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

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Message-ID: <20210830134719.GI12231@kadam>
Date:   Mon, 30 Aug 2021 16:47:19 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        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

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.

> Comparing like "> (u64) UINT_MAX" is to detect only overflow.
> 

Of course, that doesn't work on 32 bit systems.  Also the cast isn't
required because of type promotion.

regards,
dan carpenter