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

From: Eugene Shatokhin <[email-masked]>
Date: Tue, 8 Sep 2015 10:24:09 +0300

01.09.2015 17:05, Eugene Shatokhin пишет:
> 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.
>
> Locking in defer_bh() and usbnet_terminate_urbs() was revisited to avoid
> this race.
>
> Signed-off-by: Eugene Shatokhin <[email-masked]>
> ---
>   drivers/net/usb/usbnet.c | 39 ++++++++++++++++++++++++++++-----------
>   1 file changed, 28 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
> index e049857..b4cf107 100644
> --- a/drivers/net/usb/usbnet.c
> +++ b/drivers/net/usb/usbnet.c
> @@ -428,12 +428,18 @@ static enum skb_state defer_bh(struct usbnet *dev, struct sk_buff *skb,
>   	old_state = entry->state;
>   	entry->state = state;
>   	__skb_unlink(skb, list);
> -	spin_unlock(&list->lock);
> -	spin_lock(&dev->done.lock);
> +
> +	/* defer_bh() is never called with list == &dev->done.
> +	 * spin_lock_nested() tells lockdep that it is OK to take
> +	 * dev->done.lock here with list->lock held.
> +	 */
> +	spin_lock_nested(&dev->done.lock, SINGLE_DEPTH_NESTING);
> +
>   	__skb_queue_tail(&dev->done, skb);
>   	if (dev->done.qlen == 1)
>   		tasklet_schedule(&dev->bh);
> -	spin_unlock_irqrestore(&dev->done.lock, flags);
> +	spin_unlock(&dev->done.lock);
> +	spin_unlock_irqrestore(&list->lock, flags);
>   	return old_state;
>   }
>
> @@ -749,6 +755,20 @@ EXPORT_SYMBOL_GPL(usbnet_unlink_rx_urbs);
>
>   /*-------------------------------------------------------------------------*/
>
> +static void wait_skb_queue_empty(struct sk_buff_head *q)
> +{
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&q->lock, flags);
> +	while (!skb_queue_empty(q)) {
> +		spin_unlock_irqrestore(&q->lock, flags);
> +		schedule_timeout(msecs_to_jiffies(UNLINK_TIMEOUT_MS));
> +		set_current_state(TASK_UNINTERRUPTIBLE);
> +		spin_lock_irqsave(&q->lock, flags);
> +	}
> +	spin_unlock_irqrestore(&q->lock, flags);
> +}
> +
>   // precondition: never called in_interrupt
>   static void usbnet_terminate_urbs(struct usbnet *dev)
>   {
> @@ -762,14 +782,11 @@ static void usbnet_terminate_urbs(struct usbnet *dev)
>   		unlink_urbs(dev, &dev->rxq);
>
>   	/* 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);
> -	}
> +	wait_skb_queue_empty(&dev->rxq);
> +	wait_skb_queue_empty(&dev->txq);
> +	wait_skb_queue_empty(&dev->done);
> +	netif_dbg(dev, ifdown, dev->net,
> +		  "waited for %d urb completions\n", temp);
>   	set_current_state(TASK_RUNNING);
>   	remove_wait_queue(&dev->wait, &wait);
>   }
>

I resent the patch to make it separate. What is the status of this now?

Regards,
Eugene