media: vivid: Fix wrong locking that causes race conditions on streaming stop

There is the same incorrect approach to locking implemented in
vivid_stop_generating_vid_cap(), vivid_stop_generating_vid_out() and
sdr_cap_stop_streaming().

These functions are called during streaming stopping with vivid_dev.mutex
locked. And they all do the same mistake while stopping their kthreads,
which need to lock this mutex as well. See the example from
vivid_stop_generating_vid_cap():
  /* shutdown control thread */
  vivid_grab_controls(dev, false);
  mutex_unlock(&dev->mutex);
  kthread_stop(dev->kthread_vid_cap);
  dev->kthread_vid_cap = NULL;
  mutex_lock(&dev->mutex);

But when this mutex is unlocked, another vb2_fop_read() can lock it
instead of vivid_thread_vid_cap() and manipulate the buffer queue.
That causes a use-after-free access later.

To fix those issues let's:
  1. avoid unlocking the mutex in vivid_stop_generating_vid_cap(),
vivid_stop_generating_vid_out() and sdr_cap_stop_streaming();
  2. use mutex_trylock() with schedule_timeout_uninterruptible() in
the loops of the vivid kthread handlers.
Signed-off-by: Alexander Popov <alex.popov@linux.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Tested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: <stable@vger.kernel.org>      # for v3.18 and up
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/platform/vivid/vivid-kthread-cap.c

@@ -818,7 +818,11 @@ static int vivid_thread_vid_cap(void *data)
 		if (kthread_should_stop())
 			break;
 
-		mutex_lock(&dev->mutex);
+		if (!mutex_trylock(&dev->mutex)) {
+			schedule_timeout_uninterruptible(1);
+			continue;
+		}
+
 		cur_jiffies = jiffies;
 		if (dev->cap_seq_resync) {
 			dev->jiffies_vid_cap = cur_jiffies;
@@ -998,8 +1002,6 @@ void vivid_stop_generating_vid_cap(struct vivid_dev *dev, bool *pstreaming)
 
 	/* shutdown control thread */
 	vivid_grab_controls(dev, false);
-	mutex_unlock(&dev->mutex);
 	kthread_stop(dev->kthread_vid_cap);
 	dev->kthread_vid_cap = NULL;
-	mutex_lock(&dev->mutex);
 }

drivers/media/platform/vivid/vivid-kthread-out.c

@@ -166,7 +166,11 @@ static int vivid_thread_vid_out(void *data)
 		if (kthread_should_stop())
 			break;
 
-		mutex_lock(&dev->mutex);
+		if (!mutex_trylock(&dev->mutex)) {
+			schedule_timeout_uninterruptible(1);
+			continue;
+		}
+
 		cur_jiffies = jiffies;
 		if (dev->out_seq_resync) {
 			dev->jiffies_vid_out = cur_jiffies;
@@ -344,8 +348,6 @@ void vivid_stop_generating_vid_out(struct vivid_dev *dev, bool *pstreaming)
 
 	/* shutdown control thread */
 	vivid_grab_controls(dev, false);
-	mutex_unlock(&dev->mutex);
 	kthread_stop(dev->kthread_vid_out);
 	dev->kthread_vid_out = NULL;
-	mutex_lock(&dev->mutex);
 }

drivers/media/platform/vivid/vivid-sdr-cap.c

@@ -141,7 +141,11 @@ static int vivid_thread_sdr_cap(void *data)
 		if (kthread_should_stop())
 			break;
 
-		mutex_lock(&dev->mutex);
+		if (!mutex_trylock(&dev->mutex)) {
+			schedule_timeout_uninterruptible(1);
+			continue;
+		}
+
 		cur_jiffies = jiffies;
 		if (dev->sdr_cap_seq_resync) {
 			dev->jiffies_sdr_cap = cur_jiffies;
@@ -303,10 +307,8 @@ static void sdr_cap_stop_streaming(struct vb2_queue *vq)
 	}
 
 	/* shutdown control thread */
-	mutex_unlock(&dev->mutex);
 	kthread_stop(dev->kthread_sdr_cap);
 	dev->kthread_sdr_cap = NULL;
-	mutex_lock(&dev->mutex);
 }
 
 static void sdr_cap_buf_request_complete(struct vb2_buffer *vb)