Re: [PATCH] x86,seccomp,prctl: Remove PR_TSC_SIGSEGV and seccomp TSC filtering

[Peter Zijlstra <peterz@infradead.org>]

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@infradead.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@infradead.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?