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Date:   Tue, 8 Feb 2022 20:02:32 +0300
From:   Cyrill Gorcunov <gorcunov@gmail.com>
To:     Andy Lutomirski <luto@kernel.org>
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Subject: Re: [PATCH 00/35] Shadow stacks for userspace
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On Tue, Feb 08, 2022 at 08:21:20AM -0800, Andy Lutomirski wrote:
> >> But such a knob will immediately reduce the security value of the entire
> >> thing, and I don't have good ideas how to deal with it :(
> >
> > Probably a kind of latch in the task_struct which would trigger off once
> > returt to a different address happened, thus we would be able to jump inside
> > paratite code. Of course such trigger should be available under proper
> > capability only.
> 
> I'm not fully in touch with how parasite, etc works.  Are we talking about save or restore?

We use parasite code in question during checkpoint phase as far as I remember.
push addr/lret trick is used to run "injected" code (code injection itself is
done via ptrace) in compat mode at least. Dima, Andrei, I didn't look into this code
for years already, do we still need to support compat mode at all?

> If it's restore, what exactly does CRIU need to do?  Is it just that CRIU needs to return
> out from its resume code into the to-be-resumed program without tripping CET?  Would it
> be acceptable for CRIU to require that at least one shstk slot be free at save time?
> Or do we need a mechanism to atomically switch to a completely full shadow stack at resume?
> 
> Off the top of my head, a sigreturn (or sigreturn-like mechanism) that is intended for
> use for altshadowstack could safely verify a token on the altshdowstack, possibly
> compare to something in ucontext (or not -- this isn't clearly necessary) and switch
> back to the previous stack.  CRIU could use that too.  Obviously CRIU will need a way
> to populate the relevant stacks, but WRUSS can be used for that, and I think this
> is a fundamental requirement for CRIU -- CRIU restore absolutely needs a way to write
> the saved shadow stack data into the shadow stack.
> 
> So I think the only special capability that CRIU really needs is WRUSS, and
> we need to wire that up anyway.

Thanks for these notes, Andy! I can't provide any sane answer here since didn't
read tech spec for this feature yet :-)