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Subject: Re: [PATCH v28 26/32] x86/cet/shstk: Introduce shadow stack token
 setup/verify routines
To:     Dave Hansen <dave.hansen@intel.com>, <x86@kernel.org>,
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        Pengfei Xu <pengfei.xu@intel.com>,
        "Haitao Huang" <haitao.huang@intel.com>,
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References: <20210722205219.7934-1-yu-cheng.yu@intel.com>
 <20210722205219.7934-27-yu-cheng.yu@intel.com>
 <6cff5396-dff3-8db2-0883-62125252741c@intel.com>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
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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