linux-kernel - Re: [PATCH 0/6] crypto: DRBG

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Date:   Wed, 27 Oct 2021 10:40:12 +0200
From:   Nicolai Stange <nstange@...e.de>
To:     Stephan Müller <smueller@...onox.de>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Nicolai Stange <nstange@...e.de>, Torsten Duwe <duwe@...e.de>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] crypto: DRBG - improve 'nopr' reseeding

Hi Stephan,

first of all, many thanks for your prompt review!

Stephan Müller <smueller@...onox.de> writes:

> Am Montag, 25. Oktober 2021, 11:25:19 CEST schrieb Nicolai Stange:
>
>
>> - Replace the asynchronous random_ready_callback based DRBG reseeding
>>   logic with a synchronous solution leveraging rng_is_initialized().
>
> Could you please help me why replacing an async method with a sync method is 
> helpful? Which problems do you see with the async method that are alleviated 
> with the swtich to the sync method? In general, an async method is more 
> powerful, though it requires a bit more code.

There is no problem with the async method (*), I just don't see any
advantage over the less complex approach of doing all reseeding
work synchronously from drbg_generate().

Before the change, there had been two sites taking care of reseeding:
the drbg_async_seed() work handler scheduled from the
random_ready_callback and drbg_generate().

After the change, all reseeding is handled at a single place only, namely
drbg_generate(), which, in my opinion, makes it easier to reason about.
In particular, in preparation for patch 6/6 from this series introducing
yet another condition for triggering a reseed...

Thanks,

Nicolai

(*) Except for that a wait_for_random_bytes() issued by DRBG users won't
    give any guarantees with respect to a subsequent drbg_generate()
    operation, c.f. my other mail in reply to your review on 3/6 I'm
    about to write in a second. As of now, there aren't any DRBG users
    invoking wait_for_random_bytes(), but one might perhaps consider
    changing that in the future.

>>   This
>>   move simplifies the code IMO and, as a side-effect, would enable DRBG
>>   users to rely on wait_for_random_bytes() to sync properly with
>>   drbg_generate(), if desired. Implemented by patches 1-5/6.
>> - Make the 'nopr' DRBGs to reseed themselves every 5min from
>>   get_random_bytes(). This achieves at least kind of a partial prediction
>>   resistance over the time domain at almost no extra cost. Implemented
>>   by patch 6/6, the preceding patches in this series are a prerequisite
>>   for this.

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