From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc: 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 14:00:21 +0200 [thread overview]
Message-ID: <CAMuHMdV=xVhEHLEoYt3OF+kmGrLOr6t7SP1sghSmp9JqXD+3Og@mail.gmail.com> (raw)
In-Reply-To: <2fccb5d3-191c-924e-159f-1c9d423e282f@i-love.sakura.ne.jp>
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.
> vyres = maxmem / vxres;
> if (vyres < yres)
> return -ENOMEM;
>
> But I think we can check overflow in the common code like below. (Both patch fixed the oops.)
>
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 1c855145711b..8899679bbc46 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1008,6 +1008,11 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
> if (var->xres < 8 || var->yres < 8)
> return -EINVAL;
>
> + /* Don't allow u32 * u32 to overflow. */
> + if ((u64) var->xres * var->yres > (u64) UINT_MAX ||
> + (u64) var->xres_virtual * var->yres_virtual > (u64) UINT_MAX)
> + return -EINVAL;
> +
Same comment here, of course.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
next prev parent reply other threads:[~2021-08-30 12:00 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 ` Geert Uytterhoeven [this message]
2021-08-30 13:00 ` [syzbot] BUG: unable to handle kernel paging request in vga16fb_fillrect Dan Carpenter
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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