Re: [PATCH v4 11/20] KVM:x86: Save and reload GUEST_SSP to/from SMRAM

["Yang, Weijiang" <weijiang.yang@intel.com>]

On 7/24/2023 5:13 PM, Chao Gao wrote:
> On Thu, Jul 20, 2023 at 11:03:43PM -0400, Yang Weijiang wrote:
>> Save GUEST_SSP to SMRAM on SMI and reload it on RSM.
>> KVM emulates architectural behavior when guest enters/leaves SMM
>> mode, i.e., save registers to SMRAM at the entry of SMM and reload
>> them at the exit of SMM. Per SDM, GUEST_SSP is defined as one of
> To me, GUEST_SSP is confusing here. From QEMU's perspective, it reads/writes
> the SSP register. People may confuse it with the GUEST_SSP in nVMCS field.
> I prefer to rename it to MSR_KVM_SSP.

Hmm, looks a bit, I'll change it, thanks!

>
>> the fields in SMRAM for 64-bit mode, so handle the state accordingly.
>>
>> Check HF_SMM_MASK to determine whether kvm_cet_is_msr_accessible()
>> is called in SMM mode so that kvm_{set,get}_msr() works in SMM mode.
>>
>> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>> ---
>> arch/x86/kvm/smm.c | 17 +++++++++++++++++
>> arch/x86/kvm/smm.h |  2 +-
>> arch/x86/kvm/x86.c | 12 +++++++++++-
>> 3 files changed, 29 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/smm.c b/arch/x86/kvm/smm.c
>> index b42111a24cc2..a4e19d72224f 100644
>> --- a/arch/x86/kvm/smm.c
>> +++ b/arch/x86/kvm/smm.c
>> @@ -309,6 +309,15 @@ void enter_smm(struct kvm_vcpu *vcpu)
>>
>> 	kvm_smm_changed(vcpu, true);
>>
>> +#ifdef CONFIG_X86_64
>> +	if (guest_can_use(vcpu, X86_FEATURE_SHSTK)) {
>> +		u64 data;
>> +
>> +		if (!kvm_get_msr(vcpu, MSR_KVM_GUEST_SSP, &data))
>> +			smram.smram64.ssp = data;
> I don't think it is correct to continue if kvm fails to read the MSR.
>
> how about:
> 		if (kvm_get_msr(vcpu, MSR_KVM_GUEST_SSP, &smram.smram64.ssp))
> 			goto error;

Agree,  will change it.

>> +	}
>> +#endif
>> +
>> 	if (kvm_vcpu_write_guest(vcpu, vcpu->arch.smbase + 0xfe00, &smram, sizeof(smram)))
>> 		goto error;
>>
>> @@ -586,6 +595,14 @@ int emulator_leave_smm(struct x86_emulate_ctxt *ctxt)
>> 	if ((vcpu->arch.hflags & HF_SMM_INSIDE_NMI_MASK) == 0)
>> 		static_call(kvm_x86_set_nmi_mask)(vcpu, false);
>>
>> +#ifdef CONFIG_X86_64
>> +	if (guest_can_use(vcpu, X86_FEATURE_SHSTK)) {
>> +		u64 data = smram.smram64.ssp;
>> +
>> +		if (is_noncanonical_address(data, vcpu) && IS_ALIGNED(data, 4))
> shouldn't the checks be already done inside kvm_set_msr()?

Nice catch, will remove them.


>
>> +			kvm_set_msr(vcpu, MSR_KVM_GUEST_SSP, data);
> please handle the failure case. Probably just return X86EMUL_UNHANDLEABLE like other
> failure paths in this function.

OK, VM should be shutdown if this field cannot be written successfully.

>
>> +	}
>> +#endif
>> 	kvm_smm_changed(vcpu, false);
>>
>> 	/*
>> diff --git a/arch/x86/kvm/smm.h b/arch/x86/kvm/smm.h
>> index a1cf2ac5bd78..b3efef7cb1dc 100644
>> --- a/arch/x86/kvm/smm.h
>> +++ b/arch/x86/kvm/smm.h
>> @@ -116,7 +116,7 @@ struct kvm_smram_state_64 {
>> 	u32 smbase;
>> 	u32 reserved4[5];
>>
>> -	/* ssp and svm_* fields below are not implemented by KVM */
>> +	/* svm_* fields below are not implemented by KVM */
> move this comment one line downward

OK

>
>> 	u64 ssp;
>> 	u64 svm_guest_pat;
>> 	u64 svm_host_efer;
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index f7558f0f6fc0..70d7c80889d6 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -3653,8 +3653,18 @@ static bool kvm_cet_is_msr_accessible(struct kvm_vcpu *vcpu,
>> 		if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK))
>> 			return false;
>>
>> -		if (msr->index == MSR_KVM_GUEST_SSP)
>> +		/*
>> +		 * This MSR is synthesized mainly for userspace access during
>> +		 * Live Migration, it also can be accessed in SMM mode by VMM.
>> +		 * Guest is not allowed to access this MSR.
>> +		 */
>> +		if (msr->index == MSR_KVM_GUEST_SSP) {
>> +			if (IS_ENABLED(CONFIG_X86_64) &&
>> +			    !!(vcpu->arch.hflags & HF_SMM_MASK))
> use is_smm() instead.

OK.

>
>> +				return true;
>> +
>> 			return msr->host_initiated;
>> +		}
>>
>> 		return msr->host_initiated ||
>> 			guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
>> -- 
>> 2.27.0
>>