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SubjectRe: [RFC] [patch] fbcmap: integer overflow bug
On Wed, 27 Oct 2010 11:37:16 +0200
Dan Carpenter <error27@gmail.com> wrote:

> There is an integer overflow bug in the FBIOPUTCMAP ioctl if
> cmap->len * 2 overflows.
>
> It's harmless, except that it messes up the cmap until someone types
> `reset`. It could have been caught by checking the return from
> fb_copy_cmap().
>
> Or it could have been caught by limiting the size of the cmaps to one
> page. The cmaps are allocated with GFP_ATOMIC and it makes sense to
> limit them.
>
> Different drivers use different sizes of cmaps. There are about 150
> drivers. I've checked a bunch (50) of them and the larges cmap.len I've
> found is gxt4500 which maxes out at 1024 so PAGE_SIZE is about twice that
> length. For some of the 50 I wasn't sure on the limit.
>
> Is PAGE_SIZE a reasonable limit? Does anyone know or do I have to audit
> all 150 drivers?

No signed-off-by:.

> diff --git a/drivers/video/fbcmap.c b/drivers/video/fbcmap.c
> index f53b9f1..6dc5817 100644
> --- a/drivers/video/fbcmap.c
> +++ b/drivers/video/fbcmap.c
> @@ -110,7 +110,9 @@ int fb_alloc_cmap(struct fb_cmap *cmap, int len, int transp)
> }
> cmap->start = 0;
> cmap->len = len;
> - fb_copy_cmap(fb_default_cmap(len), cmap);
> + if (fb_copy_cmap(fb_default_cmap(len), cmap))
> + goto fail;
> +
> return 0;
>
> fail:
> @@ -250,6 +252,9 @@ 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;
>
> + if (cmap->len > PAGE_SIZE || cmap->len * sizeof(u16) > PAGE_SIZE)
> + return -EINVAL;
> +
> memset(&umap, 0, sizeof(struct fb_cmap));
> rc = fb_alloc_cmap(&umap, cmap->len, cmap->transp != NULL);
> if (rc)

A suitable way to fix this would be to detect the overflow only, and
then just throw the passed-in length at kmalloc(), and let kmalloc()
decide whether it is sane or not.

To do that, one would need to implement a new
fb_alloc_cmap_not_stupid() which takes a gfp_t, and pass in GFP_KERNEL.

Feel free to sanitise the fb_alloc_cmap() indenting in [patch 1/2] as well ;)


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