[PATCH] Revert "drm/core: Preserve the framebuffer after removing it."

[Daniel Vetter]

On Thu, May 19, 2016 at 5:15 PM, Hans de Goede <hdegoede at redhat.com> wrote:
> This reverts commit 13803132818c ("drm/core: Preserve the framebuffer
> after removing it.").
>
> This commit assumes that going through drm_framebuffer_remove() is not
> necessary because "the fbdev code or any system compositor should restore
> the planes anyway so there's no need to do it twice". But this is not true
> for secondary GPUs / slave outputs.
>
> This revert fixes the dgpu no longer suspending on laptops with
> switchable graphics after an external output which is connected
> to the dgpu has been used.
>
> And it fixes the WARN_ON to detect drm_framebuffer leaks in
> drm_mode_config_cleanup() triggering when unplugging an USB displaylink
> device; or when rmmod-ing the secondary GPU kms driver on laptops with
> switchable-graphics.
>
> Also this part of the reverted commit's commit-msg: "The old fb_id is
> zero'd, so there's no danger of being able to restore the fb from fb_id."
> is no longer true, the zero-ing does not happen until drm_framebuffer_free
> gets called, which does not happen until the last ref is dropped, so
> if a crtc's primary->fb is still pointing to this fb, the id will not
> get zero'd and userspace could potentially gain access to the removed
> fb again.
>
> Cc: stable at vger.kenrel.org
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>

We have a proper fix in drm-next:

commit f2d580b9a8149735cbc4b59c4a8df60173658140
Author: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Date:   Wed May 4 14:38:26 2016 +0200

    drm/core: Do not preserve framebuffer on rmfb, v4.

That thing took forever to get merged since no one seemed to have
cared and bothered with a tested-by. But it's on its way to stable
kernels now.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch