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

[Cyrill Gorcunov]

On Wed, Apr 11, 2012 at 01:42:06AM +0200, Oleg Nesterov wrote:
> On 04/10, H. Peter Anvin wrote:
> >
> > On 04/10/2012 04:08 PM, Oleg Nesterov wrote:
> > >
> > > 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.
> >
> > Because it's cheap.  "Just enough" is not what you want to shoot for,
> > ever, you want to get past the "just enough" point and then consider
> > "what can I get for cheap at this point"?
> 
> OK, I am not arguing. Just I thought that the small note like
> "we are doing this per-type to obfuscate even more" can help.
> I wouldn't have asked, but Cyrill rewrites this comment anyway.
> 
> Perhaps this is just me, but my first (and wrong) impression was
> that somehow this is needed for correctness.

Hi Oleg, would the form below sounds good?

---
Subject: Add a comment on kcmp obfuscation method

Signed-off-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
---
 kernel/kcmp.c |   14 ++++++++++++++
 1 file changed, 14 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 +17,20 @@
  * 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 odd multiplier guarantees that the product
+ * is unique ever after the high bits are truncated, since any odd
+ * number is relative prime to 2^n).
+ *
+ * The obfuscation is done with per-type cookie values to increase
+ * initial entropy of results.
+ *
+ * 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;
 
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