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

[Geert Uytterhoeven]

Hi Michel,

On Wed, Apr 5, 2023 at 10:50 AM Michel Dänzer
<michel.daenzer at mailbox.org> wrote:
> On 4/4/23 14:36, Daniel Vetter 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
> > 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>
> > ---
> >  drivers/video/fbdev/core/fbmem.c | 4 ----
> >  1 file changed, 4 deletions(-)
> >
> > diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> > index 875541ff185b..9757f4bcdf57 100644
> > --- 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;
> >       }
> >
>
> Make it pr_warn_once? 99.9...% of the benefit, without spam.

Except that it should be pr_warn_once_per_fb_info, which requires
a flag in fb_info.

If fb_info.fbops wasn't const, we could also nuke
info->fbops->check_var() ;-)

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