From: Dan Carpenter <dan.carpenter@oracle.com>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
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 16:00:00 +0300 [thread overview]
Message-ID: <20210830130000.GW7722@kadam> (raw)
In-Reply-To: <CAMuHMdV=xVhEHLEoYt3OF+kmGrLOr6t7SP1sghSmp9JqXD+3Og@mail.gmail.com>
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@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
next prev parent reply other threads:[~2021-08-30 13:00 UTC|newest]
Thread overview: 24+ 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
2021-08-30 12:00 ` [syzbot] BUG: unable to handle kernel paging request in vga16fb_fillrect Geert Uytterhoeven
2021-08-30 13:00 ` Dan Carpenter [this message]
2021-08-30 13:37 ` Tetsuo Handa
2021-08-30 13:47 ` Dan Carpenter
2021-08-30 14:25 ` Tetsuo Handa
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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