Re: [PATCH 04/10] x86/cet: Handle thread shadow stack

[Andy Lutomirski <luto@kernel.org>]

On Fri, Jun 8, 2018 at 7:53 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> On 06/07/2018 10:53 PM, Andy Lutomirski wrote:
> > On Thu, Jun 7, 2018 at 12:47 PM Florian Weimer <fweimer@redhat.com> wrote:
> >>
> >> On 06/07/2018 08:21 PM, Andy Lutomirski wrote:
> >>> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
> >>>>
> >>>> When fork() specifies CLONE_VM but not CLONE_VFORK, the child
> >>>> needs a separate program stack and a separate shadow stack.
> >>>> This patch handles allocation and freeing of the thread shadow
> >>>> stack.
> >>>
> >>> Aha -- you're trying to make this automatic.  I'm not convinced this
> >>> is a good idea.  The Linux kernel has a long and storied history of
> >>> enabling new hardware features in ways that are almost entirely
> >>> useless for userspace.
> >>>
> >>> Florian, do you have any thoughts on how the user/kernel interaction
> >>> for the shadow stack should work?
> >>
> >> I have not looked at this in detail, have not played with the emulator,
> >> and have not been privy to any discussions before these patches have
> >> been posted, however …
> >>
> >> I believe that we want as little code in userspace for shadow stack
> >> management as possible.  One concern I have is that even with the code
> >> we arguably need for various kinds of stack unwinding, we might have
> >> unwittingly built a generic trampoline that leads to full CET bypass.
> >
> > I was imagining an API like "allocate a shadow stack for the current
> > thread, fail if the current thread already has one, and turn on the
> > shadow stack".  glibc would call clone and then call this ABI pretty
> > much immediately (i.e. before making any calls from which it expects
> > to return).
>
> Ahh.  So you propose not to enable shadow stack enforcement on the new
> thread even if it is enabled for the current thread?  For the cases
> where CLONE_VM is involved?
>
> It will still need a new assembler wrapper which sets up the shadow
> stack, and it's probably required to disable signals.
>
> I think it should be reasonable safe and actually implementable.  But
> the benefits are not immediately obvious to me.

Doing it this way would have been my first incliniation.  It would
avoid all the oddities of the kernel magically creating a VMA when
clone() is called, guessing the shadow stack size, etc.  But I'm okay
with having the kernel do it automatically, too.  I think it would be
very nice to have a way for user code to find out the size of the
shadow stack and change it, though.  (And relocate it, but maybe
that's impossible.  The CET documentation doesn't have a clear
description of the shadow stack layout.)

>
> > We definitely want strong enough user control that tools like CRIU can
> > continue to work.  I haven't looked at the SDM recently enough to
> > remember for sure, but I'm reasonably confident that user code can
> > learn the address of its own shadow stack.  If nothing else, CRIU
> > needs to be able to restore from a context where there's a signal on
> > the stack and the signal frame contains a shadow stack pointer.
>
> CRIU also needs the shadow stack *contents*, which shouldn't be directly
> available to the process.  So it needs very special interfaces anyway.

True.  I proposed in a different email that ptrace() have full control
of the shadow stack (read, write, lock, unlock, etc).

>
> Does CRIU implement MPX support?

Dunno.  But given that MPX seems to be dying, I'm not sure it matters.

--Andy