Re: [Xen-devel] [PATCH net] xen-netback: disable rogue vif in kthread context

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Mar 24, 2014 at 12:49:11PM +0000, David Vrabel wrote:
> On 24/03/14 12:13, Wei Liu wrote:
> > When netback discovers frontend is sending malformed packet it will
> > disables the interface which serves that frontend.
> > 
> > However disabling a network interface involving taking a mutex which
> > cannot be done in softirq context, so we need to defer this process to
> > kthread context.
> > 
> > This patch does the following:
> > 1. introduce a flag to indicate the interface is disabled.
> > 2. check that flag in TX path, don't do any work if it's true.
> > 3. check that flag in RX path, turn off that interface if it's true.
> > 
> > The reason to disable it in RX path is because RX uses kthread. After
> > this change the behavior of netback is still consistent -- it won't do
> > any TX work for a rogue frontend, and the interface will be eventually
> > turned off.
> > 
> > Also change a "continue" to "break" after xenvif_fatal_tx_err, as it
> > doesn't make sense to continue processing packets if frontend is rogue.
> [...]
> > --- a/drivers/net/xen-netback/interface.c
> > +++ b/drivers/net/xen-netback/interface.c
> > @@ -62,6 +62,13 @@ static int xenvif_poll(struct napi_struct *napi, int 
> > budget)
> >     struct xenvif *vif = container_of(napi, struct xenvif, napi);
> >     int work_done;
> >  
> > +   /* This vif is rogue, we pretend we've used up all budget to
> > +    * deschedule it from NAPI. But this interface will be turned
> > +    * off in thread context later.
> > +    */
> > +   if (unlikely(vif->disabled))
> > +           return budget;
> 
> Shouldn't you call __napi_complete() and return 0?  Returning budget
> will make NAPI poll repeatedly (since you're pretending to do work).
> 

Yes. You're right.

> > diff --git a/drivers/net/xen-netback/netback.c 
> > b/drivers/net/xen-netback/netback.c
> > index 438d0c0..94e7261 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -655,7 +655,7 @@ static void xenvif_tx_err(struct xenvif *vif,
> >  static void xenvif_fatal_tx_err(struct xenvif *vif)
> >  {
> >     netdev_err(vif->dev, "fatal error; disabling device\n");
> > -   xenvif_carrier_off(vif);
> > +   vif->disabled = true;
> 
> Do you need to wake the thread here?
> 

That's a better approach.

> > @@ -1549,6 +1549,16 @@ int xenvif_kthread(void *data)
> >             wait_event_interruptible(vif->wq,
> >                                      rx_work_todo(vif) ||
> >                                      kthread_should_stop());
> 
>                                          || vif->disabled ?
> 
> > +
> > +           /* This frontend is found to be rogue, disable it in
> > +            * kthread context. Currently this is only set when
> > +            * netback finds out frontend sends malformed packet,
> > +            * but we cannot disable the interface in softirq
> > +            * context so we defer it here.
> > +            */
> > +           if (unlikely(vif->disabled) && netif_carrier_ok(vif->dev))
> > +                   xenvif_carrier_off(vif);
> > +
> >             if (kthread_should_stop())
> >                     break;
> >  
> 
> As an aside, since I happened to be looking at xenvif_poll(), disabling
> local irqs to avoid problems with concurrent events looks unsafe as the
> event may occur on another VCPU.
> 

Are you seeing any problem? If this is not related to this fix we should
probably discuss this in another thread.

>    __napi_complete(napi);
>    RING_FINAL_CHECK_FOR_REQUESTS(&vif->tx, more_to_do);
>    if (more_to_do)
>        napi_schedule(napi);
> 
> Would work I think.
> 

Not sure I get your suggestion. Sorry. If you're talking about the code
in xenvif_poll, there's comment up there describing a race. Again, this
should be discussed in separate thread.

Wei.

> David

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