linux-kernel - Re: [Y2038] [PATCH] crypto: Jitter RNG

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Date:	Tue, 21 Jun 2016 12:38:08 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	y2038 Mailman List <y2038@...ts.linaro.org>,
	Stephan Mueller <smueller@...onox.de>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Alexander Kuleshov <kuleshovmail@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [Y2038] [PATCH] crypto: Jitter RNG - use ktime_get_raw_ns as fallback

On Tue, Jun 21, 2016 at 12:37 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Tuesday, June 21, 2016 12:05:06 PM CEST John Stultz wrote:
>> On Tue, Jun 21, 2016 at 11:49 AM, Stephan Mueller <smueller@...onox.de> wrote:
>> > Am Dienstag, 21. Juni 2016, 11:11:42 schrieb John Stultz:
>> >
>> > Hi John,
>> >
>> >> I don't see in the above an explanation of *why* you're using
>> >> ktime_get_raw_ns() instead of ktime_get_ns().
>> >
>> > Could you help me understand what the difference is or point me to some
>> > documentation? I understood that we only talked about the _raw variant.
>>
>> Using specialized interfaces with subtle semantics w/o understanding
>> them is sort of my concern here.
>>
>> There are reasons why you might want to use the ktime_get_raw_ns()
>> interface over ktime_get_ns(), but they have not been made clear in
>> the comment. Arnd discussed some potential concerns that the freq
>> adjustment done by ntp might be somewhat predictable/controlled by
>> remote parties, which could have some effect in the calculation. That
>> feels a little overly vague to me, but I'm no crypto expert, so if
>> that is a reasonable concern, then it should be a conscious and
>> documented decision.
>
> My original patch changed __getnstimeofday() to __getnstimeofday64(),
> which kept the original semantics of not warning in case the clock
> source is suspended (which is the only different to the normal
> getnstimeofday{,64}().
>
> I did the patch a while time ago along with a number of other patches
> that I never sent out until last week, so I don't remember the
> reasoning for suggesting ktime_get_raw_fast_ns() over ktime_get_raw_ns(),
> but I sure wanted to keep the non-warning behavior, and ktime_get_ns()
> warns on timekeeping_suspended() while the other two don't.

Right. But this code isn't called in late suspend or early resume is it?


> If we don't care about the non-warning aspect, ktime_get_ns() makes
> most sense here, and the original code should probably have used
> getnstimeofday() as well.

This is what I suspect as well.  But again, I don't mind if folks use
the specialized interfaces, as long as they document a clear reason
for it. Especially for things like crypto where intuition isn't always
the best guide.

thanks
-john