[syzbot] BUG: unable to handle kernel paging request in vga16fb_fillrect

[Dan Carpenter]

On Mon, Aug 30, 2021 at 02:00:21PM +0200, Geert Uytterhoeven wrote:
> Hi Testsuo,
> 
> On Mon, Aug 30, 2021 at 4:27 AM Tetsuo Handa
> <penguin-kernel at 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.

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;

	if (vxres * vyres > maxmem) {
		...

The UINT_MAX is because vxres and vyres are u32.

This would maybe be the first time anyone ever did an integer overflow
check like this in the kernel.  It's a new idiom.

regards,
dan carpenter