Audit: send signal info if selinux is disabled

Audit will not respond to signal requests if selinux is disabled since it is
unable to translate the 0 sid from the sending process to a context.  This
patch just doesn't send the context info if there isn't any.
Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/audit.c

@@ -855,18 +855,24 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 		break;
 	}
 	case AUDIT_SIGNAL_INFO:
-		err = security_secid_to_secctx(audit_sig_sid, &ctx, &len);
-		if (err)
-			return err;
+		len = 0;
+		if (audit_sig_sid) {
+			err = security_secid_to_secctx(audit_sig_sid, &ctx, &len);
+			if (err)
+				return err;
+		}
 		sig_data = kmalloc(sizeof(*sig_data) + len, GFP_KERNEL);
 		if (!sig_data) {
-			security_release_secctx(ctx, len);
+			if (audit_sig_sid)
+				security_release_secctx(ctx, len);
 			return -ENOMEM;
 		}
 		sig_data->uid = audit_sig_uid;
 		sig_data->pid = audit_sig_pid;
-		memcpy(sig_data->ctx, ctx, len);
-		security_release_secctx(ctx, len);
+		if (audit_sig_sid) {
+			memcpy(sig_data->ctx, ctx, len);
+			security_release_secctx(ctx, len);
+		}
 		audit_send_reply(NETLINK_CB(skb).pid, seq, AUDIT_SIGNAL_INFO,
 				0, 0, sig_data, sizeof(*sig_data) + len);
 		kfree(sig_data);