Patch "[media] media: usbtv: prevent access to free'd resources" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

[media] media: usbtv: prevent access to free'd resources

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
media-usbtv-prevent-access-to-free-d-resources.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Matthew Leach <[email protected]>
Date: Fri, 8 Jul 2016 09:04:27 -0300
Subject: [media] media: usbtv: prevent access to free'd resources

From: Matthew Leach <[email protected]>

commit 2a00932f082aff93c3a55426e0c7af6d0ec03997 upstream.

When disconnecting the usbtv device, the sound card is unregistered from ALSA and the snd member of the usbtv struct is set to NULL. If the usbtv snd_trigger work is running, this can cause a race condition where the kernel will attempt to access free'd resources, shown in [1].

This patch fixes the disconnection code by cancelling any snd_trigger work before unregistering the sound card from ALSA and checking that the snd member still exists in the work function.

[1]:
usb 3-1.2: USB disconnect, device number 6

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
 IP: [<ffffffff81093850>] process_one_work+0x30/0x480
 PGD 405bbf067 PUD 405bbe067 PMD 0
 Call Trace:
  [<ffffffff81093ce8>] worker_thread+0x48/0x4e0
  [<ffffffff81093ca0>] ? process_one_work+0x480/0x480
  [<ffffffff81093ca0>] ? process_one_work+0x480/0x480
  [<ffffffff81099998>] kthread+0xd8/0xf0
  [<ffffffff815c73c2>] ret_from_fork+0x22/0x40
  [<ffffffff810998c0>] ? kthread_worker_fn+0x170/0x170
 ---[ end trace 0f3dac5c1a38e610 ]---

---
 drivers/media/usb/usbtv/usbtv-audio.c |    5 +++++
 1 file changed, 5 insertions(+)

@@ -278,6 +278,9 @@ static void snd_usbtv_trigger(struct wor
 {
 	struct usbtv *chip = container_of(work, struct usbtv, snd_trigger);
 
+	if (!chip->snd)
+		return;
+
 	if (atomic_read(&chip->snd_stream))
 		usbtv_audio_start(chip);
 	else

@@ -378,6 +381,8 @@ err:
 
 void usbtv_audio_free(struct usbtv *usbtv)
 {
+	cancel_work_sync(&usbtv->snd_trigger);
+
 	if (usbtv->snd && usbtv->udev) {
 		snd_card_free(usbtv->snd);
 		usbtv->snd = NULL;

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/media-usbtv-prevent-access-to-free-d-resources.patch