Re: [PATCH] fbmem: Reject FB_ACTIVATE_KD_TEXT from userspace

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Apr 04, 2023 at 09:39:34PM +0200, Daniel Vetter wrote:
> 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 <daniel.vetter@intel.com>
> Fixes: dc5bdb68b5b3 ("drm/fb-helper: Fix vt restore")
> Cc: Alex Deucher <alexander.deucher@amd.com>
> Cc: shlomo@fastmail.com
> Cc: Michel Dänzer <michel@daenzer.net>
> Cc: Noralf Trønnes <noralf@tronnes.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: dri-devel@lists.freedesktop.org
> Cc: <stable@vger.kernel.org> # v5.7+
> Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> Cc: Nathan Chancellor <natechancellor@gmail.com>
> Cc: Qiujun Huang <hqjagain@gmail.com>
> Cc: Peter Rosin <peda@axentia.se>
> Cc: linux-fbdev@vger.kernel.org
> Cc: Helge Deller <deller@gmx.de>
> Cc: Sam Ravnborg <sam@ravnborg.org>
> Cc: Geert Uytterhoeven <geert+renesas@glider.be>
> Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>
> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Cc: Shigeru Yoshida <syoshida@redhat.com>

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

Thanks, Daniel

> ---
>  drivers/video/fbdev/core/fbmem.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 875541ff185b..3fd95a79e4c3 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,
>  	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);
> -- 
> 2.40.0
> 

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