Hi Testsuo,
On Mon, Aug 30, 2021 at 4:27 AM Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx
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
