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

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Date:   Mon, 30 Aug 2021 23:25:51 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dan Carpenter <dan.carpenter@...cle.com>
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 2021/08/30 22:47, Dan Carpenter wrote:
> 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.

Of course, we don't allow resolutions that high. ;-)

Since I don't know possible max resolutions, I chose UINT_MAX + 1 as a common
limit for returning -EINVAL. Unless overflow happens, vga16fb_check_var() will
return -ENOMEM on such high resolutions.

> 
>> 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.

Indeed, "> UINT_MAX" seems to work on both 32bits and 64bits.

----------
#include <stdio.h>
#include <limits.h>

int main(int argc, char *argv[])
{
        unsigned int w = 0x600;
        unsigned int h = 0x10000000;
        if ((unsigned long long) w * h > UINT_MAX)
                printf("Overflowed\n");
        else
                printf("No overflow\n");
        return 0;
}
----------