Re: fbcmap: non working tests on unsigned cmap->start

Andrew Morton
2009-04-23T20:45:40Z

On Tue, 21 Apr 2009 16:26:33 +0200
Roel Kluin <roel.kluin@gmail.com> wrote:

> vi include/linux/fb.h +478
>
> Note that struct fb_cmap_user cmap->start in fb_set_user_cmap() is unsigned.
> Should there maybe be a test:
>
> if (cmap->start > MAX || ...)
>
> (and what should MAX be then?)
>
> Otherwise you may want to apply the cleanup patch below
>
> Roel
> ------------------------------>8-------------8<---------------------------------
> Remove redundant tests on unsigned
>
> Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
> ---
> diff --git a/drivers/video/fbcmap.c b/drivers/video/fbcmap.c
> index f53b9f1..958bf4e 100644
> --- a/drivers/video/fbcmap.c
> +++ b/drivers/video/fbcmap.c
> @@ -266,8 +266,7 @@ int fb_set_user_cmap(struct fb_cmap_user *cmap, struct fb_info *info)
> rc = -ENODEV;
> goto out;
> }
> - if (cmap->start < 0 || (!info->fbops->fb_setcolreg &&
> - !info->fbops->fb_setcmap)) {
> + if (!info->fbops->fb_setcolreg && !info->fbops->fb_setcmap) {
> rc = -EINVAL;
> goto out1;
> }

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.

- If this bug doesn't exist, and there _is_ userspace which is
legitimately sending in "negative" values of `start' (unlikely?) then
we will break userspace if we fix this comparison.

IOW, I don't have enough knowledge about this code to be able to know
what to do.