Re: KASAN: use-after-free Read in v4l2_fh_open

[Hillf Danton]

On Mon, 15 Feb 2021 07:18:22 -0800
> syzbot found the following issue on:
> 
> HEAD commit:    291009f6 Merge tag 'pm-5.11-rc8' of git://git.kernel.org/p..
> git tree:       upstream
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17179dd4d00000

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git  master

--- x/drivers/media/usb/em28xx/em28xx-video.c
+++ y/drivers/media/usb/em28xx/em28xx-video.c
@@ -2152,6 +2152,10 @@ static int em28xx_v4l2_open(struct file
 	if (mutex_lock_interruptible(&dev->lock))
 		return -ERESTARTSYS;
 
+	if (!dev->v4l2) {
+		mutex_unlock(&dev->lock);
+		return -ENODEV;
+	}
 	ret = v4l2_fh_open(filp);
 	if (ret) {
 		dev_err(&dev->intf->dev,
@@ -2161,6 +2165,7 @@ static int em28xx_v4l2_open(struct file
 		return ret;
 	}
 
+	v4l2 = dev->v4l2;
 	if (v4l2->users == 0) {
 		em28xx_set_mode(dev, EM28XX_ANALOG_MODE);
 
@@ -2376,10 +2381,17 @@ static const struct v4l2_ioctl_ops video
 #endif
 };
 
+static void em28xx_video_device_release(struct video_device *vd)
+{
+	struct em28xx_v4l2 *v4l2 = container_of(vd, struct em28xx_v4l2, vdev);
+
+	kref_put(&v4l2->ref, em28xx_free_v4l2);
+}
+
 static const struct video_device em28xx_video_template = {
 	.fops		= &em28xx_v4l_fops,
 	.ioctl_ops	= &video_ioctl_ops,
-	.release	= video_device_release_empty,
+	.release	= em28xx_video_device_release,
 	.tvnorms	= V4L2_STD_ALL,
 };
 
@@ -2788,6 +2800,7 @@ static int em28xx_v4l2_init(struct em28x
 			"unable to register video device (error=%i).\n", ret);
 		goto unregister_dev;
 	}
+	kref_get(&v4l2->ref); /* put by video_unregister_device() */
 
 	/* Allocate and fill vbi video_device struct */
 	if (em28xx_vbi_supported(dev) == 1) {
--