s390/kvm: validate the floating-point control before restoring it

The kvm_arch_vcpu_load() does not validate whether the floating-point
control (FPC) is valid.  Further, the return code of the restore is not
checked too.  If the FPC is invalid, the restore fails and the host FPC
value might remain.  The correct behavior would be to clear the FPC if it
is not valid.  Hence, validate the FPC value and, optionally, reset the
value before restoring it.

Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kvm/kvm-s390.c

@@ -1200,19 +1200,25 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
 
 void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 {
+	__u32 fpc;
+
 	save_fp_ctl(&vcpu->arch.host_fpregs.fpc);
 	if (test_kvm_facility(vcpu->kvm, 129))
 		save_vx_regs((__vector128 *)&vcpu->arch.host_vregs->vrs);
 	else
 		save_fp_regs(vcpu->arch.host_fpregs.fprs);
 	save_access_regs(vcpu->arch.host_acrs);
 	if (test_kvm_facility(vcpu->kvm, 129)) {
-		restore_fp_ctl(&vcpu->run->s.regs.fpc);
+		fpc = vcpu->run->s.regs.fpc;
 		restore_vx_regs((__vector128 *)&vcpu->run->s.regs.vrs);
 	} else {
-		restore_fp_ctl(&vcpu->arch.guest_fpregs.fpc);
+		fpc = vcpu->arch.guest_fpregs.fpc;
 		restore_fp_regs(vcpu->arch.guest_fpregs.fprs);
 	}
+	if (test_fp_ctl(fpc))
+		/* User space provided an invalid FPC, let's clear it */
+		fpc = 0;
+	restore_fp_ctl(&fpc);
 	restore_access_regs(vcpu->run->s.regs.acrs);
 	gmap_enable(vcpu->arch.gmap);
 	atomic_set_mask(CPUSTAT_RUNNING, &vcpu->arch.sie_block->cpuflags);