Re: KASAN: use-after-free Read in device_release_driver_internal

[Oliver Neukum]

Am Donnerstag, den 01.08.2019, 14:47 -0400 schrieb Alan Stern:
> 
> I think this must be caused by an unbalanced refcount.  That is,
> something must drop one more reference to the device than it takes.
> That would explain why the invalid access occurs inside a single
> bus_remove_device() call, between the klist_del() and
> device_release_driver().
> 
> The kernel log indicates that the device was probed by rndis_wlan,
> rndis_host, and cdc_acm, all of which got errors because of the
> device's bogus descriptors.  Probably one of them is messing up the
> refcount.

Hi,

you made me look at cdc-acm. I suspect

cae2bc768d176bfbdad7035bbcc3cdc973eb7984 ("usb: cdc-acm: Decrement tty port's refcount if probe() fail")

is buggy decrementing the refcount on the interface in destroy()
even before the refcount is increased.

Unfortunately I cannot tell from the bug report how many and which
interfaces the emulated test device has. Hence it is unclear to me,
when exactly probe() would fail cdc-acm.

If you agree. I am attaching a putative fix.

	Regards
		Oliver
From 6b31904e6cf75f89441e308b9e428a1de7728fd8 Mon Sep 17 00:00:00 2001
From: Oliver Neukum <oneukum@xxxxxxxx>
Date: Tue, 6 Aug 2019 14:34:27 +0200
Subject: [PATCH] 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.

Signed-off-by: Oliver Neukum <oneukum@xxxxxxxx>
---
 drivers/usb/class/cdc-acm.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index 183b41753c98..28e3de775ada 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -1301,10 +1301,6 @@ static int acm_probe(struct usb_interface *intf,
 	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 @@ 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 destroy() */
+
+	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 @@ 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)) {
-- 
2.16.4