KVM: SVM: hide TF/RF flags used by NMI singlestep

These flags are used internally by SVM so it's cleaner to not leak
them to callers of svm_get_rflags. This is similar to how the TF
flag is handled on KVM_GUESTDBG_SINGLESTEP by kvm_get_rflags and
kvm_set_rflags.

Without this change, the flags may propagate from host VMCB to nested
VMCB or vice versa while singlestepping over a nested VM enter/exit,
and then get stuck in inappropriate places.

Example: NMI singlestepping is enabled while running L1 guest. The
instruction to step over is VMRUN and nested vmrun emulation stashes
rflags to hsave->save.rflags. Then if singlestepping is disabled
while still in L2, TF/RF will be cleared from the nested VMCB but the
next nested VM exit will restore them from hsave->save.rflags and
cause an unexpected DB exception.
Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm.c

@@ -1725,11 +1725,24 @@ static void svm_vcpu_unblocking(struct kvm_vcpu *vcpu)
 
 static unsigned long svm_get_rflags(struct kvm_vcpu *vcpu)
 {
-	return to_svm(vcpu)->vmcb->save.rflags;
+	struct vcpu_svm *svm = to_svm(vcpu);
+	unsigned long rflags = svm->vmcb->save.rflags;
+
+	if (svm->nmi_singlestep) {
+		/* Hide our flags if they were not set by the guest */
+		if (!(svm->nmi_singlestep_guest_rflags & X86_EFLAGS_TF))
+			rflags &= ~X86_EFLAGS_TF;
+		if (!(svm->nmi_singlestep_guest_rflags & X86_EFLAGS_RF))
+			rflags &= ~X86_EFLAGS_RF;
+	}
+	return rflags;
 }
 
 static void svm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
 {
+	if (to_svm(vcpu)->nmi_singlestep)
+		rflags |= (X86_EFLAGS_TF | X86_EFLAGS_RF);
+
        /*
         * Any change of EFLAGS.VM is accompanied by a reload of SS
         * (caused by either a task switch or an inter-privilege IRET),