batman-adv: Correct rcu refcounting for neigh_node

It might be possible that 2 threads access the same data in the same
rcu grace period. The first thread calls call_rcu() to decrement the
refcount and free the data while the second thread increases the
refcount to use the data. To avoid this race condition all refcount
operations have to be atomic.

Reported-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>

1 files changed, 20 insertions, 7 deletions

diff --git a/net/batman-adv/icmp_socket.c b/net/batman-adv/icmp_socket.c
index 8e0cd8a..7fa5bb8 100644
--- a/net/batman-adv/icmp_socket.c
+++ b/net/batman-adv/icmp_socket.c
@@ -156,7 +156,8 @@ static ssize_t bat_socket_write(struct file *file, const char __user *buff,
 	struct sk_buff *skb;
 	struct icmp_packet_rr *icmp_packet;
 
-	struct orig_node *orig_node;
+	struct orig_node *orig_node = NULL;
+	struct neigh_node *neigh_node = NULL;
 	struct batman_if *batman_if;
 	size_t packet_len = sizeof(struct icmp_packet);
 	uint8_t dstaddr[ETH_ALEN];
@@ -224,17 +225,25 @@ static ssize_t bat_socket_write(struct file *file, const char __user *buff,
 	orig_node = ((struct orig_node *)hash_find(bat_priv->orig_hash,
 						   compare_orig, choose_orig,
 						   icmp_packet->dst));
-	rcu_read_unlock();
 
 	if (!orig_node)
 		goto unlock;
 
-	if (!orig_node->router)
+	kref_get(&orig_node->refcount);
+	neigh_node = orig_node->router;
+
+	if (!neigh_node)
+		goto unlock;
+
+	if (!atomic_inc_not_zero(&neigh_node->refcount)) {
+		neigh_node = NULL;
 		goto unlock;
+	}
+
+	rcu_read_unlock();
 
 	batman_if = orig_node->router->if_incoming;
 	memcpy(dstaddr, orig_node->router->addr, ETH_ALEN);
-
 	spin_unlock_bh(&bat_priv->orig_hash_lock);
 
 	if (!batman_if)
@@ -247,14 +256,14 @@ static ssize_t bat_socket_write(struct file *file, const char __user *buff,
 	       bat_priv->primary_if->net_dev->dev_addr, ETH_ALEN);
 
 	if (packet_len == sizeof(struct icmp_packet_rr))
-		memcpy(icmp_packet->rr, batman_if->net_dev->dev_addr, ETH_ALEN);
-
+		memcpy(icmp_packet->rr,
+		       batman_if->net_dev->dev_addr, ETH_ALEN);
 
 	send_skb_packet(skb, batman_if, dstaddr);
-
 	goto out;
 
 unlock:
+	rcu_read_unlock();
 	spin_unlock_bh(&bat_priv->orig_hash_lock);
 dst_unreach:
 	icmp_packet->msg_type = DESTINATION_UNREACHABLE;
@@ -262,6 +271,10 @@ dst_unreach:
 free_skb:
 	kfree_skb(skb);
 out:
+	if (neigh_node)
+		neigh_node_free_ref(neigh_node);
+	if (orig_node)
+		kref_put(&orig_node->refcount, orig_node_free_ref);
 	return len;
 }