffffffff81564ad7>] ? kasan_slab_free+0x87/0xb0
  [<ffffffff81565733>] __asan_report_store8_noabort+0x43/0x50
  [<ffffffffa0fc0f54>] ? snd_usb_audio_free+0x134/0x160 [snd_usb_audio]
  [<ffffffffa0fc0f54>] snd_usb_audio_free+0x134/0x160 [snd_usb_audio]
  [<ffffffffa0fc0fb1>] snd_usb_audio_dev_free+0x31/0x40 [snd_usb_audio]
  [<ffffffff8243c78a>] __snd_device_free+0x12a/0x210
  [<ffffffff8243d1f5>] snd_device_free_all+0x85/0xd0
  [<ffffffff8242cae4>] release_card_device+0x34/0x130
  [<ffffffff81ef1846>] device_release+0x76/0x1e0
  [<ffffffff81b37ad7>] kobject_release+0x107/0x370
  .....
 Object at ffff8801c863cc80, in cache kmalloc-2048 size: 2048
 Allocated:
  [<ffffffff810804eb>] save_stack_trace+0x2b/0x50
  [<ffffffff81564296>] save_stack+0x46/0xd0
  [<ffffffff8156450d>] kasan_kmalloc+0xad/0xe0
  [<ffffffff81560d1a>] kmem_cache_alloc_trace+0xfa/0x240
  [<ffffffff8214ea47>] usb_alloc_dev+0x57/0xc90
  [<ffffffff8216349d>] hub_event+0xf1d/0x35f0
  ....
 Freed:
  [<ffffffff810804eb>] save_stack_trace+0x2b/0x50
  [<ffffffff81564296>] save_stack+0x46/0xd0
  [<ffffffff81564ac1>] kasan_slab_free+0x71/0xb0
  [<ffffffff81560929>] kfree+0xd9/0x280
  [<ffffffff8214de6e>] usb_release_dev+0xde/0x110
  [<ffffffff81ef1846>] device_release+0x76/0x1e0
  ....

It's the code trying to clear drvdata of the assigned usb_device where
the usb_device itself was already released in usb_release_dev() after
the disconnect callback.

This patch fixes it by checking whether the code path is via the
disconnect callback, i.e. chip->shutdown flag is set.

Fixes: 79289e24 ('ALSA: usb-audio: Refer to chip->usb_id for quirks...')
Reported-and-tested-by: Shuah Khan <shuahkh@osg.samsung.com>
Cc: <stable@vger.kernel.org> # v4.6+
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/card.c

@@ -315,6 +315,7 @@ static int snd_usb_audio_free(struct snd_usb_audio *chip)
 		snd_usb_endpoint_free(ep);
 
 	mutex_destroy(&chip->mutex);
+	if (!atomic_read(&chip->shutdown))
 		dev_set_drvdata(&chip->dev->dev, NULL);
 	kfree(chip);
 	return 0;