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

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Message-ID: <1418668613.4200.4@mail.thefacebook.com>
Date:	Mon, 15 Dec 2014 13:36:53 -0500
From:	Chris Mason <clm@...com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Andy Lutomirski <luto@...capital.net>, 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



On Thu, Dec 11, 2014 at 1:36 AM, Ingo Molnar <mingo@...nel.org> wrote:
> 
> * 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.

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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