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

[Jia-Ju Bai]

On 2018/5/8 9:56, Eric Dumazet wrote:
> On 05/07/2018 05:51 PM, Jia-Ju Bai wrote:
> > 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 <baijiaju1990@xxxxxxxxx>
> > > > ---
> > > >    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 ?
>
> > So I think a data race may occur when returning "dev->stats" without lock protection.
> &dev->stats is a stable value.
>
> It wont change over the lifetime of net_device object.
>
> Adding a barrier before or after getting &dev->stats is useless, confusing and really not needed.

Yes, "&dev->stats" will not change, because it is a fixed address.
But the field data in "dev->stats" is changed (rx_frame_errors, 
rx_crc_errors and rx_missed_errors).
So if the driver returns "&dev->stats" without lock protection (like on 
line 858), the field data value of this return value can be the changed 
field data value or unchanged field data value.


Best wishes,
Jia-Ju Bai