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Date:   Wed, 17 Apr 2019 09:19:23 +0000
From:   Kloetzke Jan <Jan.Kloetzke@...h.de>
To:     Oliver Neukum <oneukum@...e.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        Kloetzke Jan <Jan.Kloetzke@...h.de>
Subject: [PATCH] usbnet: fix kernel crash after disconnect

When disconnecting cdc_ncm the kernel sporadically crashes shortly
after the disconnect:

  [   57.868812] Unable to handle kernel NULL pointer dereference at virtual address 00000000
  ...
  [   58.006653] PC is at 0x0
  [   58.009202] LR is at call_timer_fn+0xec/0x1b4
  [   58.013567] pc : [<0000000000000000>] lr : [<ffffff80080f5130>] pstate: 00000145
  [   58.020976] sp : ffffff8008003da0
  [   58.024295] x29: ffffff8008003da0 x28: 0000000000000001
  [   58.029618] x27: 000000000000000a x26: 0000000000000100
  [   58.034941] x25: 0000000000000000 x24: ffffff8008003e68
  [   58.040263] x23: 0000000000000000 x22: 0000000000000000
  [   58.045587] x21: 0000000000000000 x20: ffffffc68fac1808
  [   58.050910] x19: 0000000000000100 x18: 0000000000000000
  [   58.056232] x17: 0000007f885aff8c x16: 0000007f883a9f10
  [   58.061556] x15: 0000000000000001 x14: 000000000000006e
  [   58.066878] x13: 0000000000000000 x12: 00000000000000ba
  [   58.072201] x11: ffffffc69ff1db30 x10: 0000000000000020
  [   58.077524] x9 : 8000100008001000 x8 : 0000000000000001
  [   58.082847] x7 : 0000000000000800 x6 : ffffff8008003e70
  [   58.088169] x5 : ffffffc69ff17a28 x4 : 00000000ffff138b
  [   58.093492] x3 : 0000000000000000 x2 : 0000000000000000
  [   58.098814] x1 : 0000000000000000 x0 : 0000000000000000
  ...
  [   58.205800] [<          (null)>]           (null)
  [   58.210521] [<ffffff80080f5298>] expire_timers+0xa0/0x14c
  [   58.215937] [<ffffff80080f542c>] run_timer_softirq+0xe8/0x128
  [   58.221702] [<ffffff8008081120>] __do_softirq+0x298/0x348
  [   58.227118] [<ffffff80080a6304>] irq_exit+0x74/0xbc
  [   58.232009] [<ffffff80080e17dc>] __handle_domain_irq+0x78/0xac
  [   58.237857] [<ffffff8008080cf4>] gic_handle_irq+0x80/0xac
  ...

The crash happens roughly 125..130ms after the disconnect. This
correlates with the 'delay' timer that is started on certain USB tx/rx
errors in the URB completion handler.

The suspected problem is a race of usbnet_stop() with
usbnet_start_xmit(). In usbnet_stop() we call usbnet_terminate_urbs()
to cancel all URBs in flight. This only makes sense if no new URBs are
submitted concurrently, though. But the usbnet_start_xmit() can run at
the same time on another CPU which almost unconditionally submits an
URB. The error callback of the new URB will then schedule the timer
after it was already stopped.

The fix adds a check if the tx queue is stopped after the tx list lock
has been taken. This should reliably prevent the submission of new URBs
while usbnet_terminate_urbs() does its job. The same thing is done on
the rx side even though it might be safe due to other flags that are
checked there.

Signed-off-by: Jan Klötzke <Jan.Kloetzke@...h.de>
---
 drivers/net/usb/usbnet.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 504282af27e5..921cc0571bd0 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -506,6 +506,7 @@ static int rx_submit (struct usbnet *dev, struct urb *urb, gfp_t flags)
 
 	if (netif_running (dev->net) &&
 	    netif_device_present (dev->net) &&
+	    test_bit(EVENT_DEV_OPEN, &dev->flags) &&
 	    !test_bit (EVENT_RX_HALT, &dev->flags) &&
 	    !test_bit (EVENT_DEV_ASLEEP, &dev->flags)) {
 		switch (retval = usb_submit_urb (urb, GFP_ATOMIC)) {
@@ -1431,6 +1432,11 @@ netdev_tx_t usbnet_start_xmit (struct sk_buff *skb,
 		spin_unlock_irqrestore(&dev->txq.lock, flags);
 		goto drop;
 	}
+	if (netif_queue_stopped(net)) {
+		usb_autopm_put_interface_async(dev->intf);
+		spin_unlock_irqrestore(&dev->txq.lock, flags);
+		goto drop;
+	}
 
 #ifdef CONFIG_PM
 	/* if this triggers the device is still a sleep */
-- 
2.11.0

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