Re: [PATCH 3/3] X86: Add a thread cpu time implementation to vDSO

[Chris Mason]

On Thu, Dec 11, 2014 at 1:36 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> * Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
> >  On Wed, Dec 10, 2014 at 2:56 PM, Shaohua Li <shli@xxxxxx> wrote:
> >  > On Wed, Dec 10, 2014 at 02:13:23PM -0800, Andy Lutomirski wrote:
> >  >> On Wed, Dec 10, 2014 at 1:57 PM, Shaohua Li <shli@xxxxxx> wrote:
> >  >> > On Wed, Dec 10, 2014 at 11:10:52AM -0800, Andy Lutomirski 
> > wrote:
> >  >> >> On Sun, Dec 7, 2014 at 7:03 PM, Shaohua Li <shli@xxxxxx> 
> > wrote:
> >  >> >> > This primarily speeds up 
> > clock_gettime(CLOCK_THREAD_CPUTIME_ID, ..). We
> >  >> >> > use the following method to compute the thread cpu time:
> >  >> >>
> >  >> >> I like the idea, and I like making this type of profiling 
> > fast.  I
> >  >> >> don't love the implementation because it's an information 
> > leak (maybe
> >  >> >> we don't care) and it's ugly.
> >  >> >>
> >  >> >> The info leak could be fixed completely by having a 
> > per-process array
> >  >> >> instead of a global array.  That's currently tricky without 
> > wasting
> >  >> >> memory, but it could be created on demand if we wanted to do 
> > that,
> >  >> >> once my vvar .fault patches go in (assuming they do -- I need 
> > to ping
> >  >> >> the linux-mm people).
> >  >> >
> >  >> > those info leak really doesn't matter.
> >  >>
> >  >> Why not?
> >  >
> >  > Ofcourse I can't make sure completely, but how could this
> >  > info be used as attack?
> >
> >  It may leak interesting timing info, even from cpus that are
> >  outside your affinity mask / cpuset.  I don't know how much
> >  anyone actually cares.
>
> Finegraned timing information has been successfully used to
> recover secret keys (and sometimes even coarse timing
> information), so it can be a security issue in certain setups.

Trying to nail this down a little more clearly.  Are you worried about 
the context switch count being exported or the clock_gettime data?

-chris



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