[media] v4l: vb2: Fix race condition in vb2_fop_poll

The vb2_fop_poll() implementation tries to be clever on whether it needs
to lock the queue mutex by checking whether polling might start fileio.
The test requires reading the q->num_buffer field, which is racy if we
don't hold the queue mutex in the first place.

Remove the extra cleverness and just lock the mutex.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

1 files changed, 8 insertions, 19 deletions

diff --git a/drivers/media/v4l2-core/videobuf2-core.c b/drivers/media/v4l2-core/videobuf2-core.c
index 7aed8f22e075..2685670b20ec 100644
--- a/drivers/media/v4l2-core/videobuf2-core.c
+++ b/drivers/media/v4l2-core/videobuf2-core.c
@@ -3459,27 +3459,16 @@ unsigned int vb2_fop_poll(struct file *file, poll_table *wait)
         struct video_device *vdev = video_devdata(file);
         struct vb2_queue *q = vdev->queue;
         struct mutex *lock = q->lock ? q->lock : vdev->lock;
-        unsigned long req_events = poll_requested_events(wait);
         unsigned res;
         void *fileio;
-        bool must_lock = false;
-
-        /* Try to be smart: only lock if polling might start fileio,
-           otherwise locking will only introduce unwanted delays. */
-        if (q->num_buffers == 0 && !vb2_fileio_is_active(q)) {
-                if (!V4L2_TYPE_IS_OUTPUT(q->type) && (q->io_modes & VB2_READ) &&
-                                (req_events & (POLLIN | POLLRDNORM)))
-                        must_lock = true;
-                else if (V4L2_TYPE_IS_OUTPUT(q->type) && (q->io_modes & VB2_WRITE) &&
-                                (req_events & (POLLOUT | POLLWRNORM)))
-                        must_lock = true;
-        }
 
-        /* If locking is needed, but this helper doesn't know how, then you
-           shouldn't be using this helper but you should write your own. */
-        WARN_ON(must_lock && !lock);
+        /*
+         * If this helper doesn't know how to lock, then you shouldn't be using
+         * it but you should write your own.
+         */
+        WARN_ON(!lock);
 
-        if (must_lock && lock && mutex_lock_interruptible(lock))
+        if (lock && mutex_lock_interruptible(lock))
                 return POLLERR;
 
         fileio = q->fileio;
@@ -3487,9 +3476,9 @@ unsigned int vb2_fop_poll(struct file *file, poll_table *wait)
         res = vb2_poll(vdev->queue, file, wait);
 
         /* If fileio was started, then we have a new queue owner. */
-        if (must_lock && !fileio && q->fileio)
+        if (!fileio && q->fileio)
                 q->owner = file->private_data;
-        if (must_lock && lock)
+        if (lock)
                 mutex_unlock(lock);
         return res;
 }