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

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@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.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 183b41753c98298236ea0690cd119e231d1ae5f3..62f4fb9b362f14533c86ba324588158517787019 100644 (file)
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -1301,10 +1301,6 @@ made_compressed_probe:
        tty_port_init(&acm->port);
        acm->port.ops = &acm_port_ops;
 
-       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);
@@ -1312,6 +1308,13 @@ 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)
@@ -1458,7 +1461,6 @@ 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)) {