[PATCH v10 21/27] KVM: x86: Save and reload SSP to/from SMRAM

[Yang Weijiang <weijiang.yang@xxxxxxxxx>]

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>
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 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.
+	 */
+	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);
+
 	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;
-- 
2.43.0