batman-adv: Add inconsistent gateway netlink dump detection

The netlink dump functionality transfers a large number of entries from the
kernel to userspace. It is rather likely that the transfer has to
interrupted and later continued. During that time, it can happen that
either new entries are added or removed. The userspace could than either
receive some entries multiple times or miss entries.

Commit 670dc283 ("netlink: advertise incomplete dumps") introduced a
mechanism to inform userspace about this problem. Userspace can then decide
whether it is necessary or not to retry dumping the information again.

The netlink dump functions have to be switched to exclusive locks to avoid
changes while the current message is prepared. And an external generation
sequence counter is introduced which tracks all modifications of the list.
Reported-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/bat_iv_ogm.c

@@ -2584,13 +2584,14 @@ static void batadv_iv_gw_print(struct batadv_priv *bat_priv,
  * batadv_iv_gw_dump_entry() - Dump a gateway into a message
  * @msg: Netlink message to dump into
  * @portid: Port making netlink request
- * @seq: Sequence number of netlink message
+ * @cb: Control block containing additional options
  * @bat_priv: The bat priv with all the soft interface information
  * @gw_node: Gateway to be dumped
  *
  * Return: Error code, or 0 on success
  */
-static int batadv_iv_gw_dump_entry(struct sk_buff *msg, u32 portid, u32 seq,
+static int batadv_iv_gw_dump_entry(struct sk_buff *msg, u32 portid,
+				   struct netlink_callback *cb,
 				   struct batadv_priv *bat_priv,
 				   struct batadv_gw_node *gw_node)
 {
@@ -2610,13 +2611,16 @@ static int batadv_iv_gw_dump_entry(struct sk_buff *msg, u32 portid, u32 seq,
 
 	curr_gw = batadv_gw_get_selected_gw_node(bat_priv);
 
-	hdr = genlmsg_put(msg, portid, seq, &batadv_netlink_family,
-			  NLM_F_MULTI, BATADV_CMD_GET_GATEWAYS);
+	hdr = genlmsg_put(msg, portid, cb->nlh->nlmsg_seq,
+			  &batadv_netlink_family, NLM_F_MULTI,
+			  BATADV_CMD_GET_GATEWAYS);
 	if (!hdr) {
 		ret = -ENOBUFS;
 		goto out;
 	}
 
+	genl_dump_check_consistent(cb, hdr);
+
 	ret = -EMSGSIZE;
 
 	if (curr_gw == gw_node)
@@ -2667,13 +2671,15 @@ static void batadv_iv_gw_dump(struct sk_buff *msg, struct netlink_callback *cb,
 	int idx_skip = cb->args[0];
 	int idx = 0;
 
-	rcu_read_lock();
-	hlist_for_each_entry_rcu(gw_node, &bat_priv->gw.gateway_list, list) {
+	spin_lock_bh(&bat_priv->gw.list_lock);
+	cb->seq = bat_priv->gw.generation << 1 | 1;
+
+	hlist_for_each_entry(gw_node, &bat_priv->gw.gateway_list, list) {
 		if (idx++ < idx_skip)
 			continue;
 
-		if (batadv_iv_gw_dump_entry(msg, portid, cb->nlh->nlmsg_seq,
-					    bat_priv, gw_node)) {
+		if (batadv_iv_gw_dump_entry(msg, portid, cb, bat_priv,
+					    gw_node)) {
 			idx_skip = idx - 1;
 			goto unlock;
 		}
@@ -2681,7 +2687,7 @@ static void batadv_iv_gw_dump(struct sk_buff *msg, struct netlink_callback *cb,
 
 	idx_skip = idx;
 unlock:
-	rcu_read_unlock();
+	spin_unlock_bh(&bat_priv->gw.list_lock);
 
 	cb->args[0] = idx_skip;
 }

net/batman-adv/bat_v.c

@@ -27,11 +27,13 @@
 #include <linux/jiffies.h>
 #include <linux/kernel.h>
 #include <linux/kref.h>
+#include <linux/list.h>
 #include <linux/netdevice.h>
 #include <linux/netlink.h>
 #include <linux/rculist.h>
 #include <linux/rcupdate.h>
 #include <linux/seq_file.h>
+#include <linux/spinlock.h>
 #include <linux/stddef.h>
 #include <linux/types.h>
 #include <linux/workqueue.h>
@@ -915,13 +917,14 @@ static void batadv_v_gw_print(struct batadv_priv *bat_priv,
  * batadv_v_gw_dump_entry() - Dump a gateway into a message
  * @msg: Netlink message to dump into
  * @portid: Port making netlink request
- * @seq: Sequence number of netlink message
+ * @cb: Control block containing additional options
  * @bat_priv: The bat priv with all the soft interface information
  * @gw_node: Gateway to be dumped
  *
  * Return: Error code, or 0 on success
  */
-static int batadv_v_gw_dump_entry(struct sk_buff *msg, u32 portid, u32 seq,
+static int batadv_v_gw_dump_entry(struct sk_buff *msg, u32 portid,
+				  struct netlink_callback *cb,
 				  struct batadv_priv *bat_priv,
 				  struct batadv_gw_node *gw_node)
 {
@@ -941,13 +944,16 @@ static int batadv_v_gw_dump_entry(struct sk_buff *msg, u32 portid, u32 seq,
 
 	curr_gw = batadv_gw_get_selected_gw_node(bat_priv);
 
