Re: [PATCH] net: 8390: Fix possible data races in __ei_get_stats

[Jia-Ju Bai]

On 2018/5/7 22:15, Eric Dumazet wrote:

On 05/07/2018 07:08 AM, Jia-Ju Bai wrote:
The write operations to "dev->stats" are protected by
the spinlock on line 862-864, but the read operations to
this data on line 858 and 867 are not protected by the spinlock.
Thus, there may exist data races for "dev->stats".

To fix the data races, the read operations to "dev->stats" are
protected by the spinlock, and a local variable is used for return.

Signed-off-by: Jia-Ju Bai <baijiaju1...@gmail.com>
---
  drivers/net/ethernet/8390/lib8390.c | 14 ++++++++++----
  1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/8390/lib8390.c 
b/drivers/net/ethernet/8390/lib8390.c
index c9c55c9eab9f..198952247d30 100644
--- a/drivers/net/ethernet/8390/lib8390.c
+++ b/drivers/net/ethernet/8390/lib8390.c
@@ -852,19 +852,25 @@ static struct net_device_stats *__ei_get_stats(struct 
net_device *dev)
        unsigned long ioaddr = dev->base_addr;
        struct ei_device *ei_local = netdev_priv(dev);
        unsigned long flags;
+       struct net_device_stats *stats;
+
+       spin_lock_irqsave(&ei_local->page_lock, flags);
  
  	/* If the card is stopped, just return the present stats. */
-       if (!netif_running(dev))
-               return &dev->stats;
+       if (!netif_running(dev)) {
+               stats = &dev->stats;
+               spin_unlock_irqrestore(&ei_local->page_lock, flags);
+               return stats;
+       }
  
-	spin_lock_irqsave(&ei_local->page_lock, flags);
        /* Read the counter registers, assuming we are in page 0. */
        dev->stats.rx_frame_errors  += ei_inb_p(ioaddr + EN0_COUNTER0);
        dev->stats.rx_crc_errors    += ei_inb_p(ioaddr + EN0_COUNTER1);
        dev->stats.rx_missed_errors += ei_inb_p(ioaddr + EN0_COUNTER2);
+       stats = &dev->stats;
        spin_unlock_irqrestore(&ei_local->page_lock, flags);
  
-	return &dev->stats;
+       return stats;
  }
  
  /*

dev->stats is not a pointer, it is an array embedded in the
struct net_device

So this patch is not needed, since dev->stats can not change.

Thanks for your reply :)

I do not understand that why "dev->stats can not change".
Its data is indeed changed by the code:
     dev->stats.rx_frame_errors  += ei_inb_p(ioaddr + EN0_COUNTER0);
     dev->stats.rx_crc_errors    += ei_inb_p(ioaddr + EN0_COUNTER1);
     dev->stats.rx_missed_errors += ei_inb_p(ioaddr + EN0_COUNTER2);

So I think a data race may occur when returning "dev->stats" without 
lock protection.

By the way, I find this possible data race is similar to the previous 
commit 7b31b4deda76 for the tg3 driver.


Best wishes,
Jia-Ju Bai