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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Mon, 30 Aug 2021 14:00:21 +0200
Message-ID: <CAMuHMdV=xVhEHLEoYt3OF+kmGrLOr6t7SP1sghSmp9JqXD+3Og@mail.gmail.com>
Subject: Re: [syzbot] BUG: unable to handle kernel paging request in vga16fb_fillrect
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     Randy Dunlap <rdunlap@infradead.org>,
        syzbot <syzbot+04168c8063cfdde1db5e@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Colin King <colin.king@canonical.com>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        syzkaller-bugs@googlegroups.com
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Hi Testsuo,

On Mon, Aug 30, 2021 at 4:27 AM Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
> On 2021/08/30 9:24, Randy Dunlap wrote:
> > Note that yres_virtual is set to 0x10000000. Is there no practical limit
> > (hence limit check) that can be used here?
> >
> > Also, in vga16fb_check_var(), beginning at line 404:
> >
> >   404        if (yres > vyres)
> >   405            vyres = yres;
> >   406        if (vxres * vyres > maxmem) {
> >   407            vyres = maxmem / vxres;
> >   408            if (vyres < yres)
> >   409                return -ENOMEM;
> >   410        }
> >
> > At line 406, the product of vxres * vyres overflows 32 bits (is 0 in this
> > case/example), so any protection from this block is lost.
>
> OK. Then, we can check overflow like below.
>
> 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.

>                 vyres = maxmem / vxres;
>                 if (vyres < yres)
>                         return -ENOMEM;
>
> But I think we can check overflow in the common code like below. (Both patch fixed the oops.)
>
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 1c855145711b..8899679bbc46 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1008,6 +1008,11 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
>         if (var->xres < 8 || var->yres < 8)
>                 return -EINVAL;
>
> +       /* Don't allow u32 * u32 to overflow. */
> +       if ((u64) var->xres * var->yres > (u64) UINT_MAX ||
> +           (u64) var->xres_virtual * var->yres_virtual > (u64) UINT_MAX)
> +               return -EINVAL;
> +

Same comment here, of course.

Gr{oetje,eeting}s,

                        Geert

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