linux-kernel - Re: [PATCH v2 1/3] Add the latent

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Message-Id: <20160615003121.008b63c8152aa60c9c351530@gmail.com>
Date:	Wed, 15 Jun 2016 00:31:21 +0200
From:	Emese Revfy <re.emese@...il.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	PaX Team <pageexec@...email.hu>,
	Brad Spengler <spender@...ecurity.net>,
	Michal Marek <mmarek@...e.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Masahiro Yamada <yamada.masahiro@...ionext.com>,
	linux-kbuild <linux-kbuild@...r.kernel.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>, Jens Axboe <axboe@...nel.dk>,
	Al Viro <viro@...iv.linux.org.uk>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	bart.vanassche@...disk.com, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH v2 1/3] Add the latent_entropy gcc plugin

On Tue, 14 Jun 2016 11:27:00 -0700
Kees Cook <keescook@...omium.org> wrote:

> On Mon, Jun 13, 2016 at 2:49 PM, Emese Revfy <re.emese@...il.com> wrote:
> > On Thu, 9 Jun 2016 14:51:45 -0700
> > Kees Cook <keescook@...omium.org> wrote:
>
> >> > + * gcc plugin to help generate a little bit of entropy from program state,
> >> > + * used throughout the uptime of the kernel
> >>
> >> I think this comment needs a lot of expanding. What are all the ways
> >> that this plugin makes changes to code? Things I think I see are:
> >> pre-filling data variables with randomness, creating a local_entropy
> >> variable (local to what?), mixing stack pointer (into what?), updating
> >> latent_entropy global.
> >
> > I demonstrated the details here:
> > https://github.com/ephox-gcc-plugins/latent_entropy/commit/049acd9f478d47ee6526d8e93ab8cfcc3ff91b13
> 
> That helps, thanks. Can you also mention how __latent_entropy changes
> non-functions? (i.e. initializes them with random data.)
> 
> Also, I think this isn't accurate:
> 
>  * local_entropy ^= get_random_long();
> 
> Looking at the disassembly, it seems that static random values (i.e.
> randomly chosen at gcc runtime) are added, rather than making calls to
> the kernel's get_random_long() function.

The plugin doesn't insert calls to the kernel's get_random_long().
That was just an example (the plugin instrumentation would look like this in the kernel).
I rewrote these calls to a random constant.

-- 
Emese