[PATCH] [media] sound/usb: fix NULL dereference in usb_audio_probe()

[Nicolai Stange <nicstange@gmail.com>]

With commit

  aebb2b89bff0 ("[media] sound/usb: Use Media Controller API to share
                 media resources")

an access to quirk->media_device without checking for quirk != NULL has
been introduced in usb_audio_probe().

With a Plantronics USB headset (device ID 0x047f:0xc010) attached,
this results in the following splat at boot time:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000014
  IP: [<ffffffffa089aa6c>] usb_audio_probe+0x2cc/0x9a0 [snd_usb_audio]
  Oops: 0000 [#1] SMP
  [...]
  CPU: 2 PID: 696 Comm: systemd-udevd Not tainted 4.5.0-next-20160315 #13
  Hardware name: Dell Inc. Latitude E6540/0725FP, BIOS A10 06/26/2014
  task: ffff88021c88d7c0 ti: ffff88003d5b0000 task.ti: ffff88003d5b0000
  RIP: 0010:[<ffffffffa089aa6c>]  [<ffffffffa089aa6c>]
                                usb_audio_probe+0x2cc/0x9a0 [snd_usb_audio]
  [...]
  Call Trace:
   [<ffffffff815a8e16>] usb_probe_interface+0x136/0x2d0
   [<ffffffff81509edc>] driver_probe_device+0x22c/0x440
   [<ffffffff8150a1c1>] __driver_attach+0xd1/0xf0
   [<ffffffff8150a0f0>] ? driver_probe_device+0x440/0x440
   [<ffffffff815077ec>] bus_for_each_dev+0x6c/0xc0
   [<ffffffff815095ce>] driver_attach+0x1e/0x20
   [<ffffffff81509013>] bus_add_driver+0x1c3/0x280
   [<ffffffff8150ab10>] driver_register+0x60/0xe0
   [<ffffffff815a7711>] usb_register_driver+0x81/0x140
   [<ffffffffa08c7000>] ? 0xffffffffa08c7000
   [<ffffffffa08c701e>] usb_audio_driver_init+0x1e/0x1000 [snd_usb_audio]
   [<ffffffff81002123>] do_one_initcall+0xb3/0x1f0
   [<ffffffff811fb091>] ? __vunmap+0x81/0xd0
   [<ffffffff8121b8d2>] ? kmem_cache_alloc_trace+0x182/0x1d0
   [<ffffffff811b0267>] ? do_init_module+0x27/0x1d8
   [<ffffffff811b029f>] do_init_module+0x5f/0x1d8
   [<ffffffff8112ce35>] load_module+0x1fe5/0x27a0
   [<ffffffff81129870>] ? __symbol_put+0x60/0x60
   [<ffffffff81241690>] ? vfs_read+0x110/0x130
   [<ffffffff8112d866>] SYSC_finit_module+0xe6/0x120
   [<ffffffff8112d8be>] SyS_finit_module+0xe/0x10
   [<ffffffff81003d94>] do_syscall_64+0x64/0x110
   [<ffffffff817c0b61>] entry_SYSCALL64_slow_path+0x25/0x25

After encountering this, the system-udevd process seems to be blocked
until it is killed when hitting its timeout of 3min.

In analogy to the other accesses to members of quirk in usb_audio_probe(),
check for quirk != NULL before accessing its ->media_device.

Fixes: aebb2b89bff0 ("[media] sound/usb: Use Media Controller API to share
                      media resources")
Signed-off-by: Nicolai Stange <nicstange@gmail.com>
---
 Applicable to linux-next-20160315.

 sound/usb/card.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sound/usb/card.c b/sound/usb/card.c
index 63244bb..479621e 100644
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -612,7 +612,7 @@ static int usb_audio_probe(struct usb_interface *intf,
 	if (err < 0)
 		goto __error;
 
-	if (quirk->media_device) {
+	if (quirk && quirk->media_device) {
 		/* don't want to fail when media_snd_device_create() fails */
 		media_snd_device_create(chip, intf);
 	}
-- 
2.7.2