Re: [PATCH 1/2] media: em28xx-video: add the unregister of video/VBI entity

[Edward Adam Davis]

On Mon, 16 Mar 2026 15:03:23 +0100, Hans Verkuil wrote:
> On 11/01/2026 06:29, Edward Adam Davis wrote:
> > When creating a media graph, a failure occurred due to the lack of
> > a corresponding decoder. During the subsequent media device release
> > process, the video and VBI devices were not properly unregistered,
> > leading to a use-after-free vulnerability reported by syzbot [1].
> >
> > The fix involves adding the necessary unregister operations.
> >
> > [1]
> > BUG: KASAN: slab-use-after-free in media_device_unregister+0x141/0x430 drivers/media/mc/mc-device.c:804
> > Read of size 8 at addr ffff88807c114210 by task kworker/1:9/6093
> > Call Trace:
> >  media_device_unregister+0x141/0x430 drivers/media/mc/mc-device.c:804
> >  em28xx_unregister_media_device drivers/media/usb/em28xx/em28xx-cards.c:3511 [inline]
> >  em28xx_release_resources+0xac/0x240 drivers/media/usb/em28xx/em28xx-cards.c:3532
> >  em28xx_usb_disconnect+0x19f/0x2f0 drivers/media/usb/em28xx/em28xx-cards.c:4201
> >  usb_unbind_interface+0x26e/0x910 drivers/usb/core/driver.c:458
> >
> > Allocated by task 5932:
> >  em28xx_v4l2_init+0x10b/0x2e70 drivers/media/usb/em28xx/em28xx-video.c:2532
> >  em28xx_init_extension+0x120/0x1c0 drivers/media/usb/em28xx/em28xx-core.c:1117
> >
> > Freed by task 5932:
> >  em28xx_free_v4l2 drivers/media/usb/em28xx/em28xx-video.c:2118 [inline]
> >  kref_put include/linux/kref.h:65 [inline]
> >  em28xx_v4l2_init+0x1683/0x2e70 drivers/media/usb/em28xx/em28xx-video.c:2901
> >
> > Reported-by: syzbot+16062f26c6480975e5ed@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=16062f26c6480975e5ed
> > Tested-by: syzbot+16062f26c6480975e5ed@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
> > ---
> >  drivers/media/usb/em28xx/em28xx-video.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/media/usb/em28xx/em28xx-video.c b/drivers/media/usb/em28xx/em28xx-video.c
> > index 2dfa3242a7ab..45b68ebf2e9c 100644
> > --- a/drivers/media/usb/em28xx/em28xx-video.c
> > +++ b/drivers/media/usb/em28xx/em28xx-video.c
> > @@ -882,9 +882,12 @@ static void em28xx_v4l2_media_release(struct em28xx *dev)
> >
> >  	for (i = 0; i < MAX_EM28XX_INPUT; i++) {
> >  		if (!INPUT(i)->type)
> > -			return;
> > +			break;
> >  		media_device_unregister_entity(&dev->input_ent[i]);
> >  	}
> > +	media_device_unregister_entity(&dev->v4l2->vdev.entity);
> > +	if (em28xx_vbi_supported(dev))
> > +		media_device_unregister_entity(&dev->v4l2->vbi_dev.entity);
> 
> This is definitely wrong: these are registered and unregistered in v4l2-dev.c,
> so it makes no sense to unregister them here.
You are only half right. Under normal circumstances, these devices
would indeed be unregistered via media_device_unregister() in v4l2-dev.c.
However, did you notice that if em28xx_v4l2_init() fails to create
the media graph, it will release the corresponding v4l2, including
its members vdev and vbi_dev, before media_device_unregister() is called?

Furthermore, both vdev and vbi_dev are instances of struct video_device.
Since they are inserted into the media framework as entities, they are,
in essence, the corresponding video_device instances themselves (as the
first member of struct video_device is, in fact, the entity instance).

Therefore, the UAF vulnerability described in [1] is ultimately triggered
when media_device_unregister() attempts to remove the entity from the
mdev instance at a em28xx usb disconnect.

Why is the execution of media_device_unregister() delayed in this context?
There is a high probability that the open syscall acquires em28xx->lock
*before* the unregistration path for media devices can proceed. This
scenario is identical to the one described in patch [2]; consequently,
this issue can also be resolved by applying the fix provided in [2].

[2] https://lore.kernel.org/all/tencent_5DCCB375C3694964A3A5A44677775777E605@xxxxxx

BR,
Edward