[PATCH 1/2] media: em28xx: clear dev->v4l2 on v4l2 init failure

[Deepanshu Kartikey]

If em28xx_v4l2_init() fails after registering the video devices (for
example in v4l2_mc_create_media_graph()), the unregister_dev error path
drops the initial v4l2_device reference but leaves dev->v4l2 set.

If userspace still holds an open file handle to a video node, the
structure stays alive. On USB disconnect, em28xx_v4l2_fini() sees the
non-NULL dev->v4l2 and drops the initial reference a second time. The
refcount underflows, em28xx_free_v4l2() frees struct em28xx_v4l2, and
the later close() makes v4l2_device_release() touch a media entity in
freed memory:

  BUG: KASAN: slab-use-after-free in dev_dbg_obj+0x630/0x830
  Read of size 4 at addr ffff888023601bd0 by task v4l_id/6384

Clear dev->v4l2 in the error path after dropping the initial reference,
so that em28xx_v4l2_fini() returns early and does not put it again.

Fixes: 288254383674 ("media: em28xx: use v4l2_device release callback")
Reported-by: syzbot+54648a6c1187a7ef46a6@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=54648a6c1187a7ef46a6
Signed-off-by: Deepanshu Kartikey <kartikey406@xxxxxxxxx>
---
 drivers/media/usb/em28xx/em28xx-video.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/media/usb/em28xx/em28xx-video.c b/drivers/media/usb/em28xx/em28xx-video.c
index 84424284d486..ec4ed2b0e5fd 100644
--- a/drivers/media/usb/em28xx/em28xx-video.c
+++ b/drivers/media/usb/em28xx/em28xx-video.c
@@ -3137,6 +3137,7 @@ static int em28xx_v4l2_init(struct em28xx *dev)
 	 * v4l2_device refcount to zero.
 	 */
 	v4l2_device_put(&v4l2->v4l2_dev);
+	dev->v4l2 = NULL;
 	return ret;
 }
 
-- 
2.43.0