Re: [RFT PATCH] media: em28xx: Enable v4l2 file ops at the end of em28xx_v4l2_init()

[Soumya Negi <soumya.negi97@gmail.com>]

On Tue, Sep 06, 2022 at 03:25:58PM +0200, Hans Verkuil wrote:
> Hi Soumya,
> 
> On 29/08/2022 15:25, Soumya Negi wrote:
> > Fix syzbot bug KASAN: use-after-free Read in v4l2_fh_open
> > Syzbot link:
> > https://syzkaller.appspot.com/bug?id=0e3c97f1c4112e102c9988afd5eff079350eab7f
> > Repro: https://syzkaller.appspot.com/text?tag=ReproC&x=12663ebdd00000
> > 
> > The bug arises because the em28xx driver registers a v4l2 video
> > device(struct video_device) with the V4L2 interface in
> > em28xx_v4l2_init() but the v4l2 extension initialization eventually
> > fails and the video device is unregistered. v4l2_open() in the V4L2
> > interface views the device as registered and allows calls to
> > em28xx_v4l2_open(). Due to race between video_unregister_device() and
> > v4l2_open(), em28xx_v4l2_open() is accessing video device after it has
> > been freed(by the release callback) and is passing it on to
> > v4l2_fh_open().
> > 
> > In em28xx_v4l2_init(), temporarily disable v4l2 file operations on
> > struct video_device devices(video, vbi, radio) before registering them
> > and enable the file ops at the end when v4l2 extension initialization
> > succeeds.
> 
> That's not the right approach. The problem is the roll-your-own v4l2->ref
> refcount. Instead this should use the refcount that is built into struct
> v4l2_device.
> 
> Specifically v4l2_dev->release should be set to the release function, and
> v4l2_device_put(&v4l2->v4l2_dev); should be called in remove() or in the
> error path of em28xx_v4l2_init().
> 
> Using this the release callback of v4l2_device will be called when the last
> open file handle is closed, i.e. it keeps track of the open()s.
> 
> The roll-your-own approach in the em28xx driver (written before this v4l2_device
> refcounting existed), is not taking that into account, so that causes these
> problems.
> 
> What is also bad is that em28xx_vb2_setup() is called after the video devices
> are registered. That should happen before.
> 
> Regards,
> 
> 	Hans

Hi Hans,
Thank you for explaining both the issues in detail. I will get working
on both and get back to you. 

Regards,
Soumya

> > 
> > Reported-by: syzbot+b2391895514ed9ef4a8e@syzkaller.appspotmail.com
> > Signed-off-by: Soumya Negi <soumya.negi97@gmail.com>
> > ---
> >  drivers/media/usb/em28xx/em28xx-video.c | 45 +++++++++++++++++++++----
> >  1 file changed, 39 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/media/usb/em28xx/em28xx-video.c b/drivers/media/usb/em28xx/em28xx-video.c
> > index 8181c0e6a25b..900a1eacb1c7 100644
> > --- a/drivers/media/usb/em28xx/em28xx-video.c
> > +++ b/drivers/media/usb/em28xx/em28xx-video.c
> > @@ -2320,6 +2320,19 @@ static int em28xx_v4l2_close(struct file *filp)
> >  	return 0;
> >  }
> >  
> > +/* Used to temporarily disable file operations on video_device until successful
> > + * initialization in em28xx_v4l2_init().
> > + */
> > +static const struct v4l2_file_operations em28xx_v4l_fops_empty = {
> > +	.owner         = THIS_MODULE,
> > +	.open          = NULL,
> > +	.release       = NULL,
> > +	.read          = NULL,
> > +	.poll          = NULL,
> > +	.mmap          = NULL,
> > +	.unlocked_ioctl = NULL,
> > +};
> > +
> >  static const struct v4l2_file_operations em28xx_v4l_fops = {
> >  	.owner         = THIS_MODULE,
> >  	.open          = em28xx_v4l2_open,
> > @@ -2375,12 +2388,22 @@ static const struct v4l2_ioctl_ops video_ioctl_ops = {
> >  };
> >  
> >  static const struct video_device em28xx_video_template = {
> > -	.fops		= &em28xx_v4l_fops,
> > +	.fops		= &em28xx_v4l_fops_empty,
> >  	.ioctl_ops	= &video_ioctl_ops,
> >  	.release	= video_device_release_empty,
> >  	.tvnorms	= V4L2_STD_ALL,
> >  };
> >  
> > +/* Used to temporarily disable file operations on video_device until successful
> > + * initialization in em28xx_v4l2_init().
> > + */
> > +static const struct v4l2_file_operations radio_fops_empty = {
> > +	.owner         = THIS_MODULE,
> > +	.open          = NULL,
> > +	.release       = NULL,
> > +	.unlocked_ioctl = NULL,
> > +};
> > +
> >  static const struct v4l2_file_operations radio_fops = {
> >  	.owner         = THIS_MODULE,
> >  	.open          = em28xx_v4l2_open,
> > @@ -2404,7 +2427,7 @@ static const struct v4l2_ioctl_ops radio_ioctl_ops = {
> >  };
> >  
> >  static struct video_device em28xx_radio_template = {
> > -	.fops		= &radio_fops,
> > +	.fops		= &radio_fops_empty,
> >  	.ioctl_ops	= &radio_ioctl_ops,
> >  	.release	= video_device_release_empty,
> >  };
> > @@ -2833,9 +2856,6 @@ static int em28xx_v4l2_init(struct em28xx *dev)
> >  				"can't register radio device\n");
> >  			goto unregister_dev;
> >  		}
> > -		dev_info(&dev->intf->dev,
> > -			 "Registered radio device as %s\n",
> > -			 video_device_node_name(&v4l2->radio_dev));
> >  	}
> >  
> >  	/* Init entities at the Media Controller */
> > @@ -2851,14 +2871,27 @@ static int em28xx_v4l2_init(struct em28xx *dev)
> >  	}
> >  #endif
> >  
> > +	/* Enable v4l2 file operations for v4l2 video video_device */
> > +	v4l2->vdev.fops = &em28xx_v4l_fops;
> >  	dev_info(&dev->intf->dev,
> >  		 "V4L2 video device registered as %s\n",
> >  		 video_device_node_name(&v4l2->vdev));
> >  
> > -	if (video_is_registered(&v4l2->vbi_dev))
> > +	if (video_is_registered(&v4l2->vbi_dev)) {
> > +		/* Enable v4l2 file operations for v4l2 vbi video_device */
> > +		v4l2->vbi_dev.fops = &em28xx_v4l_fops;
> >  		dev_info(&dev->intf->dev,
> >  			 "V4L2 VBI device registered as %s\n",
> >  			 video_device_node_name(&v4l2->vbi_dev));
> > +	}
> > +
> > +	if (video_is_registered(&v4l2->radio_dev)) {
> > +		/* Enable v4l2 file operations for v4l2 radio video_device */
> > +		v4l2->radio_dev.fops = &radio_fops;
> > +		dev_info(&dev->intf->dev,
> > +			 "Registered radio device as %s\n",
> > +			 video_device_node_name(&v4l2->radio_dev));
> > +	}
> >  
> >  	/* Save some power by putting tuner to sleep */
> >  	v4l2_device_call_all(&v4l2->v4l2_dev, 0, tuner, standby);
> 
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