linux-kernel - Re: [syzbot] BUG: unable to handle kernel paging request in vga16fb

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Date:   Mon, 30 Aug 2021 16:30:36 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Dan Carpenter <dan.carpenter@...cle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        syzbot <syzbot+04168c8063cfdde1db5e@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Colin King <colin.king@...onical.com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] BUG: unable to handle kernel paging request in vga16fb_fillrect

Hi Tetsuo,

On Mon, Aug 30, 2021 at 4:26 PM Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> On 2021/08/30 22:47, Dan Carpenter wrote:
> > On Mon, Aug 30, 2021 at 10:37:31PM +0900, Tetsuo Handa wrote:
> >> On 2021/08/30 22:00, Dan Carpenter wrote:
> >>>>> diff --git a/drivers/video/fbdev/vga16fb.c b/drivers/video/fbdev/vga16fb.c
> >>>>> index e2757ff1c23d..e483a3f5fd47 100644
> >>>>> --- a/drivers/video/fbdev/vga16fb.c
> >>>>> +++ b/drivers/video/fbdev/vga16fb.c
> >>>>> @@ -403,7 +403,7 @@ static int vga16fb_check_var(struct fb_var_screeninfo *var,
> >>>>>
> >>>>>         if (yres > vyres)
> >>>>>                 vyres = yres;
> >>>>> -       if (vxres * vyres > maxmem) {
> >>>>> +       if ((u64) vxres * vyres > (u64) maxmem) {
> >>>>
> >>>> Mindlessly changing the sizes is not the solution.
> >>>> Please use e.g. the array_size() helper from <linux/overflow.h>
> >>>> instead.
> >>>
> >>> On a 64bit system the array_size() macro is going to do the exact same
> >>> casts?  But I do think this code would be easier to understand if the
> >>> integer overflow check were pull out separately and done first:
> >>>
> >>>     if (array_size(vxres, vyres) >= UINT_MAX)
> >>>             return -EINVAL;
> >>
> >> This is wrong. array_size() returns ULONG_MAX on 64bits upon overflow and
> >> returns UINT_MAX on 32bits upon overflow. However, UINT_MAX is a valid
> >> value without overflow (e.g. vxres == UINT_MAX / 15 && vyres == 15).
> >
> > Huh...  I just assumed we didn't allow resolutions that high.
>
> Of course, we don't allow resolutions that high. ;-)
>
> Since I don't know possible max resolutions, I chose UINT_MAX + 1 as a common
> limit for returning -EINVAL. Unless overflow happens, vga16fb_check_var() will
> return -ENOMEM on such high resolutions.

The highest possible value of maxmem inside vga16fb_check_var()
is 65536.

So I believe

    if (array_size(vxres, vyres) > maxmem)

should work fine.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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