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SubjectRe: [PATCH 04/10] x86/cet: Handle thread shadow stack
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On 06/07/2018 10:53 PM, Andy Lutomirski wrote:
> On Thu, Jun 7, 2018 at 12:47 PM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> On 06/07/2018 08:21 PM, Andy Lutomirski wrote:
>>> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>>>>
>>>> When fork() specifies CLONE_VM but not CLONE_VFORK, the child
>>>> needs a separate program stack and a separate shadow stack.
>>>> This patch handles allocation and freeing of the thread shadow
>>>> stack.
>>>
>>> Aha -- you're trying to make this automatic. I'm not convinced this
>>> is a good idea. The Linux kernel has a long and storied history of
>>> enabling new hardware features in ways that are almost entirely
>>> useless for userspace.
>>>
>>> Florian, do you have any thoughts on how the user/kernel interaction
>>> for the shadow stack should work?
>>
>> I have not looked at this in detail, have not played with the emulator,
>> and have not been privy to any discussions before these patches have
>> been posted, however …
>>
>> I believe that we want as little code in userspace for shadow stack
>> management as possible. One concern I have is that even with the code
>> we arguably need for various kinds of stack unwinding, we might have
>> unwittingly built a generic trampoline that leads to full CET bypass.
>
> I was imagining an API like "allocate a shadow stack for the current
> thread, fail if the current thread already has one, and turn on the
> shadow stack". glibc would call clone and then call this ABI pretty
> much immediately (i.e. before making any calls from which it expects
> to return).

Ahh. So you propose not to enable shadow stack enforcement on the new
thread even if it is enabled for the current thread? For the cases
where CLONE_VM is involved?

It will still need a new assembler wrapper which sets up the shadow
stack, and it's probably required to disable signals.

I think it should be reasonable safe and actually implementable. But
the benefits are not immediately obvious to me.

> We definitely want strong enough user control that tools like CRIU can
> continue to work. I haven't looked at the SDM recently enough to
> remember for sure, but I'm reasonably confident that user code can
> learn the address of its own shadow stack. If nothing else, CRIU
> needs to be able to restore from a context where there's a signal on
> the stack and the signal frame contains a shadow stack pointer.

CRIU also needs the shadow stack *contents*, which shouldn't be directly
available to the process. So it needs very special interfaces anyway.

Does CRIU implement MPX support?

Thanks,
Florian

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