linux-kernel - Re: [PATCH 00/35] Shadow stacks for userspace

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Date:   Wed, 9 Feb 2022 18:37:53 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "gorcunov@...il.com" <gorcunov@...il.com>
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        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH 00/35] Shadow stacks for userspace

On 2/8/22 18:18, Edgecombe, Rick P wrote:
> On Tue, 2022-02-08 at 20:02 +0300, Cyrill Gorcunov wrote:
>> 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.
> 
> 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.

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.

--Andy