fbdev: Add FBINFO_HIDE_SMEM_START flag

DRM drivers really, really, really don't want random userspace to
share buffer behind it's back, bypassing the dma-buf buffer sharing
machanism. For that reason we've ruthlessly rejected any IOCTL
exposing the physical address of any graphics buffer.

Unfortunately fbdev comes with that built-in. We could just set
smem_start to 0, but that means we'd have to hand-roll our own fb_mmap
implementation. For good reasons many drivers do that, but
smem_start/length is still super convenient.

Hence instead just stop the leak in the ioctl, to keep fb mmap working
as-is. A second patch will set this flag for all drm drivers.

Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180822085405.10787-3-daniel.vetter@ffwll.ch

1 files changed, 7 insertions, 0 deletions

diff --git a/include/linux/fb.h b/include/linux/fb.h
index b1db5c5eb671..a3cab6dc9b44 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h
@@ -456,6 +456,13 @@ struct fb_tile_ops {
  * and host endianness. Drivers should not use this flag.
  */
 #define FBINFO_BE_MATH  0x100000
+/*
+ * Hide smem_start in the FBIOGET_FSCREENINFO IOCTL. This is used by modern DRM
+ * drivers to stop userspace from trying to share buffers behind the kernel's
+ * back. Instead dma-buf based buffer sharing should be used.
+ */
+#define FBINFO_HIDE_SMEM_START  0x200000
+
 
 struct fb_info {
 	atomic_t count;