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

Alexander Graf
2022-05-11T13:20:15Z

Hi Simo,

On 11.05.22 14:59, Simo Sorce wrote:
> Hi Jason,
>
> On Wed, 2022-05-11 at 03:18 +0200, Jason A. Donenfeld wrote:
>> My proposal here is made with nonce reuse in mind, for things like
>> session keys that use sequential nonces.
> Although this makes sense the problem is that changing applications to
> do the right thing based on which situation they are in will never be
> done right or soon enough. So I would focus on a solution that makes
> the CSPRNGs in crypto libraries safe.

I think we intrinsically have 2 sets of applications: Ones that want an
event based notification and don't care about the racyness of it and
ones that want an atomic way to determine the epoch. Userspace RNGs are
naturally in the second category.

>
>> A different issue is random nonces. For these, it seems like a call to
>> getrandom() for each nonce is probably the best bet. But it sounds like
>> you're interested in a userspace RNG, akin to OpenBSD's arc4random(3). I
>> hope you saw these threads:
>>
>> - https://lore.kernel.org/lkml/YnA5CUJKvqmXJxf2@zx2c4.com/
>> - https://lore.kernel.org/lkml/Yh4+9+UpanJWAIyZ@zx2c4.com/
>> - https://lore.kernel.org/lkml/CAHmME9qHGSF8w3DoyCP+ud_N0MAJ5_8zsUWx=rxQB1mFnGcu9w@mail.gmail.com/
> 4c does sound like a decent solution, it is semantically identical to
> an epoch vmgenid, all the library needs to do is to create such a mmap
> region, stick a value on it, verify it is not zero after computing the
> next random value but before returning it to the caller.
> This reduces the race to a very small window when the machine is frozen
> right after the random value is returned to the caller but before it is
> used, but hopefully this just means that the two machines will just
> make parallel computations that yield the exact same value, so no
> catastrophic consequence will arise (there is the odd case where two
> random values are sought and the split happens between the two are
> retrieved and this has bad consequences, I think we can ignore that).

The problem with wiping memory on clone is that it means you must keep
these special wipe on clone pages always present and pinned in memory
and can no longer swap them out, compact them or move them for memory
hotplug.

We started the journey with a WIPEONSUSPEND flag and eventually
abandoned it because it seemed clunky. Happy to reopen it if we believe
it's a viable path:

https://lwn.net/Articles/825230/

Alex

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