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Date:   Mon, 4 Apr 2022 20:01:07 -0700
From:   Kees Cook <keescook@chromium.org>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, PaX Team <pageexec@freemail.hu>
Subject: Re: [PATCH v2] gcc-plugins: latent_entropy: use /dev/urandom
Message-ID: <202204041953.D7E0BA15@keescook>
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On Tue, Apr 05, 2022 at 12:47:14AM +0200, Jason A. Donenfeld wrote:
> On Mon, Apr 4, 2022 at 8:49 PM Kees Cook <keescook@chromium.org> wrote:
> > This mixes two changes: the pRNG change and the "use urandom if
> > non-deterministic" change. I think these should be split, so the pRNG
> > change can be explicitly justified.
> 
> Alright, I'll split those. Or, more probably, just drop the xorshift
> thing. There's not actually a strong reason for preferring xorshift. I
> did it because it produces more uniformity and is faster to compute and
> all that. But none of that stuff actually matters here. It was just a
> sort of "well I'm at it..." thing.

Well, it's nice to have and you already wrote it, so seems a waste to
just drop it. :)

> > > ?static struct plugin_info latent_entropy_plugin_info = {
> > > - ? ? .version ? ? ? ?= "201606141920vanilla",
> > > + ? ? .version ? ? ? ?= "202203311920vanilla",
> >
> > This doesn't really need to be versioned. We can change this to just
> > "vanilla", IMO.
> 
> Okay. I suppose you want it to be in a different patch too, right? In
> which case I'll leave it out and maybe get to it later. (I suppose one
> probably needs to double check whether it's used for anything
> interesting like dwarf debug info or whatever, where maybe it's
> helpful?)

Hm, I don't think it shows up anywhere, but you can just drop the hunk
that touch it. I can remove them all with a separate patch later.

> > > + ? ? if (deterministic_seed) {
> > > + ? ? ? ? ? ? unsigned HOST_WIDE_INT w = deterministic_seed;
> > > + ? ? ? ? ? ? w ^= w << 13;
> > > + ? ? ? ? ? ? w ^= w >> 7;
> > > + ? ? ? ? ? ? w ^= w << 17;
> > > + ? ? ? ? ? ? deterministic_seed = w;
> > > + ? ? ? ? ? ? return deterministic_seed;
> >
> > While seemingly impossible, perhaps don't reset "deterministic_seed",
> > and just continue to use "seed", so that it can never become "0" again.
> 
> Not sure I follow. It's an LFSR. The "L" is important. It'll never become
> zero. It's not "seemingly". We can prove it trivially in Magma:

Got it; yeah. I was reading too quickly. My brain misparsed and got
stuck on "left shift", but it's using rotation. Sorry for the noise.

-- 
Kees Cook