media: em28xx: fix use-after-free in em28xx_v4l2_open()

media: em28xx: fix use-after-free in em28xx_v4l2_open()

commit a66485a934c7187ae8e36517d40615fa2e961cff upstream.

em28xx_v4l2_open() reads dev->v4l2 without holding dev->lock,
creating a race with em28xx_v4l2_init()'s error path and
em28xx_v4l2_fini(), both of which free the em28xx_v4l2 struct
and set dev->v4l2 to NULL under dev->lock.

This race leads to two issues:
 - use-after-free in v4l2_fh_init() when accessing vdev->ctrl_handler,
   since the video_device is embedded in the freed em28xx_v4l2 struct.
 - NULL pointer dereference in em28xx_resolution_set() when accessing
   v4l2->norm, since dev->v4l2 has been set to NULL.

Fix this by moving the mutex_lock() before the dev->v4l2 read and
adding a NULL check for dev->v4l2 under the lock.

Reported-by: syzbot+c025d34b8eaa54c571b8@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c025d34b8eaa54c571b8
Fixes: 8139a4d583ab ("[media] em28xx: move v4l2 user counting fields from struct em28xx to struct v4l2")
Cc: stable@vger.kernel.org
Signed-off-by: Abhishek Kumar <abhishek_sts8@yahoo.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>