[PATCH v5 12/19] KVM:x86: Save and reload SSP to/from SMRAM

[Yang Weijiang]

Save CET 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, SSP is defined as one of
the fields in SMRAM for 64-bit mode, so handle the state accordingly.

Check is_smm() 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 | 11 +++++++++++
 arch/x86/kvm/smm.h |  2 +-
 arch/x86/kvm/x86.c | 11 ++++++++++-
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/smm.c b/arch/x86/kvm/smm.c
index b42111a24cc2..e0b62d211306 100644
--- a/arch/x86/kvm/smm.c
+++ b/arch/x86/kvm/smm.c
@@ -309,6 +309,12 @@ void enter_smm(struct kvm_vcpu *vcpu)
 
 	kvm_smm_changed(vcpu, true);
 
+#ifdef CONFIG_X86_64
+	if (guest_can_use(vcpu, X86_FEATURE_SHSTK) &&
+	    kvm_get_msr(vcpu, MSR_KVM_GUEST_SSP, &smram.smram64.ssp))
+		goto error;
+#endif
+
 	if (kvm_vcpu_write_guest(vcpu, vcpu->arch.smbase + 0xfe00, &smram, sizeof(smram)))
 		goto error;
 
@@ -586,6 +592,11 @@ 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) &&
+	    kvm_set_msr(vcpu, MSR_KVM_GUEST_SSP, smram.smram64.ssp))
+		return X86EMUL_UNHANDLEABLE;
+#endif
 	kvm_smm_changed(vcpu, false);
 
 	/*
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;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 98f3ff6078e6..56aa5a3d3913 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3644,8 +3644,17 @@ 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) && is_smm(vcpu))
+				return true;
+
 			return msr->host_initiated;
+		}
 
 		return msr->host_initiated ||
 			guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
-- 
2.27.0