# Re: \[PATCH v11 25/25\] x86/cet/shstk: Add arch\_prctl functions for shadow stack

- From: Yu, Yu-cheng \<hidden\>
- Date: 2020-09-16 19:27:49 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2020/09/6107702
- Thread: https://ratatoskr.run/linux-doc/2020/09/6097832/t.md

```text
On 9/16/2020 6:52 AM, Andy Lutomirski wrote:
> On Mon, Sep 14, 2020 at 2:14 PM Dave Hansen [ref] wrote:
>>
>> On 9/14/20 11:31 AM, Andy Lutomirski wrote:
>>> No matter what we do, the effects of calling vfork() are going to be a
>>> bit odd with SHSTK enabled.  I suppose we could disallow this, but
>>> that seems likely to cause its own issues.
>>
>> What's odd about it?  If you're a vfork()'d child, you can't touch the
>> stack at all, right?  If you do, you or your parent will probably die a
>> horrible death.
>>
>
> An evil program could vfork(), have the child do a bunch of returns
> and a bunch of calls, and exit.  The net effect would be to change the
> parent's shadow stack contents.  In a sufficiently strict model, this
> is potentially problematic.

When a vfork child returns, its parent's shadow stack pointer is where it was before the child starts.  To move the shadow stack pointer and re-use the content left by the child, the parent needs to use CALL, RET, INCSSP, or RSTORSSP.  This seems to be difficult.

>
> The question is: how much do we want to protect userspace from itself?
>

If any issue comes up, people can always find ways to counter it.

> --Andy
>
```