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Date:   Fri, 11 Feb 2022 10:04:38 +0200
From:   Mike Rapoport <rppt@kernel.org>
To:     "avagin@gmail.com" <avagin@gmail.com>
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Subject: Re: [PATCH 00/35] Shadow stacks for userspace
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On Thu, Feb 10, 2022 at 11:41:16PM -0800, avagin@gmail.com wrote:
> On Wed, Feb 09, 2022 at 06:37:53PM -0800, Andy Lutomirski wrote:
> >
> > An alternative would be to add a bona fide ptrace call-a-function mechanism.
> > I can think of two potentially usable variants:
> > 
> > 1. Straight call.  PTRACE_CALL_FUNCTION(addr) just emulates CALL addr,
> > shadow stack push and all.
> > 
> > 2. Signal-style.  PTRACE_CALL_FUNCTION_SIGFRAME injects an actual signal
> > frame just like a real signal is being delivered with the specified handler.
> > There could be a variant to opt-in to also using a specified altstack and
> > altshadowstack.
> 
> I think this would be ideal. In CRIU, the parasite code is executed in
> the "daemon" mode and returns back via sigreturn.  Right now, CRIU needs
> to generate a signal frame. If I understand your idea right, the signal
> frame will be generated by the kernel.
> 
> > 
> > 2 would be more expensive but would avoid the need for much in the way of
> > asm magic.  The injected code could be plain C (or Rust or Zig or whatever).
> > 
> > All of this only really handles save, not restore.  I don't understand
> > restore enough to fully understand the issue.
> 
> In a few words, it works like this: CRIU restores all required resources
> and prepares a signal frame with a target process state, then it
> switches to a small PIE blob, where it restores vma-s and calls
> rt_sigreturn.

I think it's also important to note that the stack is restored as a part of
the process memory, i.e. its contents is read from the images.
 
> > 
> > --Andy

-- 
Sincerely yours,
Mike.