linux-kernel - Re: [PATCH v8 2/3] fdt: add support for rng-seed

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Date:   Wed, 21 Aug 2019 12:21:26 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Hsin-Yi Wang <hsinyi@...omium.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        Rob Herring <robh+dt@...nel.org>,
        Devicetree List <devicetree@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Miles Chen <miles.chen@...iatek.com>,
        James Morse <james.morse@....com>,
        Andrew Murray <andrew.murray@....com>,
        Mark Rutland <mark.rutland@....com>,
        Jun Yao <yaojun8558363@...il.com>, Yu Zhao <yuzhao@...gle.com>,
        Robin Murphy <robin.murphy@....com>,
        Laura Abbott <labbott@...hat.com>,
        Stephen Boyd <swboyd@...omium.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v8 2/3] fdt: add support for rng-seed

On Wed, Aug 21, 2019 at 09:39:28AM +0300, Ard Biesheuvel wrote:
> 
> Whether to trust the firmware provided entropy is a policy decision,
> and typically, we try to avoid dictating policy in the kernel, and
> instead, we try to provide a sane default but give the user control
> over it.
> 
> So in this case, we should probably introduce
> add_firmware_randomness() with a Kconfig/cmdline option pair to decide
> whether it should be trusted or not (or reuse the one we have for
> trusting RDRAND etc)

I'd call it add_bootloader_randomness(), since we are trusting the
*bootloader*; it's the bootloader which is vouching for the security /
validity of the passed-in entropy.  Furthermore, the bootloader on
some architectures might be fetching directly from some secure
element.

And for that reason, I'd use a different Kconfig/cmdline option pair
than the one used for trusting CPU-provided randomness.

						- Ted