From: Daniel Vetter <daniel@ffwll.ch>
To: Javier Martinez Canillas <javierm@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>,
linux-kernel@vger.kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
x86@kernel.org, Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
linux-fbdev@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
Maxime Ripard <mripard@kernel.org>,
Borislav Petkov <bp@alien8.de>,
Peter Robinson <pbrobinson@gmail.com>,
Hans de Goede <hdegoede@redhat.com>,
dri-devel@lists.freedesktop.org, Ingo Molnar <mingo@redhat.com>,
David Airlie <airlied@linux.ie>
Subject: Re: [RFC PATCH 0/4] Allow to use DRM fbdev emulation layer with CONFIG_FB disabled
Date: Tue, 31 Aug 2021 14:35:45 +0200 [thread overview]
Message-ID: <YS4iIR689bAZ4QT9@phenom.ffwll.local> (raw)
In-Reply-To: <a7395626-f022-5c89-07cd-c30d0d52d3dd@redhat.com>
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
next prev parent reply other threads:[~2021-08-31 12:35 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-27 10:00 [RFC PATCH 0/4] Allow to use DRM fbdev emulation layer with CONFIG_FB disabled Javier Martinez Canillas
2021-08-27 10:00 ` [RFC PATCH 1/4] fbdev: Rename fb_*_device() functions names to match what they do Javier Martinez Canillas
2021-08-27 17:50 ` [RFC PATCH 0/4] Allow to use DRM fbdev emulation layer with CONFIG_FB disabled Thomas Zimmermann
2021-08-27 20:20 ` Daniel Vetter
2021-08-27 22:02 ` Javier Martinez Canillas
2021-08-31 12:35 ` Daniel Vetter [this message]
2021-09-01 9:08 ` Javier Martinez Canillas
2021-09-02 14:31 ` Daniel Vetter
[not found] <YS+Lhz9gg/0Caa+0@ravnborg.org>
2021-09-09 15:52 ` Noralf Trønnes
2021-09-09 16:27 ` Noralf Trønnes
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