Hi Daniel,

On Tue, Apr 11, 2023 at 3:44 PM Daniel Vetter [off-list ref] wrote:

On Tue, Apr 04, 2023 at 09:39:34PM +0200, Daniel Vetter wrote:

quoted

This is an oversight from dc5bdb68b5b3 ("drm/fb-helper: Fix vt
restore") - I failed to realize that nasty userspace could set this.

It's not pretty to mix up kernel-internal and userspace uapi flags
like this, but since the entire fb_var_screeninfo structure is uapi
we'd need to either add a new parameter to the ->fb_set_par callback
and fb_set_par() function, which has a _lot_ of users. Or some other
fairly ugly side-channel int fb_info. Neither is a pretty prospect.

Instead just correct the issue at hand by filtering out this
kernel-internal flag in the ioctl handling code.

Signed-off-by: Daniel Vetter <redacted>
Fixes: dc5bdb68b5b3 ("drm/fb-helper: Fix vt restore")

An Ack on this (or a better idea) would be great, so I can stuff it into
-fixes.

I don't understand what the original commit this fixes is doing anyway...

quoted

--- 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,
      case FBIOPUT_VSCREENINFO:
              if (copy_from_user(&var, argp, sizeof(var)))
                      return -EFAULT;
+             /* only for kernel-internal use */
+             var.activate &= ~FB_ACTIVATE_KD_TEXT;
              console_lock();
              lock_fb_info(info);
              ret = fbcon_modechange_possible(info, &var);

Perhaps FB_ACTIVATE_KD_TEXT should be removed (marked as
reserved) from include/uapi/linux/fb.h, too?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds