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Date:   Wed, 13 Apr 2022 16:16:19 +0200
From:   Lukas Wunner <>
To:     Oliver Neukum <>,
        "David S. Miller" <>,
        Jakub Kicinski <>,
        Paolo Abeni <>, Jann Horn <>,
        Oleksij Rempel <>
        Andrew Lunn <>,
        Eric Dumazet <>,
        Jacky Chou <>, Willy Tarreau <>,
        Lino Sanfilippo <>,
        Philipp Rosenberger <>,
        Heiner Kallweit <>
Subject: [PATCH] usbnet: Fix use-after-free on disconnect

Jann Horn reports a use-after-free on disconnect of a USB Ethernet
(ax88179_178a.c).  Oleksij Rempel has witnessed the same issue with a
different driver (ax88172a.c).

Jann's report (linked below) explains the root cause in great detail.
Briefly, USB Ethernet drivers schedule work (usbnet_deferred_kevent())
which in turn schedules another work (linkwatch_event()).  The problem
is that usbnet_disconnect() first synchronizes with linkwatch_event()
and only then with usbnet_deferred_kevent().  That allows
usbnet_deferred_kevent() to schedule another linkwatch_event() after
synchronization with the latter.  In other words, scheduling happens
in AB order and synchronization on disconnect happens in BA order.

The correct order is to first synchronize with usbnet_deferred_kevent()
(and prevent any future execution), then with linkwatch_event(), i.e.
in AB order.

Reported-by: Jann Horn <>
Reported-by: Oleksij Rempel <>
Signed-off-by: Lukas Wunner <>
Cc: Oliver Neukum <>
Cc: Andrew Lunn <>
 drivers/net/usb/usbnet.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 9a6450f796dc..6c67ae48afeb 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -469,6 +469,9 @@ static enum skb_state defer_bh(struct usbnet *dev, struct sk_buff *skb,
 void usbnet_defer_kevent (struct usbnet *dev, int work)
+	if (dev->intf->condition == USB_INTERFACE_UNBINDING)
+		return;
 	set_bit (work, &dev->flags);
 	if (!schedule_work (&dev->kevent))
 		netdev_dbg(dev->net, "kevent %s may have been dropped\n", usbnet_event_names[work]);
@@ -1619,11 +1622,11 @@ void usbnet_disconnect (struct usb_interface *intf)
 	if (dev->driver_info->unbind)
 		dev->driver_info->unbind(dev, intf);
+	cancel_work_sync(&dev->kevent);
 	net = dev->net;
 	unregister_netdev (net);
-	cancel_work_sync(&dev->kevent);

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