KVM: PPC: Book3S HV: Don't call real-mode XICS hypercall handlers if not enabled

When running a guest on a POWER9 system with the in-kernel XICS
emulation disabled (for example by running QEMU with the parameter
"-machine pseries,kernel_irqchip=off"), the kernel does not pass
the XICS-related hypercalls such as H_CPPR up to userspace for
emulation there as it should.

The reason for this is that the real-mode handlers for these
hypercalls don't check whether a XICS device has been instantiated
before calling the xics-on-xive code.  That code doesn't check
either, leading to potential NULL pointer dereferences because
vcpu->arch.xive_vcpu is NULL.  Those dereferences won't cause an
exception in real mode but will lead to kernel memory corruption.

This fixes it by adding kvmppc_xics_enabled() checks before calling
the XICS functions.

Cc: stable@vger.kernel.org # v4.11+
Fixes: 5af50993850a ("KVM: PPC: Book3S HV: Native usage of the XIVE interrupt controller")
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_hv_builtin.c

@@ -529,6 +529,8 @@ static inline bool is_rm(void)
 
 unsigned long kvmppc_rm_h_xirr(struct kvm_vcpu *vcpu)
 {
+	if (!kvmppc_xics_enabled(vcpu))
+		return H_TOO_HARD;
 	if (xive_enabled()) {
 		if (is_rm())
 			return xive_rm_h_xirr(vcpu);
@@ -541,6 +543,8 @@ unsigned long kvmppc_rm_h_xirr(struct kvm_vcpu *vcpu)
 
 unsigned long kvmppc_rm_h_xirr_x(struct kvm_vcpu *vcpu)
 {
+	if (!kvmppc_xics_enabled(vcpu))
+		return H_TOO_HARD;
 	vcpu->arch.gpr[5] = get_tb();
 	if (xive_enabled()) {
 		if (is_rm())
@@ -554,6 +558,8 @@ unsigned long kvmppc_rm_h_xirr_x(struct kvm_vcpu *vcpu)
 
 unsigned long kvmppc_rm_h_ipoll(struct kvm_vcpu *vcpu, unsigned long server)
 {
+	if (!kvmppc_xics_enabled(vcpu))
+		return H_TOO_HARD;
 	if (xive_enabled()) {
 		if (is_rm())
 			return xive_rm_h_ipoll(vcpu, server);
@@ -567,6 +573,8 @@ unsigned long kvmppc_rm_h_ipoll(struct kvm_vcpu *vcpu, unsigned long server)
 int kvmppc_rm_h_ipi(struct kvm_vcpu *vcpu, unsigned long server,
 		    unsigned long mfrr)
 {
+	if (!kvmppc_xics_enabled(vcpu))
+		return H_TOO_HARD;
 	if (xive_enabled()) {
 		if (is_rm())
 			return xive_rm_h_ipi(vcpu, server, mfrr);
@@ -579,6 +587,8 @@ int kvmppc_rm_h_ipi(struct kvm_vcpu *vcpu, unsigned long server,
 
 int kvmppc_rm_h_cppr(struct kvm_vcpu *vcpu, unsigned long cppr)
 {
+	if (!kvmppc_xics_enabled(vcpu))
+		return H_TOO_HARD;
 	if (xive_enabled()) {
 		if (is_rm())
 			return xive_rm_h_cppr(vcpu, cppr);
@@ -591,6 +601,8 @@ int kvmppc_rm_h_cppr(struct kvm_vcpu *vcpu, unsigned long cppr)
 
 int kvmppc_rm_h_eoi(struct kvm_vcpu *vcpu, unsigned long xirr)
 {
+	if (!kvmppc_xics_enabled(vcpu))
+		return H_TOO_HARD;
 	if (xive_enabled()) {
 		if (is_rm())
 			return xive_rm_h_eoi(vcpu, xirr);