linux-kernel - Re: Tightening up rdpmc permissions?

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Date:	Mon, 29 Sep 2014 11:42:08 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	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 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...)

--Andy
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