linux-kernel - [PATCH] media: usbtv: prevent access to free'd resources

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Message-Id: <20160708120427.29581-1-matthew@mattleach.net>
Date:	Fri,  8 Jul 2016 13:04:27 +0100
From:	Matthew Leach <matthew@...tleach.net>
To:	Mauro Carvalho Chehab <mchehab@....samsung.com>
Cc:	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peter Sutton <foxxy@...dogstudios.com>,
	Matthew Leach <matthew@...tleach.net>, stable@...r.kernel.org
Subject: [PATCH] media: usbtv: prevent access to free'd resources

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 ]---

Signed-off-by: Matthew Leach <matthew@...tleach.net>
Tested-by: Peter Sutton <foxxy@...dogstudios.com>
Cc: stable@...r.kernel.org
---
 drivers/media/usb/usbtv/usbtv-audio.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/media/usb/usbtv/usbtv-audio.c b/drivers/media/usb/usbtv/usbtv-audio.c
index 78c12d2..5dab024 100644
--- a/drivers/media/usb/usbtv/usbtv-audio.c
+++ b/drivers/media/usb/usbtv/usbtv-audio.c
@@ -278,6 +278,9 @@ static void snd_usbtv_trigger(struct work_struct *work)
 {
 	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;
-- 
2.9.0