[NETFILTER]: conntrack: fix race condition in early_drop

On SMP environments the maximum number of conntracks can be overpassed
under heavy stress situations due to an existing race condition.

        CPU A                   CPU B
     atomic_read()               ...
     early_drop()                ...
        ...                  atomic_read()
   allocate conntrack      allocate conntrack
     atomic_inc()             atomic_inc()

This patch moves the counter incrementation before the early drop stage.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/ip_conntrack_core.c

@@ -622,11 +622,15 @@ struct ip_conntrack *ip_conntrack_alloc(struct ip_conntrack_tuple *orig,
 		ip_conntrack_hash_rnd_initted = 1;
 	}
 
+	/* We don't want any race condition at early drop stage */
+	atomic_inc(&ip_conntrack_count);
+
 	if (ip_conntrack_max
-	    && atomic_read(&ip_conntrack_count) >= ip_conntrack_max) {
+	    && atomic_read(&ip_conntrack_count) > ip_conntrack_max) {
 		unsigned int hash = hash_conntrack(orig);
 		/* Try dropping from this hash chain. */
 		if (!early_drop(&ip_conntrack_hash[hash])) {
+			atomic_dec(&ip_conntrack_count);
 			if (net_ratelimit())
 				printk(KERN_WARNING
 				       "ip_conntrack: table full, dropping"
@@ -638,6 +642,7 @@ struct ip_conntrack *ip_conntrack_alloc(struct ip_conntrack_tuple *orig,
 	conntrack = kmem_cache_alloc(ip_conntrack_cachep, GFP_ATOMIC);
 	if (!conntrack) {
 		DEBUGP("Can't allocate conntrack.\n");
+		atomic_dec(&ip_conntrack_count);
 		return ERR_PTR(-ENOMEM);
 	}
 
@@ -651,8 +656,6 @@ struct ip_conntrack *ip_conntrack_alloc(struct ip_conntrack_tuple *orig,
 	conntrack->timeout.data = (unsigned long)conntrack;
 	conntrack->timeout.function = death_by_timeout;
 
-	atomic_inc(&ip_conntrack_count);
-
 	return conntrack;
 }
 

net/netfilter/nf_conntrack_core.c

@@ -848,11 +848,15 @@ __nf_conntrack_alloc(const struct nf_conntrack_tuple *orig,
 		nf_conntrack_hash_rnd_initted = 1;
 	}
 
+	/* We don't want any race condition at early drop stage */
+	atomic_inc(&nf_conntrack_count);
+
 	if (nf_conntrack_max
-	    && atomic_read(&nf_conntrack_count) >= nf_conntrack_max) {
+	    && atomic_read(&nf_conntrack_count) > nf_conntrack_max) {
 		unsigned int hash = hash_conntrack(orig);
 		/* Try dropping from this hash chain. */
 		if (!early_drop(&nf_conntrack_hash[hash])) {
+			atomic_dec(&nf_conntrack_count);
 			if (net_ratelimit())
 				printk(KERN_WARNING
 				       "nf_conntrack: table full, dropping"
@@ -903,10 +907,12 @@ __nf_conntrack_alloc(const struct nf_conntrack_tuple *orig,
 	init_timer(&conntrack->timeout);
 	conntrack->timeout.data = (unsigned long)conntrack;
 	conntrack->timeout.function = death_by_timeout;
+	read_unlock_bh(&nf_ct_cache_lock);
 
-	atomic_inc(&nf_conntrack_count);
+	return conntrack;
 out:
 	read_unlock_bh(&nf_ct_cache_lock);
+	atomic_dec(&nf_conntrack_count);
 	return conntrack;
 }