Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

[Oleg Nesterov <oleg@redhat.com>]

On 04/11, Cyrill Gorcunov wrote:
>
> --- linux-2.6.git.orig/kernel/kcmp.c
> +++ linux-2.6.git/kernel/kcmp.c
> @@ -17,6 +17,17 @@
>   * reasons, still the comparison results should be suitable for
>   * sorting. Thus, we obfuscate kernel pointers values and compare
>   * the production instead.
> + *
> + * The obfuscation is done in two steps. First -- we use xor on
> + * kernel pointer with random value, which puts pointer into
> + * a new position in reordered space. Second -- we multiply
> + * the xor production with big odd random number to permute
> + * bits even more (the oddity is important here, it allow
> + * us to have meaningful production even if multiplicants
> + * are big numbers).
> + *
> + * Note also the obfuscation itself is invisible to user-space
> + * and if needed it can be changed to any suitable scheme.
>   */
>  static unsigned long cookies[KCMP_TYPES][2] __read_mostly;

OK, since this is discussed again...

Can this comment can also explain why do we obfuscate the pointers
by type? I mean, I don't really understand why the one-dimensional
cookies[2] is "not enough" from security pov.

Oleg.