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

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

On Tue, Sep 19, 2023 at 12:11:37PM +0200, Babis Chalios wrote:
> On 19/9/23 12:01, Michael S. Tsirkin wrote:
> > On Tue, Sep 19, 2023 at 09:32:08AM +0200, Babis Chalios wrote:
> > > 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 X       <- leak event 1 will
> > > > > > > > use buffers in X
> > > > > > > > 5. avail buffers in queue X
> > > > > > > > 6. poll queue 1-X                <- leak event 2 will
> > > > > > > > use buffers in 1-X
> > > > > > > > 7. used buffers in queue 1-X
> > > > > > > > 8. avail buffers in queue 1-X
> > > > > > > >                                     <- 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
> > > 
> > Yes but notice how this is followed by:
> > 
> > 12. vcpu 1: sees used buffers in 1-X
> > 
> > Driver can notify getrandom I guess?
> 
> It could, but then we have the exact race condition that VMGENID had,
> userspace has already consumed stale entropy and there's nothing we
> can do about that.
> 
> Although this is indeed a corner case, it feels like it beats the purpose
> of having the hardware update directly userspace (via copy on leak).
> 
> How do you feel about the proposal a couple of emails back? It looks to
> me that it avoids completely the race condition.
> 
> Cheers,
> Babis

It does. The problem of course is that this means that e.g.
taking a snapshot of a guest that is stuck won't work well.
I have been thinking of adding MAP/UNMAP descriptors for
a while now. Thus it will be possible to modify
userspace memory without consuming buffers.
Would something like this solve the problem?



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