Re: Tightening up rdpmc permissions?

From: Andy Lutomirski
Date: Mon Sep 29 2014 - 14:42:36 EST

On Sep 29, 2014 10:36 AM, <Valdis.Kletnieks@xxxxxx> 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...)

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