linux-kernel - Re: [PATCH] x86,seccomp,prctl: Remove PR_TSC

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Date:	Fri, 3 Oct 2014 14:15:24 -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 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?

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.

--Andy
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