Re: [PATCH v3 3/3] media: uvcvideo: Lock video streams and queues while unregistering

[Ricardo Ribalda]

Hi Laurent

On Wed, 22 Nov 2023 at 14:14, Laurent Pinchart
<laurent.pinchart@xxxxxxxxxxxxxxxx> wrote:
>
> CC'ing Dan Williams.
>
> On Wed, Nov 22, 2023 at 10:21:04AM +0000, Sakari Ailus wrote:
> > Hi Ricardo,
> >
> > On Tue, Nov 21, 2023 at 07:53:50PM +0000, Ricardo Ribalda wrote:
> > > From: Guenter Roeck <linux@xxxxxxxxxxxx>
> > >
> > > The call to uvc_disconnect() is not protected by any mutex.
> > > This means it can and will be called while other accesses to the video
> > > device are in progress. This can cause all kinds of race conditions,
> > > including crashes such as the following.
> > >
> > > usb 1-4: USB disconnect, device number 3
> > > BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
> > > PGD 0 P4D 0
> > > Oops: 0000 [#1] PREEMPT SMP PTI
> > > CPU: 0 PID: 5633 Comm: V4L2CaptureThre Not tainted 4.19.113-08536-g5d29ca36db06 #1
> > > Hardware name: GOOGLE Edgar, BIOS Google_Edgar.7287.167.156 03/25/2019
> > > RIP: 0010:usb_ifnum_to_if+0x29/0x40
> > > Code: <...>
> > > RSP: 0018:ffffa46f42a47a80 EFLAGS: 00010246
> > > RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff904a396c9000
> > > RDX: ffff904a39641320 RSI: 0000000000000001 RDI: 0000000000000000
> > > RBP: ffffa46f42a47a80 R08: 0000000000000002 R09: 0000000000000000
> > > R10: 0000000000009975 R11: 0000000000000009 R12: 0000000000000000
> > > R13: ffff904a396b3800 R14: ffff904a39e88000 R15: 0000000000000000
> > > FS: 00007f396448e700(0000) GS:ffff904a3ba00000(0000) knlGS:0000000000000000
> > > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 0000000000000000 CR3: 000000016cb46000 CR4: 00000000001006f0
> > > Call Trace:
> > >  usb_hcd_alloc_bandwidth+0x1ee/0x30f
> > >  usb_set_interface+0x1a3/0x2b7
> > >  uvc_video_start_transfer+0x29b/0x4b8 [uvcvideo]
> > >  uvc_video_start_streaming+0x91/0xdd [uvcvideo]
> > >  uvc_start_streaming+0x28/0x5d [uvcvideo]
> > >  vb2_start_streaming+0x61/0x143 [videobuf2_common]
> > >  vb2_core_streamon+0xf7/0x10f [videobuf2_common]
> > >  uvc_queue_streamon+0x2e/0x41 [uvcvideo]
> > >  uvc_ioctl_streamon+0x42/0x5c [uvcvideo]
> > >  __video_do_ioctl+0x33d/0x42a
> > >  video_usercopy+0x34e/0x5ff
> > >  ? video_ioctl2+0x16/0x16
> > >  v4l2_ioctl+0x46/0x53
> > >  do_vfs_ioctl+0x50a/0x76f
> > >  ksys_ioctl+0x58/0x83
> > >  __x64_sys_ioctl+0x1a/0x1e
> > >  do_syscall_64+0x54/0xde
> > >
> > > usb_set_interface() should not be called after the USB device has been
> > > unregistered. However, in the above case the disconnect happened after
> > > v4l2_ioctl() was called, but before the call to usb_ifnum_to_if().
> > >
> > > Acquire various mutexes in uvc_unregister_video() to fix the majority
> > > (maybe all) of the observed race conditions.
> > >
> > > The uvc_device lock prevents races against suspend and resume calls
> > > and the poll function.
> > >
> > > The uvc_streaming lock prevents races against stream related functions;
> > > for the most part, those are ioctls. This lock also requires other
> > > functions using this lock to check if a video device is still registered
> > > after acquiring it. For example, it was observed that the video device
> > > was already unregistered by the time the stream lock was acquired in
> > > uvc_ioctl_streamon().
> > >
> > > The uvc_queue lock prevents races against queue functions, Most of
> > > those are already protected by the uvc_streaming lock, but some
> > > are called directly. This is done as added protection; an actual race
