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

[Babis Chalios <bchalios@amazon.es>]

Resending to fix e-mail formatting issues (sorry for the spam)

On 18/9/23 18:30, Babis Chalios wrote:
> > > > > Yes, that's what the driver does now in the RFC patch. However, 
> > > > > this just
> > > > > decreases
> > > > > the race window, it doesn't eliminate it. If a third leak event 
> > > > > happens it
> > > > > might not
> > > > > find any buffers to use:
> > > > >
> > > > > 1. available buffers to queue 1-X
> > > > > 2. available buffers to queue X
> > > > >
> > > > >
> > > > > 3. poll queue X
> > > > > 4. used buffers in queue XÂÂÂÂÂÂ <- leak event 1 will use buffers 
> > > > > in X
> > > > > 5. avail buffers in queue X
> > > > > 6. poll queue 1-XÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ <- leak event 2 will use buffers 
> > > > > in 1-X
> > > > > 7. used buffers in queue 1-X
> > > > > 8. avail buffers in queue 1-X
> > > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ <- leak event 3 (it needs 
> > > > > buffers in X, race with step 5)
> > > > > 9. goto 3
> > > > I don't get it. we added buffers in step 5.
> > > What if the leak event 3 arrives before step 5 had time to actually 
> > > add the
> > > buffers in X and make
> > > them visible to the device?
> >
> > Then it will see a single event in 1-X instead of two events. A leak is
> > a leak though, I don't see does it matter how many triggered.


So the scenario I have in mind is the following:

(Epoch here is terminology that I used in the Linux RFC. It is a value 
maintained by random.c
that changes every time a leak event happens).

1. add buffers to 1-X
2. add buffers to X
3. poll queue X
4. vcpu 0: get getrandom() entropy and cache epoch value
5. Device: First snapshot, uses buffers in X
6. vcpu 1: sees used buffers
7. Device: Second snapshot, uses buffers in 1-X
8. vcpu 0: getrandom() observes new epoch value & caches it
9. Device: Third snapshot, no buffers in either queue, (vcpu 1 from step 
6 has not yet finished adding new buffers).
10. vcpu 1 adds new buffer in X
11. vcpu 0: getrandom() will not see new epoch and gets stale entropy.


In this succession of events, when the third snapshot will happen, the 
device won't find
any buffers in either queue, so it won't increase the RNG epoch value. 
So, any entropy
gathered after step 8 will be the same across all snapshots. Am I 
missing something?

Cheers,
Babis