KVM: nSVM: do not forward NMI window singlestep VM exits to L1

Nested hypervisor should not see singlestep VM exits if singlestepping
was enabled internally by KVM. Windows is particularly sensitive to this
and known to bluescreen on unexpected VM exits.
Signed-off-by: NLadi Prosek <lprosek@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm.c

@@ -189,6 +189,7 @@ struct vcpu_svm {
 	struct nested_state nested;
 
 	bool nmi_singlestep;
+	u64 nmi_singlestep_guest_rflags;
 
 	unsigned int3_injected;
 	unsigned long int3_rip;
@@ -966,9 +967,13 @@ static void svm_disable_lbrv(struct vcpu_svm *svm)
 static void disable_nmi_singlestep(struct vcpu_svm *svm)
 {
 	svm->nmi_singlestep = false;
-	if (!(svm->vcpu.guest_debug & KVM_GUESTDBG_SINGLESTEP))
-		svm->vmcb->save.rflags &=
-			~(X86_EFLAGS_TF | X86_EFLAGS_RF);
+	if (!(svm->vcpu.guest_debug & KVM_GUESTDBG_SINGLESTEP)) {
+		/* Clear our flags if they were not set by the guest */
+		if (!(svm->nmi_singlestep_guest_rflags & X86_EFLAGS_TF))
+			svm->vmcb->save.rflags &= ~X86_EFLAGS_TF;
+		if (!(svm->nmi_singlestep_guest_rflags & X86_EFLAGS_RF))
+			svm->vmcb->save.rflags &= ~X86_EFLAGS_RF;
+	}
 }
 
 /* Note:
@@ -2538,6 +2543,31 @@ static int nested_svm_exit_handled_msr(struct vcpu_svm *svm)
 	return (value & mask) ? NESTED_EXIT_DONE : NESTED_EXIT_HOST;
 }
 
+/* DB exceptions for our internal use must not cause vmexit */
+static int nested_svm_intercept_db(struct vcpu_svm *svm)
+{
+	unsigned long dr6;
+
+	/* if we're not singlestepping, it's not ours */
+	if (!svm->nmi_singlestep)
+		return NESTED_EXIT_DONE;
+
+	/* if it's not a singlestep exception, it's not ours */
+	if (kvm_get_dr(&svm->vcpu, 6, &dr6))
+		return NESTED_EXIT_DONE;
+	if (!(dr6 & DR6_BS))
+		return NESTED_EXIT_DONE;
+
+	/* if the guest is singlestepping, it should get the vmexit */
+	if (svm->nmi_singlestep_guest_rflags & X86_EFLAGS_TF) {
+		disable_nmi_singlestep(svm);
+		return NESTED_EXIT_DONE;
+	}
+
+	/* it's ours, the nested hypervisor must not see this one */
+	return NESTED_EXIT_HOST;
+}
+
 static int nested_svm_exit_special(struct vcpu_svm *svm)
 {
 	u32 exit_code = svm->vmcb->control.exit_code;
@@ -2593,8 +2623,12 @@ static int nested_svm_intercept(struct vcpu_svm *svm)
 	}
 	case SVM_EXIT_EXCP_BASE ... SVM_EXIT_EXCP_BASE + 0x1f: {
 		u32 excp_bits = 1 << (exit_code - SVM_EXIT_EXCP_BASE);
-		if (svm->nested.intercept_exceptions & excp_bits)
-			vmexit = NESTED_EXIT_DONE;
+		if (svm->nested.intercept_exceptions & excp_bits) {
+			if (exit_code == SVM_EXIT_EXCP_BASE + DB_VECTOR)
+				vmexit = nested_svm_intercept_db(svm);
+			else
+				vmexit = NESTED_EXIT_DONE;
+		}
 		/* async page fault always cause vmexit */
 		else if ((exit_code == SVM_EXIT_EXCP_BASE + PF_VECTOR) &&
 			 svm->apf_reason != 0)
@@ -4635,6 +4669,7 @@ static void enable_nmi_window(struct kvm_vcpu *vcpu)
 	 * Something prevents NMI from been injected. Single step over possible
 	 * problem (IRET or exception injection or interrupt shadow)
 	 */
+	svm->nmi_singlestep_guest_rflags = svm_get_rflags(vcpu);
 	svm->nmi_singlestep = true;
 	svm->vmcb->save.rflags |= (X86_EFLAGS_TF | X86_EFLAGS_RF);
 }