[PATCH] usb: mdc800: fix circular locking dependency in probe/open

[Ivy Lopez <skunkolee@gmail.com>]

mdc800_usb_probe() acquired mdc800->io_lock before calling
usb_register_dev(), which internally takes minor_rwsem. Meanwhile
mdc800_device_open() is called through usb_open() with minor_rwsem
already held, and then tries to acquire io_lock, producing an AB-BA
lock inversion:

  probe:  io_lock -> minor_rwsem (via usb_register_dev)
  open:   minor_rwsem -> io_lock

Until usb_register_dev() succeeds and creates the device node, no
concurrent open can happen, so io_lock provides no protection during
probe before that point. Move usb_register_dev() to the end of the
probe function, after all URB and state setup, and drop io_lock
entirely from probe, eliminating the inversion.

Reported-by: syzbot+023b30ebfe0dc4bcfafe@syzkaller.appspotmail.com
Signed-off-by: Ivy Lopez <skunkolee@gmail.com>
---
 drivers/usb/image/mdc800.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/drivers/usb/image/mdc800.c b/drivers/usb/image/mdc800.c
index ca287b770e8c..976f751886f9 100644
--- a/drivers/usb/image/mdc800.c
+++ b/drivers/usb/image/mdc800.c
@@ -480,15 +480,6 @@ static int mdc800_usb_probe (struct usb_interface *intf,
 
 	dev_info(&intf->dev, "Found Mustek MDC800 on USB.\n");
 
-	mutex_lock(&mdc800->io_lock);
-
-	retval = usb_register_dev(intf, &mdc800_class);
-	if (retval) {
-		dev_err(&intf->dev, "Not able to get a minor for this device.\n");
-		mutex_unlock(&mdc800->io_lock);
-		return -ENODEV;
-	}
-
 	mdc800->dev=dev;
 	mdc800->open=0;
 
@@ -526,7 +517,11 @@ static int mdc800_usb_probe (struct usb_interface *intf,
 
 	mdc800->state=READY;
 
-	mutex_unlock(&mdc800->io_lock);
+	retval = usb_register_dev(intf, &mdc800_class);
+	if (retval) {
+		dev_err(&intf->dev, "Not able to get a minor for this device.\n");
+		return -ENODEV;
+	}
 	
 	usb_set_intfdata(intf, mdc800);
 	return 0;
-- 
2.55.0