[SCSI] libiscsi_tcp: fix max_r2t manipulation

Problem description from Xi Wang:
A large max_r2t could lead to integer overflow in subsequent call to
iscsi_tcp_r2tpool_alloc(), allocating a smaller buffer than expected
and leading to out-of-bounds write.

Signed-off-by: Mike Christie <[e-mail]>
Signed-off-by: James Bottomley <[e-mail]>

drivers/scsi/cxgbi/libcxgbi.c

···
 			enum iscsi_param param, char *buf, int buflen)
 {
 	struct iscsi_conn *conn = cls_conn->dd_data;
-	struct iscsi_session *session = conn->session;
 	struct iscsi_tcp_conn *tcp_conn = conn->dd_data;
 	struct cxgbi_conn *cconn = tcp_conn->dd_data;
 	struct cxgbi_sock *csk = cconn->cep->csk;
-	int value, err = 0;
+	int err;
 
 	log_debug(1 << CXGBI_DBG_ISCSI,
 		"cls_conn 0x%p, param %d, buf(%d) %s.\n",
···
 							conn->datadgst_en, 0);
 		break;
 	case ISCSI_PARAM_MAX_R2T:
-		sscanf(buf, "%d", &value);
-		if (value <= 0 || !is_power_of_2(value))
-			return -EINVAL;
-		if (session->max_r2t == value)
-			break;
-		iscsi_tcp_r2tpool_free(session);
-		err = iscsi_set_param(cls_conn, param, buf, buflen);
-		if (!err && iscsi_tcp_r2tpool_alloc(session))
-			return -ENOMEM;
+		return iscsi_tcp_set_max_r2t(conn, buf);
 	case ISCSI_PARAM_MAX_RECV_DLENGTH:
 		err = iscsi_set_param(cls_conn, param, buf, buflen);
 		if (!err)

drivers/scsi/iscsi_tcp.c

···
 				       int buflen)
 {
 	struct iscsi_conn *conn = cls_conn->dd_data;
-	struct iscsi_session *session = conn->session;
 	struct iscsi_tcp_conn *tcp_conn = conn->dd_data;
 	struct iscsi_sw_tcp_conn *tcp_sw_conn = tcp_conn->dd_data;
-	int value;
 
 	switch(param) {
 	case ISCSI_PARAM_HDRDGST_EN:
···
 			sock_no_sendpage : tcp_sw_conn->sock->ops->sendpage;
 		break;
 	case ISCSI_PARAM_MAX_R2T:
-		sscanf(buf, "%d", &value);
-		if (value <= 0 || !is_power_of_2(value))
-			return -EINVAL;
-		if (session->max_r2t == value)
-			break;
-		iscsi_tcp_r2tpool_free(session);
-		iscsi_set_param(cls_conn, param, buf, buflen);
-		if (iscsi_tcp_r2tpool_alloc(session))
-			return -ENOMEM;
-		break;
+		return iscsi_tcp_set_max_r2t(conn, buf);
 	default:
 		return iscsi_set_param(cls_conn, param, buf, buflen);
 	}

drivers/scsi/libiscsi.c

···
 		sscanf(buf, "%d", &session->initial_r2t_en);
 		break;
 	case ISCSI_PARAM_MAX_R2T:
-		sscanf(buf, "%d", &session->max_r2t);
+		sscanf(buf, "%hu", &session->max_r2t);
 		break;
 	case ISCSI_PARAM_IMM_DATA_EN:
 		sscanf(buf, "%d", &session->imm_data_en);

drivers/scsi/libiscsi_tcp.c

···
 }
 EXPORT_SYMBOL_GPL(iscsi_tcp_r2tpool_free);
 
+int iscsi_tcp_set_max_r2t(struct iscsi_conn *conn, char *buf)
+{
+	struct iscsi_session *session = conn->session;
+	unsigned short r2ts = 0;
+
+	sscanf(buf, "%hu", &r2ts);
+	if (session->max_r2t == r2ts)
+		return 0;
+
+	if (!r2ts || !is_power_of_2(r2ts))
+		return -EINVAL;
+
+	session->max_r2t = r2ts;
+	iscsi_tcp_r2tpool_free(session);
+	return iscsi_tcp_r2tpool_alloc(session);
+}
+EXPORT_SYMBOL_GPL(iscsi_tcp_set_max_r2t);
+
 void iscsi_tcp_conn_get_stats(struct iscsi_cls_conn *cls_conn,
 			      struct iscsi_stats *stats)
 {

include/scsi/libiscsi.h

···
 	int			lu_reset_timeout;
 	int			tgt_reset_timeout;
 	int			initial_r2t_en;
-	unsigned		max_r2t;
+	unsigned short		max_r2t;
 	int			imm_data_en;
 	unsigned		first_burst;
 	unsigned		max_burst;

include/scsi/libiscsi_tcp.h

···
 /* misc helpers */
 extern int iscsi_tcp_r2tpool_alloc(struct iscsi_session *session);
 extern void iscsi_tcp_r2tpool_free(struct iscsi_session *session);
-
+extern int iscsi_tcp_set_max_r2t(struct iscsi_conn *conn, char *buf);
 extern void iscsi_tcp_conn_get_stats(struct iscsi_cls_conn *cls_conn,
 				     struct iscsi_stats *stats);
 #endif /* LIBISCSI_TCP_H */