[NEIGH]: Fix race between neighbor lookup and table's hash_rnd update.

The neigh_hash_grow() may update the tbl->hash_rnd value, which 
is used in all tbl->hash callbacks to calculate the hashval.

Two lookup routines may race with this, since they call the 
->hash callback without the tbl->lock held. Since the hash_rnd
is changed with this lock write-locked moving the calls to ->hash
under this lock read-locked closes this gap.
Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/neighbour.c

@@ -358,11 +358,12 @@ struct neighbour *neigh_lookup(struct neigh_table *tbl, const void *pkey,
 {
 	struct neighbour *n;
 	int key_len = tbl->key_len;
-	u32 hash_val = tbl->hash(pkey, dev);
+	u32 hash_val;
 
 	NEIGH_CACHE_STAT_INC(tbl, lookups);
 
 	read_lock_bh(&tbl->lock);
+	hash_val = tbl->hash(pkey, dev);
 	for (n = tbl->hash_buckets[hash_val & tbl->hash_mask]; n; n = n->next) {
 		if (dev == n->dev && !memcmp(n->primary_key, pkey, key_len)) {
 			neigh_hold(n);
@@ -379,11 +380,12 @@ struct neighbour *neigh_lookup_nodev(struct neigh_table *tbl, struct net *net,
 {
 	struct neighbour *n;
 	int key_len = tbl->key_len;
-	u32 hash_val = tbl->hash(pkey, NULL);
+	u32 hash_val;
 
 	NEIGH_CACHE_STAT_INC(tbl, lookups);
 
 	read_lock_bh(&tbl->lock);
+	hash_val = tbl->hash(pkey, NULL);
 	for (n = tbl->hash_buckets[hash_val & tbl->hash_mask]; n; n = n->next) {
 		if (!memcmp(n->primary_key, pkey, key_len) &&
 		    (net == n->dev->nd_net)) {