USB: usblp: fix use-after-free on disconnect

A recent commit addressing a runtime PM use-count regression, introduced
a use-after-free by not making sure we held a reference to the struct
usb_interface for the lifetime of the driver data.

Fixes: 9a315358 ("USB: usblp: fix runtime PM after driver unbind")
Cc: stable <stable@vger.kernel.org>
Reported-by: syzbot+cd24df4d075c319ebfc5@syzkaller.appspotmail.com
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191015175522.18490-1-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/usblp.c

@@ -445,6 +445,7 @@ static void usblp_cleanup(struct usblp *usblp)
 	kfree(usblp->readbuf);
 	kfree(usblp->device_id_string);
 	kfree(usblp->statusbuf);
+	usb_put_intf(usblp->intf);
 	kfree(usblp);
 }
 
@@ -1113,7 +1114,7 @@ static int usblp_probe(struct usb_interface *intf,
 	init_waitqueue_head(&usblp->wwait);
 	init_usb_anchor(&usblp->urbs);
 	usblp->ifnum = intf->cur_altsetting->desc.bInterfaceNumber;
-	usblp->intf = intf;
+	usblp->intf = usb_get_intf(intf);
 
 	/* Malloc device ID string buffer to the largest expected length,
 	 * since we can re-query it on an ioctl and a dynamic string
@@ -1198,6 +1199,7 @@ static int usblp_probe(struct usb_interface *intf,
 	kfree(usblp->readbuf);
 	kfree(usblp->statusbuf);
 	kfree(usblp->device_id_string);
+	usb_put_intf(usblp->intf);
 	kfree(usblp);
 abort_ret:
 	return retval;