em28xx_v4l2_open() reads dev->v4l2 without holding dev->lock,
creating a race with em28xx_v4l2_init()'s error path and
em28xx_v4l2_fini(), both of which free the em28xx_v4l2 struct
and set dev->v4l2 to NULL under dev->lock.

This race leads to two issues:
 - use-after-free in v4l2_fh_init() when accessing vdev->ctrl_handler,
   since the video_device is embedded in the freed em28xx_v4l2 struct.
 - NULL pointer dereference in em28xx_resolution_set() when accessing
   v4l2->norm, since dev->v4l2 has been set to NULL.

Fix this by moving the mutex_lock() before the dev->v4l2 read and
adding a NULL check for dev->v4l2 under the lock.

Reported-by: syzbot+c025d34b8eaa54c571b8@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=c025d34b8eaa54c571b8
Fixes: 8139a4d583ab ("[media] em28xx: move v4l2 user counting fields from struct em28xx to struct v4l2")
Cc: stable@vger.kernel.org
Signed-off-by: Abhishek Kumar <abhishek_sts8@yahoo.com>
---
 drivers/media/usb/em28xx/em28xx-video.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/media/usb/em28xx/em28xx-video.c b/drivers/media/usb/em28xx/em28xx-video.c
index b0c184f237a7..5f13f63fbdee 100644
--- a/drivers/media/usb/em28xx/em28xx-video.c
+++ b/drivers/media/usb/em28xx/em28xx-video.c
@@ -2126,7 +2126,7 @@ static int em28xx_v4l2_open(struct file *filp)
 {
 	struct video_device *vdev = video_devdata(filp);
 	struct em28xx *dev = video_drvdata(filp);
-	struct em28xx_v4l2 *v4l2 = dev->v4l2;
+	struct em28xx_v4l2 *v4l2;
 	enum v4l2_buf_type fh_type = 0;
 	int ret;
 
@@ -2143,13 +2143,19 @@ static int em28xx_v4l2_open(struct file *filp)
 		return -EINVAL;
 	}
 
+	if (mutex_lock_interruptible(&dev->lock))
+		return -ERESTARTSYS;
+
+	v4l2 = dev->v4l2;
+	if (!v4l2) {
+		mutex_unlock(&dev->lock);
+		return -ENODEV;
+	}
+
 	em28xx_videodbg("open dev=%s type=%s users=%d\n",
 			video_device_node_name(vdev), v4l2_type_names[fh_type],
 			v4l2->users);
 
-	if (mutex_lock_interruptible(&dev->lock))
-		return -ERESTARTSYS;
-
 	ret = v4l2_fh_open(filp);
 	if (ret) {
 		dev_err(&dev->intf->dev,
-- 
2.43.0