[RFC PATCH 0/4] Allow to use DRM fbdev emulation layer with CONFIG_FB disabled

[Daniel Vetter]

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