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

On Tue, 14 Jun 2016 11:27:00 -0700
Kees Cook [off-list ref] wrote:

On Mon, Jun 13, 2016 at 2:49 PM, Emese Revfy [off-list ref] wrote:

quoted

On Thu, 9 Jun 2016 14:51:45 -0700
Kees Cook [off-list ref] wrote:

quoted

quoted

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+ * 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