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Date:	Mon, 6 Oct 2014 09:44:27 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Erik Bosman <ebn310@....vu.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linux API <linux-api@...r.kernel.org>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	X86 ML <x86@...nel.org>
Subject: Re: [PATCH] x86,seccomp,prctl: Remove PR_TSC_SIGSEGV and seccomp TSC filtering

On Sat, Oct 4, 2014 at 1:13 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Oct 03, 2014 at 02:15:24PM -0700, Andy Lutomirski wrote:
>> On Fri, Oct 3, 2014 at 2:12 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Fri, Oct 03, 2014 at 02:04:53PM -0700, Andy Lutomirski wrote:
>> >> On Fri, Oct 3, 2014 at 2:02 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> >
>> >> > Something like so.. slightly less ugly and possibly with more
>> >> > complicated conditions setting the cr4 if you want to fix tsc vs seccomp
>> >> > as well.
>> >>
>> >> This will crash anything that tries rdpmc in an allow-everything
>> >> seccomp sandbox.  It's also not very compatible with my grand scheme
>> >> of allowing rdtsc to be turned off without breaking clock_gettime. :)
>> >
>> > Well, we clear cap_user_rdpmc, so everybody who still tries it gets what
>> > he deserves, no problem there.
>>
>> Oh, interesting.
>>
>> To continue playing devil's advocate, what if you do perf_event_open,
>> then mmap it, then start the seccomp sandbox?
>
> We update that cap bit on every update to the self-monitor state, and in
> a perfect world people would also check the cap bit every time they try
> and read it, and fall back to the syscall. So we could just clear it..
> but I can imagine reality ruining things here.

If nothing else, the fact that rdpmc fails with SIGSEGV instead of
with some nonsense value means that this will always be racy.

>
>> My draft patches are currently tracking the number of perf_event mmaps
>> per mm.  I'm not thrilled with it, but it's straightforward.  And I
>> still need to benchmark cr4 writes, which is tedious, because I can't
>> do it from user code.
>
> Should be fairly straight fwd from kernel space, get a tsc stamp,
> read+write cr4 1000 times, get another tsc read, and maybe do that
> several times. No?

I tried it.  Rough numbers on my 2.7 GHz Sandy Bridge laptop

Writing to cr4 in VMX non-root (changing PCE) takes ~48ns.  RMW cr4
takes rougly 51ns.  IMO neither of these is enough to be worth
worrying *that* much about when switching into or out of a perf-using
task.  But you might disagree with me.

Changing TSD takes 700ns, because KVM has the VMCS programmed wrong.
I'll send a patch.

I suspect that the same experiment on bare metal would run faster.


--Andy
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