SMACK: Add the rcu synchronization mechanism in ipv6 hooks

Add the rcu synchronization mechanism for accessing smk_ipv6_port_list
in smack IPv6 hooks. Access to the port list is vulnerable to a race
condition issue,it does not apply proper synchronization methods while
working on critical section. It is possible that when one thread is
reading the list, at the same time another thread is modifying the
same port list, which can cause the major problems.

To ensure proper synchronization between two threads, rcu mechanism
has been applied while accessing and modifying the port list. RCU will
also not affect the performance, as there are more accesses than
modification where RCU is most effective synchronization mechanism.
Signed-off-by: Vishal Goel <vishal.goel@samsung.com>
Signed-off-by: Himanshu Shukla <himanshu.sh@samsung.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smack_lsm.c

@@ -52,6 +52,7 @@
 #define SMK_SENDING	2
 
 #ifdef SMACK_IPV6_PORT_LABELING
+DEFINE_MUTEX(smack_ipv6_lock);
 static LIST_HEAD(smk_ipv6_port_list);
 #endif
 static struct kmem_cache *smack_inode_cache;
@@ -2603,17 +2604,20 @@ static void smk_ipv6_port_label(struct socket *sock, struct sockaddr *address)
 		 * on the bound socket. Take the changes to the port
 		 * as well.
 		 */
-		list_for_each_entry(spp, &smk_ipv6_port_list, list) {
+		rcu_read_lock();
+		list_for_each_entry_rcu(spp, &smk_ipv6_port_list, list) {
 			if (sk != spp->smk_sock)
 				continue;
 			spp->smk_in = ssp->smk_in;
 			spp->smk_out = ssp->smk_out;
+			rcu_read_unlock();
 			return;
 		}
 		/*
 		 * A NULL address is only used for updating existing
 		 * bound entries. If there isn't one, it's OK.
 		 */
+		rcu_read_unlock();
 		return;
 	}
 
@@ -2629,16 +2633,18 @@ static void smk_ipv6_port_label(struct socket *sock, struct sockaddr *address)
 	 * Look for an existing port list entry.
 	 * This is an indication that a port is getting reused.
 	 */
-	list_for_each_entry(spp, &smk_ipv6_port_list, list) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(spp, &smk_ipv6_port_list, list) {
 		if (spp->smk_port != port)
 			continue;
 		spp->smk_port = port;
 		spp->smk_sock = sk;
 		spp->smk_in = ssp->smk_in;
 		spp->smk_out = ssp->smk_out;
+		rcu_read_unlock();
 		return;
 	}
-
+	rcu_read_unlock();
 	/*
 	 * A new port entry is required.
 	 */
@@ -2651,7 +2657,9 @@ static void smk_ipv6_port_label(struct socket *sock, struct sockaddr *address)
 	spp->smk_in = ssp->smk_in;
 	spp->smk_out = ssp->smk_out;
 
-	list_add(&spp->list, &smk_ipv6_port_list);
+	mutex_lock(&smack_ipv6_lock);
+	list_add_rcu(&spp->list, &smk_ipv6_port_list);
+	mutex_unlock(&smack_ipv6_lock);
 	return;
 }
 
@@ -2702,7 +2710,8 @@ static int smk_ipv6_port_check(struct sock *sk, struct sockaddr_in6 *address,
 		return 0;
 
 	port = ntohs(address->sin6_port);
-	list_for_each_entry(spp, &smk_ipv6_port_list, list) {
+	rcu_read_lock();
+	list_for_each_entry_rcu(spp, &smk_ipv6_port_list, list) {
 		if (spp->smk_port != port)
 			continue;
 		object = spp->smk_in;
@@ -2710,6 +2719,7 @@ static int smk_ipv6_port_check(struct sock *sk, struct sockaddr_in6 *address,
 			ssp->smk_packet = spp->smk_out;
 		break;
 	}
+	rcu_read_unlock();
 
 	return smk_ipv6_check(skp, object, address, act);
 }