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Date:   Mon, 28 Feb 2022 12:30:41 -0800
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Mike Rapoport" <rppt@kernel.org>
Cc:     "Rick P Edgecombe" <rick.p.edgecombe@intel.com>,
        "Cyrill Gorcunov" <gorcunov@gmail.com>,
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Subject: Re: [PATCH 00/35] Shadow stacks for userspace
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On Mon, Feb 28, 2022, at 12:27 PM, Mike Rapoport wrote:
> On Wed, Feb 09, 2022 at 06:37:53PM -0800, Andy Lutomirski wrote:
>> On 2/8/22 18:18, Edgecombe, Rick P wrote:
>> > On Tue, 2022-02-08 at 20:02 +0300, Cyrill Gorcunov wrote:
>> > 
>> > Still wrapping my head around the CRIU save and restore steps, but
>> > another general approach might be to give ptrace the ability to
>> > temporarily pause/resume/set CET enablement and SSP for a stopped
>> > thread. Then injected code doesn't need to jump through any hoops or
>> > possibly run into road blocks. I'm not sure how much this opens things
>> > up if the thread has to be stopped...
>> 
>> Hmm, that's maybe not insane.
>> 
>> 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.
>
> Using ptrace() will not solve CRIU's issue with sigreturn because sigreturn
> is called from the victim context rather than from the criu process that
> controls the dump and uses ptrace().

I'm not sure I follow.

>
> Even with the current shadow stack interface Rick proposed, CRIU can restore
> the victim using ptrace without any additional knobs, but we loose an
> important ability to "self-cure" the victim from the parasite in case
> anything goes wrong with criu control process.
>
> Moreover, the issue with backward compatibility is not with ptrace but with
> sigreturn and it seems that criu is not its only user.

So we need an ability for a tracer to cause the tracee to call a function and to return successfully.  Apparently a gdb branch can already do this with shstk, and my PTRACE_CALL_FUNCTION_SIGFRAME should also do the trick.  I don't see why we need a sigretur-but-dont-verify -- we just need this mechanism to create a frame such that sigreturn actually works.

--Andy