fbmem: Reject FB_ACTIVATE_KD_TEXT from userspace

This is an oversight from dc5bdb6 ("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.

Reviewed-by: Javier Martinez Canillas <[email protected]>
Acked-by: Maarten Lankhorst <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Fixes: dc5bdb6 ("drm/fb-helper: Fix vt restore")
Cc: Alex Deucher <[email protected]>
Cc: [email protected]
Cc: Michel Dänzer <[email protected]>
Cc: Noralf Trønnes <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: [email protected]
Cc: <[email protected]> # v5.7+
Cc: Bartlomiej Zolnierkiewicz <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Qiujun Huang <[email protected]>
Cc: Peter Rosin <[email protected]>
Cc: [email protected]
Cc: Helge Deller <[email protected]>
Cc: Sam Ravnborg <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Samuel Thibault <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Shigeru Yoshida <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: danvet

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);