Re: [PATCH] fbmem: don't allow too huge resolutions

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Tetsuo,

Thanks for your patch!

On Mon, Aug 30, 2021 at 6:05 PM Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
> syzbot is reporting page fault at vga16fb_fillrect() [1], for
> vga16fb_check_var() is failing to detect multiplication overflow.
>
>   if (vxres * vyres > maxmem) {
>     vyres = maxmem / vxres;
>     if (vyres < yres)
>       return -ENOMEM;
>   }

IMHO that should be fixed in vga16fb, too.

> Since no module would accept too huge resolutions where multiplication
> overflow happens, let's reject in the common path.
>
> This patch does not use array_size(), for array_size() is allowed to
> return UINT_MAX on 32bits even if overflow did not happen. We want to
> detect only overflow here, for individual module will recheck with more
> strict limits as needed.

Which is IMHO not really an issue, as I believe on 32-bit you cannot
use a very large frame buffer, long before you reach UINT_MAX.

> Link: https://syzkaller.appspot.com/bug?extid=04168c8063cfdde1db5e [1]
> Reported-by: syzbot <syzbot+04168c8063cfdde1db5e@syzkaller.appspotmail.com>
> Debugged-by: Randy Dunlap <rdunlap@infradead.org>
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Tested-by: syzbot <syzbot+04168c8063cfdde1db5e@syzkaller.appspotmail.com>
> ---
>  drivers/video/fbdev/core/fbmem.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 1c855145711b..9f5075dc2345 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 > UINT_MAX ||
> +           (u64) var->xres_virtual * var->yres_virtual > UINT_MAX)
> +               return -EINVAL;
> +

I think it would still be better to use check_mul_overflow(), as that
makes it clear and explicit what is being done, even without a comment.

Furthermore, this restricts the virtual frame buffer size on 64-bit,
too, while graphics cards can have much more than 4 GiB of RAM.

>         ret = info->fbops->fb_check_var(var, info);
>
>         if (ret)

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