Messages in this thread Patch in this message | | | Date | Wed, 11 Apr 2012 10:39:57 +0400 | From | Cyrill Gorcunov <> | Subject | Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree |
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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@openvz.org> --- 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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