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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