Re: [PATCH v18 05/25] x86/fpu/xstate: Introduce CET MSR and XSAVES supervisor states

[Dave Hansen]

On 1/29/21 2:35 PM, Yu, Yu-cheng wrote:
>> Andy Cooper just mentioned on IRC about this nugget in the spec:
>>
>>     XRSTORS on CET state will do reserved bit and canonicality
>>     checks on the state in similar manner as done by the WRMSR to
>>     these state elements.
>>
>> We're using copy_kernel_to_xregs_err(), so the #GP *should* be OK.
>> Could we prove this out in practice, please?
>>>
> Do we want to verify that setting reserved bits in CET XSAVES states
> triggers GP?  Then, yes, I just verified it again.  Thanks for
> reminding.  Do we have any particular case relating to this?

I want to confirm that it triggers #GP and kills userspace without the
kernel WARN'ing or otherwise being visibly unhappy.

What about the return-to-userspace path after a ptracer writes content
to the CET fields?   I don't see the same tolerance for errors in
__fpregs_load_activate(), for instance.