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

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Message-ID: <20141211063621.GC5059@gmail.com>
Date:	Thu, 11 Dec 2014 07:36:21 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Shaohua Li <shli@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, kernel-team@...com,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 3/3] X86: Add a thread cpu time implementation to vDSO


* Andy Lutomirski <luto@...capital.net> wrote:

> On Wed, Dec 10, 2014 at 2:56 PM, Shaohua Li <shli@...com> 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@...com> 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@...com> 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.

Thanks,

	Ingo
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