Re: [PATCH] arm64: add support for rng-seed

From: Kees Cook
Date: Thu May 09 2019 - 17:59:39 EST

On Thu, May 9, 2019 at 1:00 AM Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx> wrote:
> This early added entropy is also going to be used for stack canary. At
> the time it's created there's not be much entropy (before
> boot_init_stack_canary(), there's only add_latent_entropy() and
> command_line).
> On arm64, there is a single canary for all tasks. If RNG is weak or
> the seed can be read, it might be easier to figure out the canary.

With newer compilers[1] there will be a per-task canary on arm64[2],
which will improve this situation, but many architectures lack a
per-task canary, unfortunately. I've also recently rearranged the RNG
initialization[3] which should also help with better entropy mixing.
But each of these are kind of band-aids against not having sufficient
initial entropy, which leaves the canary potentially exposed.



Kees Cook