Re: [PATCH v9 21/27] KVM: x86: Save and reload SSP to/from SMRAM

[Chao Gao]

On Tue, Jan 23, 2024 at 06:41:54PM -0800, 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@xxxxxxxxxx>
>Suggested-by: Chao Gao <chao.gao@xxxxxxxxx>
>Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
>Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
>---
> 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 3ab133530573..95233b0879a3 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.
>+	 */
>+	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)))

F(LM) and F(SHSTK) are kernel-defined feature bits; they are not
bit masks defined by the CPU. You can use cpuid_entry_has() instead.

Shouldn't we do this only if SMM is supported, i.e., when

static_call(kvm_x86_has_emulated_msr)(kvm, MSR_IA32_SMBASE) is true.

>+			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);
>+
> 	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;
>-- 
>2.39.3
>