Messages in this thread | | | From | Eugene Shatokhin <> | Subject | Several races in "usbnet" module (kernel 4.1.x) | Date | Mon, 20 Jul 2015 21:13:21 +0300 |
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Hi,
I have recently found several data races in "usbnet" module, checked on vanilla kernel 4.1.0 on x86_64. The races do actually happen, I have confirmed it by adding delays and using hardware breakpoints to detect the conflicting memory accesses (with RaceHound tool, https://github.com/winnukem/racehound).
I have not analyzed yet how harmful these races are (if they are), but it is better to report them anyway, I think.
Everything was checked using YOTA 4G LTE Modem that works via "usbnet" and "cdc_ether" kernel modules. --------------------------
[Race #1]
Race on skb_queue ('next' pointer) between usbnet_stop() and rx_complete().
Reproduced that by unplugging the device while the system was downloading a large file from the Net.
Here is part of the call stack with the code where the changes to the queue happen:
#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 the same 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);
For example, it is possible 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. --------------------------
Unrelated the that race, if the goal of that while loop in usbnet_terminate_urbs() is to wait till all three queues (dev->rxq, dev->txq, dev->done) become empty, perhaps the code should be changed as follows:
while (!skb_queue_empty(&dev->rxq) - && !skb_queue_empty(&dev->txq) - && !skb_queue_empty(&dev->done)) { + || !skb_queue_empty(&dev->txq) + || !skb_queue_empty(&dev->done)) { schedule_timeout(...)); --------------------------
[Race #2]
Races on dev->rx_qlen. Reproduced these by repeatedly changing MTU (1500 <-> 1400) while downloading large files.
dev->rx_qlen is written to here: #0 usbnet_update_max_qlen (usbnet.c:351) case USB_SPEED_HIGH: dev->rx_qlen = MAX_QUEUE_MEMORY / dev->rx_urb_size; #1 __handle_link_change (usbnet.c:1049) /* hard_mtu or rx_urb_size may change during link change */ usbnet_update_max_qlen(dev); #2 usbnet_deferred_kevent (usbnet.c:1172) if (test_bit (EVENT_LINK_CHANGE, &dev->flags)) __handle_link_change(dev);
Here are the conflicting reads from dev->rx_qlen (via RX_QLEN(dev)), 3 code locations:
* usbnet_bh (usbnet.c:1492) if (temp < RX_QLEN(dev)) { ... * usbnet_bh (usbnet.c:1499) if (dev->rxq.qlen < RX_QLEN(dev)) ... * rx_alloc_submit (usbnet.c:1431) for (i = 0; i < 10 && dev->rxq.qlen < RX_QLEN(dev); i++) { ... --------------------------
[Race #3]
Similar to race #2 but on dev->tx_qlen. I reproduced it the same way.
dev->tx_qlen is written to here: #0 usbnet_update_max_qlen (usbnet.c:352) case USB_SPEED_HIGH: dev->rx_qlen = MAX_QUEUE_MEMORY / dev->rx_urb_size; dev->tx_qlen = MAX_QUEUE_MEMORY / dev->hard_mtu; #1 __handle_link_change (usbnet.c:1049) /* hard_mtu or rx_urb_size may change during link change */ usbnet_update_max_qlen(dev); #2 usbnet_deferred_kevent (usbnet.c:1172) if (test_bit (EVENT_LINK_CHANGE, &dev->flags)) __handle_link_change(dev);
Here are the conflicting reads from dev->tx_qlen (via TX_QLEN(dev)), 2 code locations:
* usbnet_bh (usbnet.c:1502) if (dev->txq.qlen < TX_QLEN (dev)) netif_wake_queue (dev->net); * usbnet_start_xmit (usbnet.c:1398) if (dev->txq.qlen >= TX_QLEN (dev)) netif_stop_queue (net); --------------------------
[Race #4]
Race on dev->flags. It happened when I unplugged the device while a large file was being downloaded.
dev->flags is set to 0 here:
#0 usbnet_stop (usbnet.c:816) /* 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. */ dev->flags = 0; del_timer_sync (&dev->delay); tasklet_kill (&dev->bh);
And here, the code clears EVENT_RX_KILL bit in dev->flags, which may execute concurrently with the above operation: #0 clear_bit (bitops.h:113, inlined) #1 usbnet_bh (usbnet.c:1475) /* restart RX again after disabling due to high error rate */ clear_bit(EVENT_RX_KILL, &dev->flags); If clear_bit() is atomic w.r.t. setting dev->flags to 0, this race is not a problem, I guess. Otherwise, it may be. --------------------------
[Race #5]
Race on dev->rx_urb_size. I reproduced it a similar way as the races #2 and #3 (changing MTU while downloading files).
dev->rx_urb_size is written to here: #0 usbnet_change_mtu (usbnet.c:392) dev->rx_urb_size = dev->hard_mtu;
Here is the conflicting read from dev->rx_urb_size: * rx_submit (usbnet.c:467) size_t size = dev->rx_urb_size; --------------------------
Regards, Eugene
-- Eugene Shatokhin, ROSA www.rosalab.com
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