[PATCH] KVM: nSVM: Add missing CET VMRUN consistency checks

[Jim Mattson <jmattson@google.com>]

Per the APM, "Canonicalization and Consistency Checks,"

  [t]he following conditions are considered illegal state combinations:

   - CR4.CET=1 when CR0.WP=0
   - Any reserved bit set in S_CET
   - CR4.CET=1 and U_CET.SS=1 when EFLAGS.VM=1

Check these conditions in software before trying to VMRUN vmcb02, so that
the illegal states result in a #VMEXIT to L1 rather than an exit to
userspace with KVM_EXIT_FAIL_ENTRY.

Fixes: c5ba49458513 ("KVM: nSVM: Save/load CET Shadow Stack state to/from vmcb12/vmcb02")
Signed-off-by: Jim Mattson <jmattson@google.com>
---
 arch/x86/kvm/svm/nested.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index af0ef678433d..20ede403b62d 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -469,6 +469,21 @@ static bool nested_vmcb_check_save(struct kvm_vcpu *vcpu,
 	if (CC(!__kvm_is_valid_cr4(vcpu, save->cr4)))
 		return false;
 
+	if (CC((save->cr4 & X86_CR4_CET) && !(save->cr0 & X86_CR0_WP)))
+		return false;
+
+	if (guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK) &&
+	    CC(!kvm_is_valid_u_s_cet(vcpu, save->s_cet)))
+		return false;
+
+	if ((save->cr4 & X86_CR4_CET) && (save->rflags & X86_EFLAGS_VM)) {
+		u64 u_cet = 0;
+
+		if (!__kvm_emulate_msr_read(vcpu, MSR_IA32_U_CET, &u_cet) &&
+		    CC(u_cet & CET_SHSTK_EN))
+			return false;
+	}
+
 	if (CC(!kvm_valid_efer(vcpu, save->efer)))
 		return false;
 
-- 
2.56.0.506.g506a7eabc5-goog