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 670dc2833d14 ("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>

1 files changed, 17 insertions, 9 deletions

diff --git a/net/batman-adv/bat_v.c b/net/batman-adv/bat_v.c
index 6baec4e68898..90e33f84d37a 100644
--- a/net/batman-adv/bat_v.c
+++ b/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;
 }