linux-kernel - Re: [PATCH v3 6/7] docs: gpio: Add GPIO Aggregator/Repeater documentation

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Date:   Thu, 12 Dec 2019 15:42:01 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Geert Uytterhoeven <geert+renesas@...der.be>
Cc:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Jonathan Corbet <corbet@....net>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Harish Jenny K N <harish_kandiga@...tor.com>,
        Eugeniu Rosca <erosca@...adit-jv.com>,
        Alexander Graf <graf@...zon.com>,
        Peter Maydell <peter.maydell@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Phil Reid <preid@...ctromag.com.au>,
        Marc Zyngier <marc.zyngier@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        Magnus Damm <magnus.damm@...il.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>
Subject: Re: [PATCH v3 6/7] docs: gpio: Add GPIO Aggregator/Repeater documentation

On Wed, Nov 27, 2019 at 9:43 AM Geert Uytterhoeven
<geert+renesas@...der.be> wrote:

> +The GPIO Aggregator allows access control for individual GPIOs, by aggregating
> +them into a new gpio_chip, which can be assigned to a group or user using
> +standard UNIX file ownership and permissions.  Furthermore, this simplifies and
> +hardens exporting GPIOs to a virtual machine, as the VM can just grab the full
> +GPIO controller, and no longer needs to care about which GPIOs to grab and
> +which not, reducing the attack surface.
> +
> +Aggregated GPIO controllers are instantiated and destroyed by writing to
> +write-only attribute files in sysfs.

I suppose virtual machines will have a lengthy config file where
they specify which GPIO lines to pick and use for their GPIO
aggregator, and that will all be fine, the VM starts and the aggregator
is there and we can start executing.

I would perhaps point out a weakness as with all sysfs and with the current
gpio sysfs: if a process creates an aggregator device, and then that
process crashes, what happens when you try to restart the process and
run e.g. your VM again?

Time for a hard reboot? Or should we add some design guidelines for
these machines so that they can cleanly tear down aggregators
previously created by the crashed VM?

Yours,
Linus Walleij