Re: [virtio-dev] [PATCH RFC 3/3] rng: leak detection support

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Sep 13, 2023 at 11:32:57AM +0200, Babis Chalios wrote:
> > I do not understand why this matters though. we know there was a leak,
> > why does it matter whether there was one or two leaks?
> >
> > > In the last RFC implementing this in Linux we sent to LKML [1] we avoid
> the
> > > issue by pre-populating both
> > > queues, but that does not solve the problem if a third entropy leak
> event
> > > arrives. The probability of this
> > > happening is indeed small, but we thought of a potential solution to
> this.
> > >
> > > What if we modify the spec here to instruct the VMM to deny taking a
> > > snapshot if there are not any buffers
> > > in the active leak queue? If we did this, we could even simplify the
> spec to
> > > just introduce a single entropy
> > > leak queue, so we could avoid the complexity of switching between active
> > > leak queues in the driver and
> > > the device. WDYT?
> >
> > here's the problem:
> >
> > - driver adds batch 1 of buffers
> > - leak
> > - device starts using buffers from batch 1
> > - driver sees some buffers and starts adding batch 2
> 
> If understand this clause:
> 
> > > +\item Upon detecting that buffers have been used, driver
> > > +      switches to another leak queue making it active
> > > +      (e.g. from \field{leakq1} to \field{leakq2} or vice versa).
> > > +      It then starts adding buffers to the new leak queue.
> 
> correctly:
> 
> At this point, the driver will first switch active leak queue and
> then add batch 2 to the new leak queue.
> 
> and due to this:
> 
> > > +\item Device will keep using buffers in the active leak queue
> > > +      until it detects that both the current leak queue is empty and
> another
> > > +      leak queue has buffers. At that point device switches to
> > > +      another leak queue, making it active.
> > > +\item After the switch, buffers from the new leak queue are not
> > > +      used until an information leak is detected.
> > > +\end{enumerate}
> 
> the following won't happen:
> 
> > - device sees batch 2 and thinks this is part of batch 1
> >    consumes them all
> 
> Does it make sense?
> 
> Cheers,
> Babis

yes, the queue switch is used as a barrier to detect a new leak event.

-- 
MST