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

[Yang Weijiang]

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
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@xxxxxxxxx>
---
 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;
+	}
+#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))
+			kvm_set_msr(vcpu, MSR_KVM_GUEST_SSP, data);
+	}
+#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 */
 	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))
+				return true;
+
 			return msr->host_initiated;
+		}
 
 		return msr->host_initiated ||
 			guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
-- 
2.27.0