# Re: \[PATCH v27 06/10\] x86/cet/ibt: Update arch\_prctl functions for Indirect Branch Tracking

- From: Yu, Yu-cheng \<hidden\>
- Date: 2021-07-20 17:13:26 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2021/07/1452650
- Thread: https://ratatoskr.run/linux-arch/2021/05/1388137/t.md

```text
On 7/19/2021 11:21 AM, Edgecombe, Rick P wrote:
> On Fri, 2021-05-21 at 15:15 -0700, Yu-cheng Yu wrote:
>> From: "H.J. Lu" <redacted>
>>
>> Update ARCH_X86_CET_STATUS and ARCH_X86_CET_DISABLE for Indirect
>> Branch
>> Tracking.
>>
>> Signed-off-by: H.J. Lu <redacted>
>> Signed-off-by: Yu-cheng Yu <redacted>
>> Reviewed-by: Kees Cook <redacted>
>> ---
>>   arch/x86/kernel/cet_prctl.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git a/arch/x86/kernel/cet_prctl.c
>> b/arch/x86/kernel/cet_prctl.c
>> index b426d200e070..bd3c80d402e7 100644
>> --- a/arch/x86/kernel/cet_prctl.c
>> +++ b/arch/x86/kernel/cet_prctl.c
>> @@ -22,6 +22,9 @@ static int cet_copy_status_to_user(struct
>> thread_shstk *shstk, u64 __user *ubuf)
>>                  buf[2] = shstk->size;
>>          }
>>   
>> +       if (shstk->ibt)
>> +               buf[0] |= GNU_PROPERTY_X86_FEATURE_1_IBT;
>> +
> Can you have IBT enabled but not shadow stack via kernel parameters?
> Outside this diff it has:
> if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> 	return -ENOTSUPP;

If shadow stack is disabled by the kernel parameter, IBT is also disabled.

> So if "no_user_shstk" is set, this can't be used for IBT. But the
> kernel would attempt to enable IBT.

It will not.

> Also if so, the CR4 bit enabling logic needs adjusting in this IBT
> series. If not, we should probably mention this in the docs and enforce
> it. It would then follow the logic in Kconfig, so maybe the simplest.
> Like maybe instead of no_user_shstk, just no_user_cet?

If shadow stack is disabled (from either Kconfig or kernel command-line), then IBT is also disabled.  However, we still need two kernel parameters because no_user_ibt can be useful sometimes.  I will add a sentence in the document to indicate that IBT depends on shadow stack.

Thanks,
Yu-cheng
```