[PATCH] fbdev: Don't spam dmesg on bad userspace ioctl input

[Geert Uytterhoeven]

Hi Daniel,

CC vkmsdrm maintainer

Thanks for your patch!

On Tue, Apr 4, 2023 at 2:36 PM Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> There's a few reasons the kernel should not spam dmesg on bad
> userspace ioctl input:
> - at warning level it results in CI false positives
> - it allows userspace to drown dmesg output, potentially hiding real
>   issues.
>
> None of the other generic EINVAL checks report in the
> FBIOPUT_VSCREENINFO ioctl do this, so it's also inconsistent.
>
> I guess the intent of the patch which introduced this warning was that
> the drivers ->fb_check_var routine should fail in that case. Reality
> is that there's too many fbdev drivers and not enough people
> maintaining them by far, and so over the past few years we've simply
> handled all these validation gaps by tighning the checks in the core,
> because that's realistically really all that will ever happen.
>
> Reported-by: syzbot+20dcf81733d43ddff661 at syzkaller.appspotmail.com
> Link: https://syzkaller.appspot.com/bug?id=c5faf983bfa4a607de530cd3bb008888bf06cefc

    WARNING: fbcon: Driver 'vkmsdrmfb' missed to adjust virtual screen
size (0x0 vs. 64x768)

This is a bug in the vkmsdrmfb driver and/or DRM helpers.

The message was added to make sure the individual drivers are fixed.
Perhaps it should be changed to BUG() instead, so dmesg output
cannot be drown?

> Fixes: 6c11df58fd1a ("fbmem: Check virtual screen sizes in fb_set_var()")
> Cc: Helge Deller <deller at gmx.de>
> Cc: Geert Uytterhoeven <geert at linux-m68k.org>
> Cc: stable at vger.kernel.org # v5.4+
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: Javier Martinez Canillas <javierm at redhat.com>
> Cc: Thomas Zimmermann <tzimmermann at suse.de>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>

NAKed-by: Geert Uytterhoeven <geert at linux-m68k.org>

> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1021,10 +1021,6 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
>         /* verify that virtual resolution >= physical resolution */
>         if (var->xres_virtual < var->xres ||
>             var->yres_virtual < var->yres) {
> -               pr_warn("WARNING: fbcon: Driver '%s' missed to adjust virtual screen size (%ux%u vs. %ux%u)\n",
> -                       info->fix.id,
> -                       var->xres_virtual, var->yres_virtual,
> -                       var->xres, var->yres);
>                 return -EINVAL;
>         }

Gr{oetje,eeting}s,

                        Geert


--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at 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