linux-kernel - Re: [PATCH v4 09/20] KVM:x86: Add common code of CET MSR access

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Date:   Thu, 27 Jul 2023 15:41:08 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     "Yang, Weijiang" <weijiang.yang@...el.com>
CC:     <seanjc@...gle.com>, <pbonzini@...hat.com>, <peterz@...radead.org>,
        <john.allen@....com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <rick.p.edgecombe@...el.com>,
        <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH v4 09/20] KVM:x86: Add common code of CET MSR access

>> -	case MSR_KVM_GUEST_SSP:
>> -	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
>> 	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
>> 		if (!kvm_cet_is_msr_accessible(vcpu, msr_info))
>> 			return 1;
>> 		if (is_noncanonical_address(data, vcpu))
>> 			return 1;
>> 		if (!IS_ALIGNED(data, 4))
>> 			return 1;
>> 		if (msr == MSR_IA32_PL0_SSP || msr == MSR_IA32_PL1_SSP ||
>> 		    msr == MSR_IA32_PL2_SSP) {
>> 			vcpu->arch.cet_s_ssp[msr - MSR_IA32_PL0_SSP] = data;
>> 		} else if (msr == MSR_IA32_PL3_SSP) {
>> 			kvm_set_xsave_msr(msr_info);
>> 		}
>> 		break;
>> 
>> 
>> 
>> BTW, shouldn't bit2:0 of MSR_KVM_GUEST_SSP be 0? i.e., for MSR_KVM_GUEST_SSP,
>> the alignment check should be IS_ALIGNED(data, 8).
>
>The check for GUEST_SSP should be consistent with that of PLx_SSPs, otherwise
>there would be issues

OK. I had the question because Gil said in a previous email:

	IDT event delivery, when changing to rings 0-2 will load SSP from the
	MSR corresponding to the new ring.  These transitions check that bits
	2:0 of the new value are all zero and will generate a nested fault if
	any of those bits are set.  (Far CALL using a call gate also checks this
	if changing CPL.)

it sounds to me, at least for CPL0-2, SSP (or the synethic
MSR_KVM_GUEST_SSP) should be 8-byte aligned. Otherwise, there will be a
nested fault when trying to load SSP.

I might be overly cautious. No objection to do IS_ALIGNED(data, 4) for SSP.