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Subject[PATCH 3/4] fbdev: Add FBINFO_HIDE_SMEM_START flag
Date
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.

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>
---
drivers/video/fbdev/core/fbmem.c | 4 ++++
include/linux/fb.h | 7 +++++++
2 files changed, 11 insertions(+)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 609438d2465b..549d0f86fcf3 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1116,6 +1116,8 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
if (!lock_fb_info(info))
return -ENODEV;
fix = info->fix;
+ if (info->flags & FBINFO_HIDE_SMEM_START)
+ fix.smem_start = 0;
unlock_fb_info(info);

ret = copy_to_user(argp, &fix, sizeof(fix)) ? -EFAULT : 0;
@@ -1326,6 +1328,8 @@ static int fb_get_fscreeninfo(struct fb_info *info, unsigned int cmd,
if (!lock_fb_info(info))
return -ENODEV;
fix = info->fix;
+ if (info->flags & FBINFO_HIDE_SMEM_START)
+ fix.smem_start = 0;
unlock_fb_info(info);
return do_fscreeninfo_to_user(&fix, compat_ptr(arg));
}
diff --git a/include/linux/fb.h b/include/linux/fb.h
index fa8c6f9c9c3a..f42b09ca71f8 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;
--
2.18.0
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