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

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Message-ID: <8ed0ca59-226b-2d0e-b1ae-82305202558d@i-love.sakura.ne.jp>
Date:   Mon, 30 Aug 2021 22:37:31 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dan Carpenter <dan.carpenter@...cle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     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 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).
Comparing like "> (u64) UINT_MAX" is to detect only overflow.

array_size() would be helpful for forcing memory allocation to fail
(instead of allocating smaller than actually required).

> 
> 	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
>