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

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Date:	Sat, 4 Oct 2014 10:13:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
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 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.

> 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?
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