linux-kernel - Re: [Linux-fbdev-devel] fbcmap: non working tests on unsigned cmap->start

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Date:	Mon, 27 Apr 2009 14:21:47 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Roel Kluin <roel.kluin@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fbdev-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, adaplas@...il.com
Subject: Re: [Linux-fbdev-devel] fbcmap: non working tests on unsigned 
	cmap->start

On Mon, Apr 27, 2009 at 13:58, Roel Kluin <roel.kluin@...il.com> wrote:
> Ville Syrjälä wrote:
>> On Sun, Apr 26, 2009 at 02:20:47PM +0200, Roel Kluin wrote:
>>> cmap->start is unsigned,
>
>>>> On Thu, Apr 23, 2009 at 01:41:10PM -0700, Andrew Morton wrote:
>>>>> argh.
>>>>>
>>>>> - Perhaps userspace can kill the kernel by sending a "negative"
>>>>>   `start'.  Removing the test will make it even less likely that we'll
>>>>>   fix this bug.
>>>> Shouldn't happen. 'start' is used as the starting index for the hardware
>>>> palette, 'start+len-1' is the last index. All drivers should already check
>>>> the passed values since the maximum index depends on the display mode.
>>>> And I suppose the worst thing that could happen if the driver fails to
>>>> check the values would be incorrect colors.
>
>> You would rather need something like
>> 'if (start+len > 1 << max(red.len, green.len, blue.len, transp.len))'
>
> what do you mean with `red.len'? is that `info->var.red.length'?

That's correct.

>> and a check to make sure that start+len doesn't overflow.
>>
>> Oh and I guess it should also check that the visual is pseudocolor or
>> directcolor.
>
> I am fairly new so please review carefully.
>
> Not yet signed off-by: Roel Kluin <roel.kluin@...il.com>

Looks OK, thx!

> ---
> diff --git a/drivers/video/fbcmap.c b/drivers/video/fbcmap.c
> index f53b9f1..b34e74e 100644
> --- a/drivers/video/fbcmap.c
> +++ b/drivers/video/fbcmap.c
> @@ -249,6 +249,7 @@ int fb_set_user_cmap(struct fb_cmap_user *cmap, struct fb_info *info)
>  {
>        int rc, size = cmap->len * sizeof(u16);
>        struct fb_cmap umap;
> +       __u32 rgba_max = 0;
>
>        memset(&umap, 0, sizeof(struct fb_cmap));
>        rc = fb_alloc_cmap(&umap, cmap->len, cmap->transp != NULL);
> @@ -261,13 +262,27 @@ int fb_set_user_cmap(struct fb_cmap_user *cmap, struct fb_info *info)
>                rc = -EFAULT;
>                goto out;
>        }
> +
> +       if (cmap->start + cmap->len < cmap->start) {
> +               rc = -EINVAL;
> +               goto out;
> +       }
> +
>        umap.start = cmap->start;
>        if (!lock_fb_info(info)) {
>                rc = -ENODEV;
>                goto out;
>        }
> -       if (cmap->start < 0 || (!info->fbops->fb_setcolreg &&
> -                               !info->fbops->fb_setcmap)) {
> +
> +       rgba_max = max(info->var.red.length, info->var.green.length);
> +       rgba_max = max(rgba_max, info->var.blue.length);
> +       rgba_max = max(rgba_max, info->var.transp.length);
> +
> +       if (cmap->start + cmap->len > 1 << rgba_max ||
> +                       !(info->fbops->fb_setcolreg ||
> +                               info->fbops->fb_setcmap) ||
> +                       !(info->fix.visual == FB_VISUAL_PSEUDOCOLOR ||
> +                               info->fix.visual == FB_VISUAL_TRUECOLOR)) {
>                rc = -EINVAL;
>                goto out1;
>        }
>
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-- 
Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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
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