[PATCH V4] fbcon: fix fbcon out-of-bounds write in sys_imageblit

[Geert Uytterhoeven]

Hi Tetsuo,

On Mon, Aug 30, 2021 at 12:25 PM Tetsuo Handa
<penguin-kernel at i-love.sakura.ne.jp> wrote:
> On 2021/08/30 17:16, Daniel Vetter wrote:
> > On Mon, Aug 30, 2021 at 11:30:23AM +0800, tcs.kernel at gmail.com wrote:
> >> From: Haimin Zhang <tcs_kernel at tencent.com>
> >>
> >> yres and vyres can be controlled by user mode parameters, and cause
> >> p->vrows to become a negative value. While this value be passed to real_y
> >> function, the ypos will be out of screen range.This is an out-of-bounds
> >> write bug.
> >> some driver will check xres and yres in fb_check_var callback,but some not
> >> so we add a common check after that callback.
> >>
> >> Signed-off-by: Haimin Zhang <tcs_kernel at tencent.com>
> >> Signed-off-by: Tetsuo Handa <penguin-kernel at i-love.sakura.ne.jp>
>
> Please s/Signed-off-by: Tetsuo Handa/Suggested-by: Tetsuo Handa/ .
> It is Haimin who debugged this problem and wrote this patch.
>
> >
> > Does this fix a syzbot crash or how was this discovered?
>
> Yes, Haimin's team is running syzkaller locally and found this bug.
> Therefore, no Reported-by: syzbot tag for this patch.
>
>
> -------- Forwarded Message --------
> Subject: Re: [PATCH v2] fbcon: fix Out-Of-Bounds write in sys_imageblit
> Message-ID: <33fc0e30-b94c-939f-a708-4b939af43100 at gmail.com>
> Date: Mon, 2 Aug 2021 14:50:24 +0800
>
> hi, Tetsuo Handa
>
> i made a test with your suggested code, it can block the out-of-bound bug.
>
> where to add this check logic, i suggest to add it after the driver's
> fb_check_var callback.because what we plan to add is a common check by
> framework,but some driver can fault-tolerant invalid parameters(like
> yres_virtual > yres)
>
>         /* exist common check */
>         if (var->xres < 8 || var->yres < 8)
>                 return -EINVAL;
>
>         /* callback to drivers, some driver can fix invalid virtual
> xres or virtual yres */

Yes. Fbdev drivers are supposed to round up invalid or unsupported
values, or return -EINVAL.

>         ret = info->fbops->fb_check_var(var, info);
>         if (ret)
>                 return ret;
>         /* we add a more check here, if some drivers can't fix invalid x,y
> virtual values, we return a -EINVAL */
>         if (var->yres_virtual < var->yres || var->xres_virtual < var->xres)
>                 return -EINVAL;
>
> how about this fix ?  i can make a v3 patch.
>
>
>
> diff --git a/drivers/video/fbdev/core/fbmem.c
> b/drivers/video/fbdev/core/fbmem.c
> index 1c85514..9fb7e94 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1012,6 +1012,10 @@ static int fb_check_caps(struct fb_info *info,
> struct fb_var_screeninfo *var,
>
>         if (ret)
>                 return ret;
> +
> +       /* Virtual resolution cannot be smaller than visible resolution. */
> +       if (var->yres_virtual < var->yres || var->xres_virtual < var->xres)
> +               return -EINVAL;

So if this happens, it's a driver bug, not a userspace bug.

>
>         if ((var->activate & FB_ACTIVATE_MASK) != FB_ACTIVATE_NOW)
>                 return 0;

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at 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