Re: [PATCH 2/2] random: add fork_event sysctl for polling VM forks

Alexander Graf
2022-05-11T13:22:20Z

On 11.05.22 03:18, Jason A. Donenfeld wrote:
> Hi Simo,
>
> On Tue, May 10, 2022 at 08:40:48PM -0400, Simo Sorce wrote:
>> At your request teleporting here the answer I gave on a different
>> thread, reinforced by some thinking.
>>
>> As a user space crypto library person I think the only reasonable
>> interface is something like a vDSO.
>>
>> Poll() interfaces are nice and all for system programs that have full
>> control of their event loop and do not have to react immediately to
>> this event, however crypto libraries do not have the luxury of
>> controlling the main loop of the application.
>>
>> Additionally crypto libraries really need to ensure the value they
>> return from their PRNG is fine, which means they do not return a value
>> if the vmgenid has changed before they can reseed, or there could be
>> catastrophic duplication of "random" values used in IVs or ECDSA
>> Signatures or ids/cookies or whatever.
>>
>> For crypto libraries it is much simpler to poll for this information
>> than using notifications of any kind given libraries are
>> generally not in full control of what the process does.
>>
>> This needs to be polled fast as well, because the whole point of
>> initializing a PRNG in the library is that asking /dev/urandom all the
>> time is too slow (due to context switches and syscall overhead), so
>> anything that would require a context switch in order to pull data from
>> the PRNG would not really fly.
>>
>> A vDSO or similar would allow to pull the vmgenid or whatever epoch
>> value in before generating the random numbers and then barrier-style
>> check that the value is still unchanged before returning the random
>> data to the caller. This will reduce the race condition (which simply
>> cannot be completely avoided) to a very unlikely event.
> It sounds like your library issue is somewhat similar to what Alex was
> talking about with regards to having a hard time using poll in s2n. I'm
> still waiting to hear if Amazon figured out some way that this is
> possible (with, e.g., a thread). But anyway, it seems like this is

Sounds like I didn't reply with my findings - sorry about that. Our s2n
people *could* build something based on a thread, but are afraid that
it's racy and would introduce creating a thread which the library does
not do today.

So in a nutshell, possible yes, desirable no.

I think we're maybe a bit too scared of building something from scratch
here. What would the best case situation be? Let's roll backwards from
that then.

From what I read, we want a "VMGenID v2" device that gives us the
ability to

* Get an IRQ on VM clone
* Store and update an RNG seed value (128bit? Configurable len?) at a
physical address or stand alone page on clone
* Store and update a unique-to-this-VM rolling 32bit identifier at a
physical address or stand alone page on clone

We can either make the device expose these values as individual pages
(like VMGenID does today) or as guest physical addresses that it needs
to store into (like virtio-rng). The latter makes protection from DMA
attacks of the hypervisor and kexec slightly more complicated, but it
would be doable.

VMGenID has 2 out of 3 features above. So why don't we just go the easy
route, add a second property to VMGenID that gives us another page with
that 32bit value and then provide a /dev/vmgenid device node you can
open and mmap() that 32bit value page from?

User space libraries can then try to open on init and determine their epoch.
For the async event, we add the poll() logic again to /dev/vmgenid and
make networkd for example use that.

Alex

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