linux-kernel - [PATCH 4/4] drm/fb: Stop leaking physical address

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Date:   Wed, 22 Aug 2018 10:54:05 +0200
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     DRI Development <dri-devel@...ts.freedesktop.org>
Cc:     Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        John Stultz <john.stultz@...aro.org>,
        Noralf Trønnes <noralf@...nnes.org>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: [PATCH 4/4] drm/fb: Stop leaking physical address

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@...ovan.org>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
Cc: Sean Paul <sean@...rly.run>
Cc: David Airlie <airlied@...ux.ie>
Cc: John Stultz <john.stultz@...aro.org>
Cc: Noralf Trønnes <noralf@...nnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
---
 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