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

[Florian Weimer]

On 06/07/2018 10:53 PM, Andy Lutomirski wrote:
> On Thu, Jun 7, 2018 at 12:47 PM Florian Weimer <fweimer@xxxxxxxxxx> 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@xxxxxxxxx> 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 â
> >
> > 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.

> 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.

Does CRIU implement MPX support?

Thanks,
Florian