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

[Cyrill Gorcunov <gorcunov@openvz.org>]

On Mon, Apr 09, 2012 at 04:22:38PM -0700, H. Peter Anvin wrote:
> > The obfuscation logic was done with great help from hpa@. And the main
> > idea was to have ordered results after obfuscation. Per-type noise increase
> > randomization of results. So Andrew, I actually dont know what to add
> > here. We don't want to provide kernel order back to user-space in
> > naked manner.
> > 
> 
> The obfuscation logic is to provide a 1:1 mapping but which doesn't
> preserve ordering, thereby avoid leaking information of kernel pointers
> to user space.
> 

OK, Peter, would the following comment bring light on the obfuscation
procedure?
---
Add a comment on kcmp obfuscation method

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
---
 kernel/kcmp.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

Index: linux-2.6.git/kernel/kcmp.c
===================================================================
--- 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;