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From:   Kees Cook <keescook@chromium.org>
Date:   Thu, 9 May 2019 14:58:07 -0700
Message-ID: <CAGXu5j+S3tQ3DFtmTJT_O1rNx4ofZWvaFpPrES9peHRhMqRGjg@mail.gmail.com>
Subject: Re: [PATCH] arm64: add support for rng-seed
To:     Hsin-Yi Wang <hsinyi@chromium.org>
Cc:     Rob Herring <robh+dt@kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Frank Rowand <frowand.list@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Michal Hocko <mhocko@suse.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        James Morse <james.morse@arm.com>,
        Andrew Murray <andrew.murray@arm.com>,
        devicetree <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Stephen Boyd <swboyd@chromium.org>,
        Architecture Mailman List <boot-architecture@lists.linaro.org>,
        Kees Cook <keescook@chromium.org>
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On Thu, May 9, 2019 at 1:00 AM Hsin-Yi Wang <hsinyi@chromium.org> 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

[1] https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=359c1bf35e3109d2f3882980b47a5eae46123259
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0a1213fa7432778b71a1c0166bf56660a3aab030
[3] https://git.kernel.org/pub/scm/linux/kernel/git/tytso/random.git/commit/?h=dev&id=d55535232c3dbde9a523a9d10d68670f5fe5dec3

-- 
Kees Cook