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

[Maxim Levitsky]

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)

Best regards,
	Maxim Levitsky