linux-kernel - Re: [RFC PATCH 0/4] Allow to use DRM fbdev emulation layer with CONFIG

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Date:   Tue, 31 Aug 2021 14:35:45 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Javier Martinez Canillas <javierm@...hat.com>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org, "H . Peter Anvin" <hpa@...or.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        x86@...nel.org, Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-fbdev@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Maxime Ripard <mripard@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Robinson <pbrobinson@...il.com>,
        Hans de Goede <hdegoede@...hat.com>,
        dri-devel@...ts.freedesktop.org, Ingo Molnar <mingo@...hat.com>,
        David Airlie <airlied@...ux.ie>
Subject: Re: [RFC PATCH 0/4] Allow to use DRM fbdev emulation layer with
 CONFIG_FB disabled

On Sat, Aug 28, 2021 at 12:02:21AM +0200, Javier Martinez Canillas wrote:
> Hello Daniel and Thomas,
> 
> On 8/27/21 10:20 PM, Daniel Vetter wrote:
> > On Fri, Aug 27, 2021 at 07:50:23PM +0200, Thomas Zimmermann wrote:
> >> Hi
> >>
> >> Am 27.08.21 um 12:00 schrieb Javier Martinez Canillas:
> >>> This patch series splits the fbdev core support in two different Kconfig
> >>> symbols: FB and FB_CORE. The motivation for this is to allow CONFIG_FB to
> >>> be disabled, while still using fbcon with the DRM fbdev emulation layer.
> >>
> >> I'm skeptical. DRM's fbdev emulation is not just the console emulation, it's
> >> a full fbdev device. You can see the related device file as /dev/fb*.
> >> Providing the file while having CONFIG_FB disabled doesn't make much sense
> >> to me. I know it's not pretty, but it's consistent at least.
> >>
> >> If you want to remove fbdev, you could try to untangle fbdev and the console
> >> emulation such that DRM can set up a console by itself. Old fbdev drives
> >> would also set up the console individually.
> > 
> > Yeah given the horrendous security track record of all that code, and the
> > maze of handover we have (stuff like flicker free boot and all that) I'm
> > wondering whether typing a new drmcon wouldn't be faster and a lot more
> > maintainable.
> > 
> 
> We talked about a drmcon with Peter Robinson as well but then decided that a
> way to disable CONFIG_FB but still having the DRM fbdev emulation could be a
> intermediary step, hence these RFC patches.
> 
> But yes, I agree that a drmcon would be the proper approach for this, to not
> need any fbdev support at all. We will just keep the explicit disable for the
> fbdev drivers then in the meantime.

I think the only intermediate step would be to disable the fbdev uapi
(char node and anything in sysfs), while still registering against the
fbcon layer so you have a console.

But looking at the things syzbot finds the really problematic code is all
in the fbcon and console layer in general, and /dev/fb0 seems pretty
solid.

I think for a substantial improvement here in robustness what you really
want is
- kmscon in userspace
- disable FB layer
- ideally also disable console/vt layer in the kernel
- have a minimal emergency/boot-up log thing in drm, patches for that
  floated around a few times

Otherwise it feels a bit like we're just doing Kconfig bikeshedding and
no real improvement on the attack surface :-/
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch