Re: [PATCH v18 05/25] x86/fpu/xstate: Introduce CET MSR and XSAVES supervisor states
From: Dave Hansen
Date: Sun Jan 31 2021 - 18:34:24 EST
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.