# Re: \[PATCH v29 23/32\] x86/cet/shstk: Add user-mode shadow stack support

- From: Yu, Yu-cheng \<hidden\>
- Date: 2021-08-27 18:10:45 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2021/08/1490798
- Thread: https://ratatoskr.run/linux-doc/2021/08/1484313/t.md

```text
On 8/26/2021 9:25 AM, Borislav Petkov wrote:
> On Fri, Aug 20, 2021 at 11:11:52AM -0700, Yu-cheng Yu wrote:
[...]
>> +
>> +int shstk_setup(void)
>> +{
>> +	struct thread_shstk *shstk = &current->thread.shstk;
>> +	unsigned long addr, size;
>> +	int err;
>> +
>> +	if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
>> +		return 0;
>> +
>> +	size = round_up(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G), PAGE_SIZE);
>> +	addr = alloc_shstk(size);
>> +	if (IS_ERR_VALUE(addr))
>> +		return PTR_ERR((void *)addr);
>> +
>> +	start_update_msrs();
>
> You're setting CET_U with the MSR writes below. Why do you need to do
> XRSTOR here? To zero out PL[012]_SSP?
>
> If so, you can WRMSR those too - no need for a full XRSTOR...
>

Because on context switches the whole xstates are switched together, we need to make sure all are in registers.

>> +	err = wrmsrl_safe(MSR_IA32_PL3_SSP, addr + size);
>> +	if (!err)
>> +		wrmsrl_safe(MSR_IA32_U_CET, CET_SHSTK_EN);
>> +	end_update_msrs();
>> +
>> +	if (!err) {
>> +		shstk->base = addr;
>> +		shstk->size = size;
>> +	}
>> +
>> +	return err;
>> +}
>
```