Subject: Re: [PATCH 2/2] usbnet: Fix a race between usbnet_stop() and the BH

From: Eugene Shatokhin <[email-masked]>
Date: Fri, 28 Aug 2015 11:09:52 +0300

25.08.2015 00:01, Bjørn Mork пишет:
> Eugene Shatokhin <[email-masked]> writes:
>
>> The race may happen when a device (e.g. YOTA 4G LTE Modem) is
>> unplugged while the system is downloading a large file from the Net.
>>
>> Hardware breakpoints and Kprobes with delays were used to confirm that
>> the race does actually happen.
>>
>> The race is on skb_queue ('next' pointer) between usbnet_stop()
>> and rx_complete(), which, in turn, calls usbnet_bh().
>>
>> Here is a part of the call stack with the code where the changes to the
>> queue happen. The line numbers are for the kernel 4.1.0:
>>
>> *0 __skb_unlink (skbuff.h:1517)
>>      prev->next = next;
>> *1 defer_bh (usbnet.c:430)
>>      spin_lock_irqsave(&list->lock, flags);
>>      old_state = entry->state;
>>      entry->state = state;
>>      __skb_unlink(skb, list);
>>      spin_unlock(&list->lock);
>>      spin_lock(&dev->done.lock);
>>      __skb_queue_tail(&dev->done, skb);
>>      if (dev->done.qlen == 1)
>>          tasklet_schedule(&dev->bh);
>>      spin_unlock_irqrestore(&dev->done.lock, flags);
>> *2 rx_complete (usbnet.c:640)
>>      state = defer_bh(dev, skb, &dev->rxq, state);
>>
>> At the same time, the following code repeatedly checks if the queue is
>> empty and reads these values concurrently with the above changes:
>>
>> *0  usbnet_terminate_urbs (usbnet.c:765)
>>      /* maybe wait for deletions to finish. */
>>      while (!skb_queue_empty(&dev->rxq)
>>          && !skb_queue_empty(&dev->txq)
>>          && !skb_queue_empty(&dev->done)) {
>>              schedule_timeout(msecs_to_jiffies(UNLINK_TIMEOUT_MS));
>>              set_current_state(TASK_UNINTERRUPTIBLE);
>>              netif_dbg(dev, ifdown, dev->net,
>>                    "waited for %d urb completions\n", temp);
>>      }
>> *1  usbnet_stop (usbnet.c:806)
>>      if (!(info->flags & FLAG_AVOID_UNLINK_URBS))
>>          usbnet_terminate_urbs(dev);
>>
>> As a result, it is possible, for example, that the skb is removed from
>> dev->rxq by __skb_unlink() before the check
>> "!skb_queue_empty(&dev->rxq)" in usbnet_terminate_urbs() is made. It is
>> also possible in this case that the skb is added to dev->done queue
>> after "!skb_queue_empty(&dev->done)" is checked. So
>> usbnet_terminate_urbs() may stop waiting and return while dev->done
>> queue still has an item.
>
> Exactly what problem will that result in?  The tasklet_kill() will wait
> for the processing of the single element done queue, and everything will
> be fine.  Or?

Given enough time, what prevents defer_bh() from calling 
tasklet_schedule(&dev->bh) *after* usbnet_stop() calls tasklet_kill()?

Consider the following situation (assuming '&&' are changed to '||' in 
that while loop in usbnet_terminate_urbs() as they should be):

CPU0                            CPU1
usbnet_stop()                   defer_bh() with list == dev->rxq
   usbnet_terminate_urbs()
                                 __skb_unlink() removes the last
                                 skb from dev->rxq.
                                 dev->rxq, dev->txq and dev->done
                                 are now empty.
   while (!skb_queue_empty()...)
     The loop ends because all 3
     queues are now empty.

   usbnet_terminate_urbs() ends.

usbnet_stop() continues:
   usbnet_status_stop(dev);
   ...
   del_timer_sync (&dev->delay);
   tasklet_kill (&dev->bh);
                                 __skb_queue_tail(&dev->done, skb);
                                 if (dev->done.qlen == 1)
                                   tasklet_schedule(&dev->bh);

The BH is scheduled at this point, which is not what was intended. The 
race window is small, but still.

Regards,
Eugene