Re: [RFC PATCH 0/2] Propagating reseed notifications to user space

Alexander Graf
2023-09-18T08:33:38Z

Hey Yann!

On 17.09.23 15:34, Yann Droneaud wrote:
>
> Hi,
>
> Le 23/08/2023 à 11:01, Babis Chalios a écrit :
>> User space often implements PRNGs that use /dev/random as entropy
>> source. We can not expect that this randomness sources stay completely
>> unknown forever. For various reasons, the originating PRNG seed may
>> become known at which point the PRNG becomes insecure for further random
>> number generation. Events that can lead to that are for example fast
>> computers reversing the PRNG function using a number of inputs or
>> Virtual Machine clones which carry seed values into their clones.
>>
>> During LPC 2022 Jason, Alex, Michael and me brainstormed on how to
>> atomically expose a notification to user space that it should reseed.
>> Atomicity is key for the VM clone case. This patchset implements a
>> potential path to do so.
>>
>> This patchset introduces an epoch value as the means of communicating to
>> the guest the need to reseed. The epoch is a 32bit value with the
>> following form:
>>
>>                RNG epoch
>> *-------------*---------------------*
>> | notifier id | epoch counter value |
>> *-------------*---------------------*
>>       8 bits           24 bits
>>
>> Changes in this value signal moments in time that PRNGs need to be
>> re-seeded. As a result, the intended use of the epoch from user space
>> PRNGs is to cache the epoch value every time they reseed using kernel
>> entropy, then control that its value hasn't changed before giving out
>> random numbers. If the value has changed the PRNG needs to reseed before
>> producing any more random bits.
>>
>> The API for getting hold of this value is offered through
>> /dev/(u)random. We introduce a new ioctl for these devices, which
>> creates an anonymous file descriptor. User processes can call the
>> ioctl() to get the anon fd and then mmap it to a single page. That page
>> contains the value of the epoch at offset 0.
>>
>> Naturally, random.c is the component that maintains the RNG epoch.
>> During initialization it allocates a single global page which holds the
>> epoch value. Moreover, it exposes an API to kernel subsystems
>> (notifiers) which can report events that require PRNG reseeding.
>> Notifiers register with random.c and receive an 8-bit notifier id (up to
>> 256 subscribers should be enough) and a pointer to the epoch. Notifying,
>> then, is equivalent to writing in the epoch address a new epoch value.
>>
>> Notifiers write epoch values that include the notifier ID on the higher
>> 8 bits and increasing counter values on the 24 remaining bits. This
>> guarantees that two notifiers cannot ever write the same epoch value,
>> since notificator IDs are unique.
>>
>> The first patch of this series implements the epoch mechanism. It adds
>> the logic in the random.c to maintain the epoch page and expose the
>> user space facing API. It also adds the internal API that allows kernel
>> systems to register as notifiers.
>
> From userspace point of view, having to open /dev/random, ioctl, and
> mmap()
> is a no-go for a (CS)PRNG embedded in libc for arc4random().

Could you please elaborate on why it's a no-go? With any approach we
take, someone somewhere needs to map and expose data to user space that
we are in a new "epoch". With this patch set, you do that explicitly
from user space through an fd that you keep open plus an mmap that you
keep active. With vgetrandom, the kernel does it implicitly for you.

So with this patch set's approach, the first call to arc4random() would
need to establish the epoch mmap and leave it open. After that epoch
handling is (almost) free - it's just a 32bit value compare.

Are you saying that there is a problem with keeping track of that
additional state? As mentioned above, we need to keep track of some
state somewhere: Either in the vdso plus kernel page map logic or in the
library that consumes epochs.

If this is the problem, maybe the fundamental issue is that arc4random()
assumes you always have everything in place to receive randomness
without a handle that could go through an open/close (init/destroy)
cycle? I suppose you could change that?

> I'm biased, as I proposed to expose such seed epoch value to userspace
> through
> getrandom() directly, relying on vDSO for reasonable performances,
> because
> current's glibc arc4random() is somewhat to slow to be a general
> replacement
> rand().
>
> See
> https://lore.kernel.org/all/cover.1673539719.git.ydroneaud@opteya.com/
> https://lore.kernel.org/all/ae35afa5b824dc76c5ded98efcabc117e6dd3d70@opteya.com/
>

There are more problems with coupling epochs to the vgetrandom approach:
Not everyone will want to or can use Linux's rng as the sole source of
entropy for various reasons (NIST, FIPS, TLS recommendations to not rely
on a single source, real time requirements, etc) but still require
knowledge of epoch changes.

That means we need an alternative path for these applications
regardless. May as well start with that :). If we then still conclude
that vgetrandom is the best path forward to accelerate access to
/dev/urandom in user space, we can just always map this patch set's
epoch page into the vDSO range and then make vgetrandom consume it,
similar to how a user space library would.

I genuinely don't understand how vgetrandom and this patch set
contradict each other.

Alex

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