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Date: Wed, 1 May 2024 15:50:11 -0700
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Subject: Re: [PATCH v10 21/27] KVM: x86: Save and reload SSP to/from SMRAM
From: Sean Christopherson <seanjc@google.com>
To: Yang Weijiang <weijiang.yang@intel.com>
Cc: pbonzini@redhat.com, dave.hansen@intel.com, x86@kernel.org, 
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org, peterz@infradead.org, 
	chao.gao@intel.com, rick.p.edgecombe@intel.com, mlevitsk@redhat.com, 
	john.allen@amd.com
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On Sun, Feb 18, 2024, 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 64-bit Arch, and add the support for SSP. Note,
> on 32-bit Arch, SSP is not defined in SMRAM, so fail 32-bit CET guest
> launch.
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  arch/x86/kvm/cpuid.c | 11 +++++++++++
>  arch/x86/kvm/smm.c   |  8 ++++++++
>  arch/x86/kvm/smm.h   |  2 +-
>  3 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 2bb1931103ad..c0e13040e35b 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -149,6 +149,17 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
>  		if (vaddr_bits != 48 && vaddr_bits != 57 && vaddr_bits != 0)
>  			return -EINVAL;
>  	}
> +	/*
> +	 * Prevent 32-bit guest launch if shadow stack is exposed as SSP
> +	 * state is not defined for 32-bit SMRAM.

Why?  Lack of save/restore for SSP on 32-bit guests is a gap in Intel's
architecture, I don't see why KVM should diverge from hardware.  I.e. just do
nothing for SSP on SMI/RSM, because that's exactly what the architecture says
will happen.

> +	 */
> +	best = cpuid_entry2_find(entries, nent, 0x80000001,
> +				 KVM_CPUID_INDEX_NOT_SIGNIFICANT);
> +	if (best && !(best->edx & F(LM))) {
> +		best = cpuid_entry2_find(entries, nent, 0x7, 0);
> +		if (best && (best->ecx & F(SHSTK)))
> +			return -EINVAL;
> +	}
>  
>  	/*
>  	 * Exposing dynamic xfeatures to the guest requires additional
> 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);


This should synthesize triple-fault, not WARN and kill the VM, as the value to
be restored is guest controlled (the guest can scribble SMRAM from within the
SMI handler).

At that point, I would just synthesize triple-fault for the read path too.