Re: Tightening up rdpmc permissions?

From: Andy Lutomirski
Date: Wed Oct 08 2014 - 19:47:58 EST

On Mon, Sep 29, 2014 at 11:42 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> 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...)

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 Lutomirski
AMA Capital Management, LLC
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