kvm: Fix nonsense handling of compat ioctl

KVM_SET_SIGNAL_MASK passed a NULL argument leaves the on stack signal
sets uninitialized. It then passes them through to
kvm_vcpu_ioctl_set_sigmask.

We should be passing a NULL in this case not translated garbage.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

virt/kvm/kvm_main.c

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 2468523..d617f69 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1976,9 +1976,10 @@
 			if (copy_from_user(&csigset, sigmask_arg->sigset,
 					   sizeof csigset))
 				goto out;
-		}
-		sigset_from_compat(&sigset, &csigset);
-		r = kvm_vcpu_ioctl_set_sigmask(vcpu, &sigset);
+			sigset_from_compat(&sigset, &csigset);
+			r = kvm_vcpu_ioctl_set_sigmask(vcpu, &sigset);
+		} else
+			r = kvm_vcpu_ioctl_set_sigmask(vcpu, NULL);
 		break;
 	}
 	default: