Re: [PATCH v7 21/26] KVM: x86: Save and reload SSP to/from SMRAM

[Chao Gao]

On Thu, Nov 30, 2023 at 07:42:44PM +0200, Maxim Levitsky wrote:
>On Fri, 2023-11-24 at 00:53 -0500, Yang Weijiang wrote:
>> Save CET SSP to SMRAM on SMI and reload it on RSM. KVM emulates HW arch
>> behavior when guest enters/leaves SMM mode,i.e., save registers to SMRAM
>> at the entry of SMM and reload them at the exit to SMM. Per SDM, SSP is
>> one of such registers on 64bit Arch, so add the support for SSP.
>> 
>> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
>> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
>> ---
>>  arch/x86/kvm/smm.c | 8 ++++++++
>>  arch/x86/kvm/smm.h | 2 +-
>>  2 files changed, 9 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/kvm/smm.c b/arch/x86/kvm/smm.c
>> index 45c855389ea7..7aac9c54c353 100644
>> --- a/arch/x86/kvm/smm.c
>> +++ b/arch/x86/kvm/smm.c
>> @@ -275,6 +275,10 @@ static void enter_smm_save_state_64(struct kvm_vcpu *vcpu,
>>  	enter_smm_save_seg_64(vcpu, &smram->gs, VCPU_SREG_GS);
>>  
>>  	smram->int_shadow = static_call(kvm_x86_get_interrupt_shadow)(vcpu);
>> +
>> +	if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
>> +		KVM_BUG_ON(kvm_msr_read(vcpu, MSR_KVM_SSP, &smram->ssp),
>> +			   vcpu->kvm);
>>  }
>>  #endif
>>  
>> @@ -564,6 +568,10 @@ static int rsm_load_state_64(struct x86_emulate_ctxt *ctxt,
>>  	static_call(kvm_x86_set_interrupt_shadow)(vcpu, 0);
>>  	ctxt->interruptibility = (u8)smstate->int_shadow;
>>  
>> +	if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
>> +		KVM_BUG_ON(kvm_msr_write(vcpu, MSR_KVM_SSP, smstate->ssp),
>> +			   vcpu->kvm);
>> +
>>  	return X86EMUL_CONTINUE;
>>  }
>>  #endif
>> diff --git a/arch/x86/kvm/smm.h b/arch/x86/kvm/smm.h
>> index a1cf2ac5bd78..1e2a3e18207f 100644
>> --- a/arch/x86/kvm/smm.h
>> +++ b/arch/x86/kvm/smm.h
>> @@ -116,8 +116,8 @@ struct kvm_smram_state_64 {
>>  	u32 smbase;
>>  	u32 reserved4[5];
>>  
>> -	/* ssp and svm_* fields below are not implemented by KVM */
>>  	u64 ssp;
>> +	/* svm_* fields below are not implemented by KVM */
>>  	u64 svm_guest_pat;
>>  	u64 svm_host_efer;
>>  	u64 svm_host_cr4;
>
>
>My review feedback from the previous patch series still applies, and I don't
>know why it was not addressed/replied to:
>
>I still think that it is worth it to have a check that CET is not enabled in
>enter_smm_save_state_32 which is called for pure 32 bit guests (guests that don't
>have X86_FEATURE_LM enabled)

can KVM just reject a KVM_SET_CPUID ioctl which attempts to expose shadow stack
(or even any CET feature) to 32-bit guest in the first place? I think it is simpler.