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From:   Stephen Boyd <swboyd@chromium.org>
Subject: Re: [PATCH] arm64: add support for rng-seed
Cc:     "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@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Architecture Mailman List <boot-architecture@lists.linaro.org>,
        Kees Cook <keescook@chromium.org>
To:     Hsin-Yi Wang <hsinyi@chromium.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Rob Herring <robh+dt@kernel.org>
Message-ID: <155754891575.14659.7326257870940088515@swboyd.mtv.corp.google.com>
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Date:   Fri, 10 May 2019 21:28:35 -0700
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Quoting Rasmus Villemoes (2019-05-09 23:14:00)
>=20
> So, why not just have the bootloader add whatever entropy it has via the
> commandline, which already gets mixed in? That requires no kernel
> changes, and works for all architectures.
>=20
> If anything, perhaps instead of just adding gobbledygook=3Dabc123, make an
> official command line parameter (there was talk about this at some
> point), and have the kernel overwrite the value with xxx so it's not
> visible in /proc/cmdline.
>=20

Why is using the commandline desired? Just for ease of implementation
and cross-architecture support because we already mix in the
commandline?=20

The kernel commandline is limited in size so we would waste around
64-bytes of the buffer to get a random chunk of data from the bootloader
into the kernel instead of allowing more parameters. Or if we wanted a
large chunk of random bytes then we would start running into the length
limit. Given that EFI based systems already have a way to inject more
randomness into the kernel's RNG very early by means of an RNG seed EFI
protocol it looks irrelevant to want to be cross-architecture in this
way because EFI platforms wouldn't use it.=20

If DT based systems can all get support for this in the generic DT code
then we're able to make things work on both EFI and DT platforms with a
little extra __init code while keeping things away from the commandline.
That sounds like a win to me because the commandline is limited in size
and meant to pass things like parameters and flags to the kernel, not
raw data like seeds and binary gook.