linux-kernel - Re: Tightening up rdpmc permissions?

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Message-ID: <CALCETrX9KAFMyQNhx5-qvZGPZQxDimLByr7u4Nj5tLuBsG_Tvw@mail.gmail.com>
Date:	Wed, 8 Oct 2014 16:47:26 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Valdis Kletnieks <Valdis.Kletnieks@...edu>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: Tightening up rdpmc permissions?

On Mon, Sep 29, 2014 at 11:42 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Sep 29, 2014 10:36 AM, <Valdis.Kletnieks@...edu> wrote:
>>
>> On Mon, 29 Sep 2014 09:39:16 -0700, Andy Lutomirski said:
>>
>> > Would it make sense to restrict rdpmc to tasks that are in mms that
>> > have a perf_event mapping?  After all, unless I misunderstand
>> > something, user code can't reliably use rdpmc unless they've mapped a
>> > perf_event object to check the rdpmc bit and figure out what ecx value
>> > to use.
>>
>> Wouldn't that be trivially easy for an attacker to bypass? Just map a dummy
>> perf_event object  and then go to town?
>
> Depends on the paranoia setting.  We could require that the mapped
> object actually have an rdpmc-able counter running.
>
> Seccomp can (and often does) block access to perf_event_open entirely.
> We could certainly change the code to only twiddle CR4 if TIF_SECCOMP
> or TIF_NOTSC is set.  I think that the real thing we should optimize
> for is to minimize the chance that a given context switch actually
> needs to *change* cr4.  Since perf_event maps are relatively unusual,
> at least only perf-using programs would have overhead if we just gated
> it on the existence of a useful rdpmc-able mapping.
>
> (Also, why on earth is TIF_NOTSC a thread_info flag?  Wouldn't just
> adding a field "cr4" to task_struct or something be simpler and quite
> possibly faster?  We have a branch anyway...)

I have a prototype patch that seems to work and should have relatively
little overhead.  I'll send it either when the merge window closes or
when some pending dependencies get resolved.

--Andy

>
> --Andy



-- 
Andy Lutomirski
AMA Capital Management, LLC
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