Re: [PATCH v28 26/32] x86/cet/shstk: Introduce shadow stack token setup/verify routines

Yu, Yu-cheng
2021-07-23T18:01:38Z

On 7/22/2021 2:15 PM, Dave Hansen wrote:
> On 7/22/21 1:52 PM, Yu-cheng Yu wrote:
>> + if (fpregs_state_valid(fpu, smp_processor_id())) {
>> + rdmsrl(MSR_IA32_PL3_SSP, ssp);
>> + } else {
>> + struct cet_user_state *p;
>> +
>> + /*
>> + * When !fpregs_state_valid() and get_xsave_addr() returns
>> + * null, XFEAUTRE_CET_USER is in init state. Shadow stack
>> + * pointer is null in this case, so return zero.
>> + */
>> + p = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
>> + if (p)
>> + ssp = p->user_ssp;
>> + }
>> +
>> + fpregs_unlock();
>
> Why are we even calling into this code if shadow stacks might be
> disabled? Seems like we should have just errored out long before
> getting here.
>

That is true. When this function is called, shadow stack is enabled.
If get_xsave_addr() returns null, it is possible xstates is messed up.
Maybe I can update the comments to explain it?

Thanks,
Yu-cheng