Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 8 Oct 2014 16:47:26 -0700 | Subject | Re: Tightening up rdpmc permissions? |
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On Mon, Sep 29, 2014 at 11:42 AM, Andy Lutomirski <luto@amacapital.net> wrote: > On Sep 29, 2014 10:36 AM, <Valdis.Kletnieks@vt.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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