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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
hifive-unleashed-5.1
Daniel Vetter 2018-08-22 10:54:04 +02:00
parent 8d7fc2994f
commit da6c7707ca
2 changed files with 11 additions and 0 deletions
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4
drivers/video/fbdev/core/fbmem.c
View File

@ -1117,6 +1117,8 @@ 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;
@ -1327,6 +1329,8 @@ 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));
}

7
include/linux/fb.h
View File

@ -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;