Re: [PATCH v2 1/3] Add the latent_entropy gcc plugin

Emese Revfy
2016-06-14T22:24:43Z

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

> On Mon, Jun 13, 2016 at 2:49 PM, Emese Revfy <re.emese@gmail.com> wrote:
> > On Thu, 9 Jun 2016 14:51:45 -0700
> > Kees Cook <keescook@chromium.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