Message-ID: <CANiDSCsP74bJJZcT8cKUWCOcTT1h3RiPiFKdYfFNQ-WRkBugEA@mail.gmail.com>
Date: Mon, 25 Mar 2024 17:37:43 +0100
From: Ricardo Ribalda <ribalda@...omium.org>
To: Hans Verkuil <hverkuil-cisco@...all.nl>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>, Guenter Roeck <linux@...ck-us.net>, 
	Tomasz Figa <tfiga@...omium.org>, Laurent Pinchart <laurent.pinchart@...asonboard.com>, 
	Alan Stern <stern@...land.harvard.edu>, linux-media@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Sean Paul <seanpaul@...omium.org>, 
	Sakari Ailus <sakari.ailus@...ux.intel.com>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH v5 1/3] media: uvcvideo: Lock video streams and queues
 while unregistering

Hi Hans

On Thu, 21 Mar 2024 at 16:47, Hans Verkuil <hverkuil-cisco@...all.nl> wrote:
>
> Hi Ricardo,
>
> On 22/11/2023 12:45 pm, Ricardo Ribalda wrote:
> > From: Guenter Roeck <linux@...ck-us.net>
> >
> > 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@...asonboard.com>
> > Cc: Alan Stern <stern@...land.harvard.edu>
> > Cc: Hans Verkuil <hverkuil-cisco@...all.nl>
> > Reviewed-by: Tomasz Figa <tfiga@...omium.org>
> > Reviewed-by: Sean Paul <seanpaul@...omium.org>
> > Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> > Reviewed-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > ---
> >  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 08fcd2ffa727..ded2cb6ce14f 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);
>
> Part of the problem here is that video_unregister_device does not stop streaming.
>
> For 'normal' drivers that leave most of the locking to the core framework and
> that use the standard vb2_fop_* and vb2_ioctl_* helpers, instead of calling
> video_unregister_device, they call vb2_video_unregister_device(). This will
> safely unregister the device and stop streaming at the same time.
>
> Since after the device is unregistered the only file operation that is accepted
> is close(), it will be impossible to restart streaming.
>
> In other words, this guarantees that when the disconnect function exits there
> is nothing streaming anymore.

I have tested this approach and it seems to work :)

I have updated and sent for review, and in the meantime I will
continue torturing my device to figure out if we are missing any other
race.

Thanks!

>
> This makes it much easier to deal with disconnects, and I think this should
> happen here as well. I wonder if this will obsolete patch 3/3 and perhaps
> 2/3.
>
> I don't think taking the queue.mutex is needed, especially if you stop
> streaming here.
>
> Regards,
>
>         Hans
>
> >       }
> >
> >       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,
> >
>


-- 
Ricardo Ribalda

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