[PATCH 4.9 034/101] usbnet: Fix linkwatch use-after-free on disconnect

[Greg Kroah-Hartman]

From: Lukas Wunner <lukas@xxxxxxxxx>

commit a69e617e533edddf3fa3123149900f36e0a6dc74 upstream.

usbnet uses the work usbnet_deferred_kevent() to perform tasks which may
sleep.  On disconnect, completion of the work was originally awaited in
->ndo_stop().  But in 2003, that was moved to ->disconnect() by historic
commit "[PATCH] USB: usbnet, prevent exotic rtnl deadlock":

  https://git.kernel.org/tglx/history/c/0f138bbfd83c

The change was made because back then, the kernel's workqueue
implementation did not allow waiting for a single work.  One had to wait
for completion of *all* work by calling flush_scheduled_work(), and that
could deadlock when waiting for usbnet_deferred_kevent() with rtnl_mutex
held in ->ndo_stop().

The commit solved one problem but created another:  It causes a
use-after-free in USB Ethernet drivers aqc111.c, asix_devices.c,
ax88179_178a.c, ch9200.c and smsc75xx.c:

* If the drivers receive a link change interrupt immediately before
  disconnect, they raise EVENT_LINK_RESET in their (non-sleepable)
  ->status() callback and schedule usbnet_deferred_kevent().
* usbnet_deferred_kevent() invokes the driver's ->link_reset() callback,
  which calls netif_carrier_{on,off}().
* That in turn schedules the work linkwatch_event().

Because usbnet_deferred_kevent() is awaited after unregister_netdev(),
netif_carrier_{on,off}() may operate on an unregistered netdev and
linkwatch_event() may run after free_netdev(), causing a use-after-free.

In 2010, usbnet was changed to only wait for a single instance of
usbnet_deferred_kevent() instead of *all* work by commit 23f333a2bfaf
("drivers/net: don't use flush_scheduled_work()").

Unfortunately the commit neglected to move the wait back to
->ndo_stop().  Rectify that omission at long last.

Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Link: https://lore.kernel.org/netdev/CAG48ez0MHBbENX5gCdHAUXZ7h7s20LnepBF-pa5M=7Bi-jZrEA@xxxxxxxxxxxxxx/
Reported-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/netdev/20220315113841.GA22337@xxxxxxxxxxxxxx/
Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Acked-by: Oliver Neukum <oneukum@xxxxxxxx>
Link: https://lore.kernel.org/r/d1c87ebe9fc502bffcd1576e238d685ad08321e4.1655987888.git.lukas@xxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/net/usb/usbnet.c |    8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -847,13 +847,11 @@ int usbnet_stop (struct net_device *net)
 
 	mpn = !test_and_clear_bit(EVENT_NO_RUNTIME_PM, &dev->flags);
 
-	/* deferred work (task, timer, softirq) must also stop.
-	 * can't flush_scheduled_work() until we drop rtnl (later),
-	 * else workers could deadlock; so make workers a NOP.
-	 */
+	/* deferred work (timer, softirq, task) must also stop */
 	dev->flags = 0;
 	del_timer_sync (&dev->delay);
 	tasklet_kill (&dev->bh);
+	cancel_work_sync(&dev->kevent);
 	if (!pm)
 		usb_autopm_put_interface(dev->intf);
 
@@ -1577,8 +1575,6 @@ void usbnet_disconnect (struct usb_inter
 	net = dev->net;
 	unregister_netdev (net);
 
-	cancel_work_sync(&dev->kevent);
-
 	usb_scuttle_anchored_urbs(&dev->deferred);
 
 	if (dev->driver_info->unbind)