Revert "rules: allow users to access frame buffer devices" (#3333)

Revert "rules: allow users to access frame buffer devices" (#3333)

This reverts commit 483d8bbb4c0190f419bf9fba57fb0feb1a56bea6.

In [1] Michel Dänzer and Daniel Vetter wrote:
>> The scenario you describe isn't possible if the Wayland compositor
>> directly uses the KMS API of /dev/dri/card*, but it may be possible if
>> the Wayland compositor uses the fbdev API of /dev/fb* instead (e.g. if
>> weston uses its fbdev backend).
>
> Yeah, if both weston and your screen grabber uses native fbdev API you can
> now screenshot your desktop. And since fbdev has no concept of "current
> owner of the display hw" like the drm master, I think this is not fixable.
> At least not just in userspace. Also even with native KMS compositors
> fbdev still doesn't have the concept of ownership, which is why it doesn't
> bother clearing it's buffer before KMS takes over. I agree that this
> should be reverted or at least hidden better.

TBH, I think that privilege separation between processes running under the same
UID is tenuous. Even with drm, in common setups any user process can ptrace the
"current owner of the display" and call DROP_MASTER or do whatever. It *is*
possible to prevent that, e.g. by disabling ptrace using yama.ptrace_scope, or
selinux, and so on, but afaik this is not commonly done. E.g. all Fedora
systems pull in elfutils-default-yama-scope.rpm through dependencies which sets
yama.ptrace_scope=0. And even assuming that ptrace was disabled, it is trivial
to modify files on disk, communicate through dbus, etc; there is just to many
ways for a non-sandboxed process to interact maliciously with the display shell
to close them all off. To achieve real protection, some sort of sandboxing
must be implemented, and in that case there is no need to rely on access mode
on the device files, since much more stringent measures have to be implemented
anyway.

The situation is similar for framebuffer devices. It is common to add
framebuffer users to video group to allow them unlimited access to /dev/fb*.
Using uaccess would be better solution in that case. Also, since there is no
"current owner" limitation like in DRM, processes running under the same UID
should be able to access /proc/<pid-of-display-server>/fd/* and gain access to
the devices. Nevertheless, weston implements a suid wrapper to access the
devices and then drop privileges, and this patch would make this daemon
pointless. So if the weston developers feel that this change reduces security,
I prefer to revert it.

[1] https://lists.freedesktop.org/archives/wayland-devel/2016-May/029017.html