> > > was not (yet) observed.
> > >
> > > Cc: Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
> > > Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> > > Cc: Hans Verkuil <hverkuil-cisco@xxxxxxxxx>
> > > Reviewed-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> > > Reviewed-by: Sean Paul <seanpaul@xxxxxxxxxxxx>
> > > Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > > Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
> > > ---
> > >  drivers/media/usb/uvc/uvc_driver.c | 9 +++++++++
> > >  1 file changed, 9 insertions(+)
> > >
> > > diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> > > index 413c32867617..3408b865d346 100644
> > > --- a/drivers/media/usb/uvc/uvc_driver.c
> > > +++ b/drivers/media/usb/uvc/uvc_driver.c
> > > @@ -1907,14 +1907,22 @@ static void uvc_unregister_video(struct uvc_device *dev)
> > >  {
> > >     struct uvc_streaming *stream;
> > >
> > > +   mutex_lock(&dev->lock);
> > > +
> > >     list_for_each_entry(stream, &dev->streams, list) {
> > >             if (!video_is_registered(&stream->vdev))
> > >                     continue;
> > >
> > > +           mutex_lock(&stream->mutex);
> > > +           mutex_lock(&stream->queue.mutex);
> > > +
> > >             video_unregister_device(&stream->vdev);
> > >             video_unregister_device(&stream->meta.vdev);
> > >
> > >             uvc_debugfs_cleanup_stream(stream);
> > > +
> > > +           mutex_unlock(&stream->queue.mutex);
> > > +           mutex_unlock(&stream->mutex);
> > >     }
> > >
> > >     uvc_status_unregister(dev);
> > > @@ -1925,6 +1933,7 @@ static void uvc_unregister_video(struct uvc_device *dev)
> > >     if (media_devnode_is_registered(dev->mdev.devnode))
> > >             media_device_unregister(&dev->mdev);
> > >  #endif
> > > +   mutex_unlock(&dev->lock);
> > >  }
> > >
> > >  int uvc_register_video_device(struct uvc_device *dev,
> > >
> >
> > Instead of acquiring all the possible locks, could you instead take the
> > same approach as you do with ISP? It should use refcount_t btw.
> > <URL:https://chromium.googlesource.com/chromiumos/third_party/kernel/+/refs/heads/kcam-6.1/drivers/isp/isp-device.c:
>
> The right approach, as I've said countless of times, is to fix this at
> the cdev and V4L2 level. Dan Williams has done some ground work on this
> a while ago ([1]), and before that I posted an RFC series that
> overlapped with Dan's work (with a more naive and less efficient
> implementation of refcounting, see [2]).
>
> [1] https://lore.kernel.org/all/161117153776.2853729.6944617921517514510.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> [2] https://lore.kernel.org/linux-media/20171116003349.19235-2-laurent.pinchart+renesas@xxxxxxxxxxxxxxxx/

The UVC driver is violating the USB driver API [1] causing crashes and
probably security vulnerabilities. It has to be fixed.

If there was a different way TODAY to do this, I would very happily
implement it differently. But your patchset is 6 years old and Dan's 2
and they have not landed. How many kernel versions is ok to put our
users willingly at risk?

This patch simply serialises unregister_video? How can this patch
affect the viability of your patchset or Dan's patch? If this patch is
not needed in the future we can simply revert it.

When/If there is a better way to implement this, I would very happily
send a follow-up patch to use that framework.

[1] https://www.kernel.org/doc/html/latest/driver-api/usb/callbacks.html#the-disconnect-callback









>
> --
> Regards,
>
> Laurent Pinchart



--
Ricardo Ribalda