Re: [virtio-dev] [PATCH RFC 3/3] rng: leak detection support
Babis Chalios <[email protected]>
| Newsgroups | dev.linux.lists.virtio-dev |
|---|---|
| Message-ID | <[email protected]> |
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 --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]