usb: cdc-acm: make sure a refcount is taken early enough

BugLink: https://bugs.launchpad.net/bugs/1845038

commit c52873e5 upstream.

destroy() will decrement the refcount on the interface, so that
it needs to be taken so early that it never undercounts.

Fixes: 7fb57a01 ("USB: cdc-acm: Fix potential deadlock (lockdep warning)")
Cc: stable <stable@vger.kernel.org>
Reported-and-tested-by: syzbot+1b2449b7b5dc240d107a@syzkaller.appspotmail.com
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Link: https://lore.kernel.org/r/20190808142119.7998-1-oneukum@suse.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/usb/class/cdc-acm.c

@@ -1319,13 +1319,6 @@ static int acm_probe(struct usb_interface *intf,
 	if (acm == NULL)
 		goto alloc_fail;
 
-	minor = acm_alloc_minor(acm);
-	if (minor < 0) {
-		dev_err(&intf->dev, "no more free acm devices\n");
-		kfree(acm);
-		return -ENODEV;
-	}
-
 	ctrlsize = usb_endpoint_maxp(epctrl);
 	readsize = usb_endpoint_maxp(epread) *
 				(quirks == SINGLE_RX_URB ? 1 : 2);
@@ -1333,6 +1326,16 @@ static int acm_probe(struct usb_interface *intf,
 	acm->writesize = usb_endpoint_maxp(epwrite) * 20;
 	acm->control = control_interface;
 	acm->data = data_interface;
+
+	usb_get_intf(acm->control); /* undone in destruct() */
+
+	minor = acm_alloc_minor(acm);
+	if (minor < 0) {
+		dev_err(&intf->dev, "no more free acm devices\n");
+		kfree(acm);
+		return -ENODEV;
+	}
+
 	acm->minor = minor;
 	acm->dev = usb_dev;
 	acm->ctrl_caps = ac_management_function;
@@ -1474,7 +1477,6 @@ static int acm_probe(struct usb_interface *intf,
 	usb_driver_claim_interface(&acm_driver, data_interface, acm);
 	usb_set_intfdata(data_interface, acm);
 
-	usb_get_intf(control_interface);
 	tty_dev = tty_port_register_device(&acm->port, acm_tty_driver, minor,
 			&control_interface->dev);
 	if (IS_ERR(tty_dev)) {