-	hdr = genlmsg_put(msg, portid, seq, &batadv_netlink_family,
-			  NLM_F_MULTI, BATADV_CMD_GET_GATEWAYS);
+	hdr = genlmsg_put(msg, portid, cb->nlh->nlmsg_seq,
+			  &batadv_netlink_family, NLM_F_MULTI,
+			  BATADV_CMD_GET_GATEWAYS);
 	if (!hdr) {
 		ret = -ENOBUFS;
 		goto out;
 	}
 
+	genl_dump_check_consistent(cb, hdr);
+
 	ret = -EMSGSIZE;
 
 	if (curr_gw == gw_node) {
@@ -1018,13 +1024,15 @@ static void batadv_v_gw_dump(struct sk_buff *msg, struct netlink_callback *cb,
 	int idx_skip = cb->args[0];
 	int idx = 0;
 
-	rcu_read_lock();
-	hlist_for_each_entry_rcu(gw_node, &bat_priv->gw.gateway_list, list) {
+	spin_lock_bh(&bat_priv->gw.list_lock);
+	cb->seq = bat_priv->gw.generation << 1 | 1;
+
+	hlist_for_each_entry(gw_node, &bat_priv->gw.gateway_list, list) {
 		if (idx++ < idx_skip)
 			continue;
 
-		if (batadv_v_gw_dump_entry(msg, portid, cb->nlh->nlmsg_seq,
-					   bat_priv, gw_node)) {
+		if (batadv_v_gw_dump_entry(msg, portid, cb, bat_priv,
+					   gw_node)) {
 			idx_skip = idx - 1;
 			goto unlock;
 		}
@@ -1032,7 +1040,7 @@ static void batadv_v_gw_dump(struct sk_buff *msg, struct netlink_callback *cb,
 
 	idx_skip = idx;
 unlock:
-	rcu_read_unlock();
+	spin_unlock_bh(&bat_priv->gw.list_lock);
 
 	cb->args[0] = idx_skip;
 }

net/batman-adv/gateway_client.c

@@ -377,6 +377,7 @@ static void batadv_gw_node_add(struct batadv_priv *bat_priv,
 
 	kref_get(&gw_node->refcount);
 	hlist_add_head_rcu(&gw_node->list, &bat_priv->gw.gateway_list);
+	bat_priv->gw.generation++;
 
 	batadv_dbg(BATADV_DBG_BATMAN, bat_priv,
 		   "Found new gateway %pM -> gw bandwidth: %u.%u/%u.%u MBit\n",
@@ -472,6 +473,7 @@ void batadv_gw_node_update(struct batadv_priv *bat_priv,
 		if (!hlist_unhashed(&gw_node->list)) {
 			hlist_del_init_rcu(&gw_node->list);
 			batadv_gw_node_put(gw_node);
+			bat_priv->gw.generation++;
 		}
 		spin_unlock_bh(&bat_priv->gw.list_lock);
 
@@ -518,6 +520,7 @@ void batadv_gw_node_free(struct batadv_priv *bat_priv)
 				  &bat_priv->gw.gateway_list, list) {
 		hlist_del_init_rcu(&gw_node->list);
 		batadv_gw_node_put(gw_node);
+		bat_priv->gw.generation++;
 	}
 	spin_unlock_bh(&bat_priv->gw.list_lock);
 }

net/batman-adv/main.c

@@ -186,6 +186,8 @@ int batadv_mesh_init(struct net_device *soft_iface)
 	INIT_HLIST_HEAD(&bat_priv->softif_vlan_list);
 	INIT_HLIST_HEAD(&bat_priv->tp_list);
 
+	bat_priv->gw.generation = 0;
+
 	ret = batadv_v_mesh_init(bat_priv);
 	if (ret < 0)
 		goto err;

net/batman-adv/types.h

@@ -1096,12 +1096,15 @@ struct batadv_priv_gw {
 	/** @gateway_list: list of available gateway nodes */
 	struct hlist_head gateway_list;
 
-	/** @list_lock: lock protecting gateway_list & curr_gw */
+	/** @list_lock: lock protecting gateway_list, curr_gw, generation */
 	spinlock_t list_lock;
 
 	/** @curr_gw: pointer to currently selected gateway node */
 	struct batadv_gw_node __rcu *curr_gw;
 
+	/** @generation: current (generation) sequence number */
+	unsigned int generation;
+
 	/**
 	 * @mode: gateway operation: off, client or server (see batadv_gw_modes)
 	 */