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 > > Reported-by: syzbot+b2391895514ed9ef4a8e@xxxxxxxxxxxxxxxxxxxxxxxxx > Signed-off-by: Soumya Negi <soumya.negi97@xxxxxxxxx> > --- > 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);
