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

Hi Michael,

On 19/9/23 12:11, 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.

Any thoughts on this? Sorry for pushing. I want to finalize the details 
on this,
so I can close open fronts on the LKML patch.

Cheers,
Babis

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