Re: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack

[Cyrill Gorcunov]

On Fri, Jun 08, 2018 at 07:57:22AM -0700, Andy Lutomirski wrote:
> On Fri, Jun 8, 2018 at 5:24 AM H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
> >
> > On Thu, Jun 7, 2018 at 9:38 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > > On Thu, Jun 7, 2018 at 9:10 PM H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
> > >>
> > >> On Thu, Jun 7, 2018 at 4:01 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > >>
> > >
> > > By the time malicious code issue its own syscalls, you've already lost
> > > the battle.  I could probably be convinced that a lock-CET-on feature
> > > that applies *only* to the calling thread and is not inherited by
> > > clone() is a decent idea, but I'd want to see someone who understands
> > > the state of the art in exploit design justify it.  You're also going
> > > to need to figure out how to make CRIU work if you allow locking CET
> > > on.
> > >
> > > A priori, I think we should just not provide a lock mechanism.
> >
> > We need a door for CET.  But it is a very bad idea to leave it open
> > all the time.  I don't know much about CRIU,  If it is Checkpoint/Restore
> > In Userspace.  Can you free any application with AVX512 on AVX512
> > machine and restore it on non-AVX512 machine?
> 
> Presumably not -- if the program uses AVX512 and AVX512 goes away,
> then the program won't be happy.

Yes. In most scenarios we require the fpu capability to be the same
on both machines (in case of migration) or/and not being changed
between c/r cycles.
...
> As an aside, where are the latest CET docs?  I've found the "CET
> technology preview 2.0", but it doesn't seem to be very clear or
> entirely complete.

+1