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SubjectRe: [PATCH 3/4] fbdev: Add FBINFO_HIDE_SMEM_START flag
On Wed, Aug 22, 2018 at 10:54:04AM +0200, Daniel Vetter wrote:
> 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>

Any comments from the fbdev side on this?
-Daniel

> ---
> 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
>

--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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