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Date:   Fri, 8 Jun 2018 18:52:25 +0300
From:   Cyrill Gorcunov <gorcunov@gmail.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     "H. J. Lu" <hjl.tools@gmail.com>,
        Dmitry Safonov <dsafonov@virtuozzo.com>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-doc@vger.kernel.org,
        Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Oleg Nesterov <oleg@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
        mike.kravetz@oracle.com
Subject: Re: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack
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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@gmail.com> wrote:
> >
> > On Thu, Jun 7, 2018 at 9:38 PM, Andy Lutomirski <luto@kernel.org> wrote:
> > > On Thu, Jun 7, 2018 at 9:10 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > >>
> > >> On Thu, Jun 7, 2018 at 4:01 PM, Andy Lutomirski <luto@kernel.org> 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