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

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Message-ID: <ZMKiLI2oEHivN000@google.com>
Date:   Thu, 27 Jul 2023 09:58:20 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Gao <chao.gao@...el.com>
Cc:     Weijiang Yang <weijiang.yang@...el.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

On Thu, Jul 27, 2023, Chao Gao wrote:
> >> -	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.

Yes, but that's the guest's problem.  KVM's responsibility is purely to faithfully
emulate hardware, which in this case means requiring that bits 1:0 be cleared on
the WRMSR.  *Architecturally*, software is allowed to set bit 2, and only if/when
the vCPU consumes the "bad" value by transitioning to the relevant CPL will the
CPU generate a fault.