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

Andy Lutomirski
2022-02-28T20:31:12Z

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