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

Cyrill Gorcunov
2012-04-09T22:24:50Z

On Mon, Apr 09, 2012 at 03:10:27PM -0700, Andrew Morton wrote:
> Back on to kcmp.
>
> On Wed, 15 Feb 2012 20:27:52 +0400
> Cyrill Gorcunov <gorcunov@openvz.org> wrote:
>
> > On Wed, Feb 15, 2012 at 05:06:52PM +0100, Oleg Nesterov wrote:
> > > Not a comment, but the question. I am just curious...
> > >
> > > > +/*
> > > > + * We don't expose real in-memory order of objects for security
> > > > + * reasons, still the comparison results should be suitable for
> > > > + * sorting. Thus, we obfuscate kernel pointers values and compare
> > > > + * the production instead.
> > > > + */
> > > > +static unsigned long cookies[KCMP_TYPES][2] __read_mostly;
> > > > +
> > > > +static long kptr_obfuscate(long v, int type)
> > > > +{
> > > > + return (v ^ cookies[type][0]) * cookies[type][1];
> > > > +}
> > >
> > > OK, but why do we need this per type? Just to add more obfuscation
> > > or there is another reason?
> >
> > Just to add more obfuscation.
>
> Having re-read most of the (enormous) email discussion on the kcmp()
> syscall patch, I'm thinking:
>
> - Nobody seems to understand the obfuscation logic. Jon sounded
> confused, Oleg sounds confused and it's rather unclear what it does,
> how it does it and why it does it.

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.

>
> - Lots of people have looked at the code and made comments and there
> have been lots of changes. But we presently have zero Acked-by's and
> Reviewed-by's.
>

I guess I can ask hpa@ and Eric for Reviewed-by or Acked-by tag?

> I guess this means that at present nobody is aware of any issues with
> the proposal, btu nobody is terribly excisted about it either?
>

I would rather say not much people yet use it.

> So what do people think? Any issues? Any nacks? Should I sneak it
> into Linus this week or do we need to go another round with it all?
>
> I'd like to at least have a fighting chance of understnading what's
> going on with that obfuscation code.

Cyrill