From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
To: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>,
Randy Dunlap <rdunlap@infradead.org>,
syzbot <syzbot+04168c8063cfdde1db5e@syzkaller.appspotmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
Colin King <colin.king@canonical.com>,
DRI Development <dri-devel@lists.freedesktop.org>,
Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Masahiro Yamada <masahiroy@kernel.org>,
syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] BUG: unable to handle kernel paging request in vga16fb_fillrect
Date: Mon, 30 Aug 2021 23:25:51 +0900 [thread overview]
Message-ID: <03d0f549-9731-8b06-1393-60d4bef27884@i-love.sakura.ne.jp> (raw)
In-Reply-To: <20210830134719.GI12231@kadam>
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;
}
----------
next prev parent reply other threads:[~2021-08-30 14:26 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-14 5:16 [syzbot] BUG: unable to handle kernel paging request in vga16fb_fillrect syzbot
2021-08-30 0:24 ` Randy Dunlap
2021-08-30 2:27 ` Tetsuo Handa
2021-08-30 2:31 ` Randy Dunlap
2021-08-30 16:05 ` [PATCH] fbmem: don't allow too huge resolutions Tetsuo Handa
2021-08-31 6:48 ` Geert Uytterhoeven
2021-08-31 15:23 ` Tetsuo Handa
2021-08-31 16:20 ` Daniel Vetter
2021-08-31 17:19 ` Geert Uytterhoeven
2021-08-31 18:53 ` Daniel Vetter
2021-08-31 18:56 ` Geert Uytterhoeven
2021-08-31 19:04 ` Daniel Vetter
2021-09-01 1:14 ` [PATCH v2] " Tetsuo Handa
2021-09-01 7:12 ` Geert Uytterhoeven
2021-09-08 10:27 ` [PATCH v2 (repost)] " Tetsuo Handa
2021-09-08 16:52 ` Daniel Vetter
2021-08-30 12:00 ` [syzbot] BUG: unable to handle kernel paging request in vga16fb_fillrect Geert Uytterhoeven
2021-08-30 13:00 ` Dan Carpenter
2021-08-30 13:37 ` Tetsuo Handa
2021-08-30 13:47 ` Dan Carpenter
2021-08-30 14:25 ` Tetsuo Handa [this message]
2021-08-30 14:29 ` Dan Carpenter
2021-08-30 14:30 ` Geert Uytterhoeven
2021-08-30 14:38 ` Tetsuo Handa
2021-08-30 14:53 ` Geert Uytterhoeven
2021-08-30 15:00 ` Geert Uytterhoeven
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