Re: [Security] [PATCH] fix mmap random address range on x86

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 18 Feb 2011 14:15:57 +0100
Ludwig Nussel <ludwig.nussel@suse.de> wrote:

> On x86 casting the unsigned int result of get_random_int() to long
> may result in a negative value. On x86 the range of mmap_rnd()
> therefore was -255 to 255. The 32bit mode on x86_64 used 0 to 255 as
> intended.
> 
> The bug was introduced by commit 675a081 in January 2008.
> 
> Signed-off-by: Ludwig Nussel <ludwig.nussel@suse.de>
> ---
>  arch/x86/mm/mmap.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> index 1dab519..f927429 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -87,9 +87,9 @@ static unsigned long mmap_rnd(void)
>  	*/
>  	if (current->flags & PF_RANDOMIZE) {
>  		if (mmap_is_ia32())
> -			rnd = (long)get_random_int() % (1<<8);
> +			rnd = get_random_int() % (1<<8);
>  		else
> -			rnd = (long)(get_random_int() % (1<<28));
> +			rnd = get_random_int() % (1<<28);
>  	}
>  	return rnd << PAGE_SHIFT;
>  }

The changelog didn't describe the user-visible consequences of this
bug, so readers must try to work this out for themselves.  That's not a
very desirable or efficient thing, so please do prepare more complete
changelogs.

afacit the effects are very small: the mmap base may fall slightly
below MIN_GAP, but that won't really affect anything?