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Date:   Wed, 22 Aug 2018 08:57:36 -0400
From:   Sean Paul <sean@poorly.run>
To:     Daniel Vetter <daniel.vetter@ffwll.ch>
Cc:     DRI Development <dri-devel@lists.freedesktop.org>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Gustavo Padovan <gustavo@padovan.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Sean Paul <sean@poorly.run>, David Airlie <airlied@linux.ie>,
        John Stultz <john.stultz@linaro.org>,
        Noralf =?iso-8859-1?Q?Tr=F8nnes?= <noralf@tronnes.org>,
        Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH 4/4] drm/fb: Stop leaking physical address
Message-ID: <20180822125736.GV164269@art_vandelay>
References: <20180822085405.10787-1-daniel.vetter@ffwll.ch>
 <20180822085405.10787-4-daniel.vetter@ffwll.ch>
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On Wed, Aug 22, 2018 at 10:54:05AM +0200, Daniel Vetter wrote:
> For buffer sharing, use dma-buf instead. We can't set smem_start to 0
> unconditionally since that's used by the fbdev mmap default
> implementation. And we have plenty of userspace which would like to
> keep that working.
> 
> This might break legit userspace - if it does we need to look at a
> case-by-cases basis how to handle that. Worst case I expect overrides
> for only specific drivers, since anything remotely modern should be
> using dma-buf/prime now (which is about 7 years old now for DRM
> drivers).
> 
> This issue was uncovered because Noralf's rework to implement a
> generic fb_probe also implements it's own fb_mmap callback. Which
> means smem_start didn't have to be set anymore, which blew up some
> blob in userspace rather badly.
> 
> Cc: Gustavo Padovan <gustavo@padovan.org>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Sean Paul <sean@poorly.run>
> Cc: David Airlie <airlied@linux.ie>
> Cc: John Stultz <john.stultz@linaro.org>
> Cc: Noralf Tr?nnes <noralf@tronnes.org>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

After clarifying on IRC, I think this makes sense. The chance to break userspace
requires a very specific configuration that I don't expect will be common.

Acked-by: Sean Paul <seanpaul@chromium.org>

> ---
>  drivers/gpu/drm/drm_fb_helper.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 4b0dd20bccb8..bcb78693c4f7 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -2673,6 +2673,8 @@ __drm_fb_helper_initial_config_and_unlock(struct drm_fb_helper *fb_helper,
>  
>  	info = fb_helper->fbdev;
>  	info->var.pixclock = 0;
> +	/* don't leak any physical addresses to userspace */
> +	info->flags |= FBINFO_HIDE_SMEM_START;
>  
>  	/* Need to drop locks to avoid recursive deadlock in
>  	 * register_framebuffer. This is ok because the only thing left to do is
> -- 
> 2.18.0
> 

-- 
Sean Paul, Software Engineer, Google / Chromium OS