Re: [Qemu-devel] [PATCH v2 3/3] Add rate limiting of RTC_CHANGE, BALLOON_CHANGE & WATCHDOG events

[Daniel P. Berrange]

On Mon, Jun 11, 2012 at 02:22:05PM -0300, Luiz Capitulino wrote:
> On Fri, 8 Jun 2012 17:48:56 +0100
> "Daniel P. Berrange" <address@hidden> wrote:
> 
> > On Wed, May 30, 2012 at 03:50:37PM -0300, Luiz Capitulino wrote:
> > > On Mon, 21 May 2012 17:59:53 +0100
> > > "Daniel P. Berrange" <address@hidden> wrote:
> > 
> > > > +/* Global, one-time initializer to configure the rate limiting
> > > > + * and initialize state */
> > > > +static void monitor_protocol_event_init(void)
> > > > +{
> > > > +    qemu_mutex_init(&monitor_event_state_lock);
> > > > +    /* Limit RTC & BALLOON events to 1 per second */
> > > > +    monitor_protocol_event_throttle(QEVENT_RTC_CHANGE, 1000);
> > > > +    monitor_protocol_event_throttle(QEVENT_BALLOON_CHANGE, 1000);
> > > > +    monitor_protocol_event_throttle(QEVENT_WATCHDOG, 1000);
> > > 
> > > What about SUSPENDED and BLOCK_IO_ERROR? Couldn't the former be also
> > > used by a malicious guest to cause a DoS? The former is already emitted
> > > several times for virtio.
> > 
> > This can't be used to filter BLOCK_IO_ERROR, since that event
> > contains per-device state information. Filtering this would
> > need to be done in the block layer, so it can done per device.
> 
> That's right.
> 
> > I don't think SUSPEND can be used to DoS, since once the VM
> > is in the suspend state, a monitor command is required to wake
> > it up again before the guest OS can trigger a new suspend.
> 
> Can't the guest OS awake itself?

I didn't think so. Even if it can, we can't rate limit SUSPEND
events in isolation, because they must be strictly ordered
wrt RESUME events.

Daniel
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