[PATCH 4.9 084/103] usb: cdc-acm: make sure a refcount is taken early enough

[Greg Kroah-Hartman]

From: Oliver Neukum <oneukum@xxxxxxxx>

commit c52873e5a1ef72f845526d9f6a50704433f9c625 upstream.

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

Fixes: 7fb57a019f94e ("USB: cdc-acm: Fix potential deadlock (lockdep warning)")
Cc: stable <stable@xxxxxxxxxxxxxxx>
Reported-and-tested-by: syzbot+1b2449b7b5dc240d107a@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Oliver Neukum <oneukum@xxxxxxxx>
Link: https://lore.kernel.org/r/20190808142119.7998-1-oneukum@xxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/usb/class/cdc-acm.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -1264,10 +1264,6 @@ made_compressed_probe:
 	if (acm == NULL)
 		goto alloc_fail;
 
-	minor = acm_alloc_minor(acm);
-	if (minor < 0)
-		goto alloc_fail1;
-
 	ctrlsize = usb_endpoint_maxp(epctrl);
 	readsize = usb_endpoint_maxp(epread) *
 				(quirks == SINGLE_RX_URB ? 1 : 2);
@@ -1275,6 +1271,13 @@ made_compressed_probe:
 	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)
+		goto alloc_fail1;
+
 	acm->minor = minor;
 	acm->dev = usb_dev;
 	if (h.usb_cdc_acm_descriptor)
@@ -1420,7 +1423,6 @@ skip_countries:
 	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)) {