# Re: \[PATCH v27 24/31\] x86/cet/shstk: Handle thread shadow stack

- From: Yu, Yu-cheng \<hidden\>
- Date: 2021-05-25 15:05:45 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2021/05/1391494
- Thread: https://ratatoskr.run/linux-doc/2021/05/1388103/t.md

```text
On 5/22/2021 4:39 PM, Andy Lutomirski wrote:
> On Fri, May 21, 2021 at 3:14 PM Yu-cheng Yu [ref] wrote:
>> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
>> index 5ea2b494e9f9..8e5f772181b9 100644
>> --- a/arch/x86/kernel/shstk.c
>> +++ b/arch/x86/kernel/shstk.c
>> @@ -71,6 +71,53 @@ int shstk_setup(void)
>>          return 0;
>>   }
>>
>> +int shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
>> +                            unsigned long stack_size)
>> +{
>
> ...
>
>> +       state = get_xsave_addr(&tsk->thread.fpu.state.xsave, XFEATURE_CET_USER);
>> +       if (!state)
>> +               return -EINVAL;
>> +
>
> The get_xsave_addr() API is horrible, and we already have one
> egregiously buggy instance in the kernel.  Let's not add another
> dubious use case.
>
> If state == NULL, this means that CET_USER is in its init state.
> CET_USER in the init state should behave identically regardless of
> whether XINUSE[CET_USER] is set.  Can you please adjust this code
> accordingly?
>

I will work on that.

Thanks,
Yu-cheng
```