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 <[e-mail]>
Cc: Bartlomiej Zolnierkiewicz <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: [e-mail]
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/video/fbdev/core/fbmem.c

···
 		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;
···
 	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));
 }

include/linux/fb.h

···
  * 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;