packet: race condition in packet_bind

There is a race conditions between packet_notifier and packet_bind{_spkt}.

It happens if packet_notifier(NETDEV_UNREGISTER) executes between the
time packet_bind{_spkt} takes a reference on the new netdevice and the
time packet_do_bind sets po->ifindex.
In this case the notification can be missed.
If this happens during a dev_change_net_namespace this can result in the
netdevice to be moved to the new namespace while the packet_sock in the
old namespace still holds a reference on it. When the netdevice is later
deleted in the new namespace the deletion hangs since the packet_sock
is not found in the new namespace' &net->packet.sklist.
It can be reproduced with the script below.

This patch makes packet_do_bind check again for the presence of the
netdevice in the packet_sock's namespace after the synchronize_net
in unregister_prot_hook.
More in general it also uses the rcu lock for the duration of the bind
to stop dev_change_net_namespace/rollback_registered_many from
going past the synchronize_net following unlist_netdevice, so that
no NETDEV_UNREGISTER notifications can happen on the new netdevice
while the bind is executing. In order to do this some code from
packet_bind{_spkt} is consolidated into packet_do_dev.

import socket, os, time, sys
proto=7
realDev='em1'
vlanId=400
if len(sys.argv) > 1:
   vlanId=int(sys.argv[1])
dev='vlan%d' % vlanId

os.system('taskset -p 0x10 %d' % os.getpid())

s = socket.socket(socket.PF_PACKET, socket.SOCK_RAW, proto)
os.system('ip link add link %s name %s type vlan id %d' %
          (realDev, dev, vlanId))
os.system('ip netns add dummy')

pid=os.fork()

if pid == 0:
   # dev should be moved while packet_do_bind is in synchronize net
   os.system('taskset -p 0x20000 %d' % os.getpid())
   os.system('ip link set %s netns dummy' % dev)
   os.system('ip netns exec dummy ip link del %s' % dev)
   s.close()
   sys.exit(0)

time.sleep(.004)
try:
   s.bind(('%s' % dev, proto+1))
except:
   print 'Could not bind socket'
   s.close()
   os.system('ip netns del dummy')
   sys.exit(0)

os.waitpid(pid, 0)
s.close()
os.system('ip netns del dummy')
sys.exit(0)
Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -2911,22 +2911,40 @@ static int packet_release(struct socket *sock)
  *	Attach a packet hook.
  */
 
-static int packet_do_bind(struct sock *sk, struct net_device *dev, __be16 proto)
+static int packet_do_bind(struct sock *sk, const char *name, int ifindex,
+			  __be16 proto)
 {
 	struct packet_sock *po = pkt_sk(sk);
 	struct net_device *dev_curr;
 	__be16 proto_curr;
 	bool need_rehook;
+	struct net_device *dev = NULL;
+	int ret = 0;
+	bool unlisted = false;
 
-	if (po->fanout) {
-		if (dev)
-			dev_put(dev);
-
+	if (po->fanout)
 		return -EINVAL;
-	}
 
 	lock_sock(sk);
 	spin_lock(&po->bind_lock);
+	rcu_read_lock();
+
+	if (name) {
+		dev = dev_get_by_name_rcu(sock_net(sk), name);
+		if (!dev) {
+			ret = -ENODEV;
+			goto out_unlock;
+		}
+	} else if (ifindex) {
+		dev = dev_get_by_index_rcu(sock_net(sk), ifindex);
+		if (!dev) {
+			ret = -ENODEV;
+			goto out_unlock;
+		}
+	}
+
+	if (dev)
+		dev_hold(dev);
 
 	proto_curr = po->prot_hook.type;
 	dev_curr = po->prot_hook.dev;
@@ -2934,14 +2952,29 @@ static int packet_do_bind(struct sock *sk, struct net_device *dev, __be16 proto)
 	need_rehook = proto_curr != proto || dev_curr != dev;
 
 	if (need_rehook) {
-		unregister_prot_hook(sk, true);
+		if (po->running) {
+			rcu_read_unlock();
+			__unregister_prot_hook(sk, true);
+			rcu_read_lock();
+			dev_curr = po->prot_hook.dev;
+			if (dev)
+				unlisted = !dev_get_by_index_rcu(sock_net(sk),
+								 dev->ifindex);
+		}
 
 		po->num = proto;
 		po->prot_hook.type = proto;
-		po->prot_hook.dev = dev;
 
-		po->ifindex = dev ? dev->ifindex : 0;
-		packet_cached_dev_assign(po, dev);
+		if (unlikely(unlisted)) {
+			dev_put(dev);
+			po->prot_hook.dev = NULL;
+			po->ifindex = -1;
+			packet_cached_dev_reset(po);
+		} else {
+			po->prot_hook.dev = dev;
+			po->ifindex = dev ? dev->ifindex : 0;
+			packet_cached_dev_assign(po, dev);
+		}
 	}
 	if (dev_curr)
 		dev_put(dev_curr);
@@ -2949,7 +2982,7 @@ static int packet_do_bind(struct sock *sk, struct net_device *dev, __be16 proto)
 	if (proto == 0 || !need_rehook)
 		goto out_unlock;
 
-	if (!dev || (dev->flags & IFF_UP)) {
+	if (!unlisted && (!dev || (dev->flags & IFF_UP))) {
 		register_prot_hook(sk);
 	} else {
 		sk->sk_err = ENETDOWN;
@@ -2958,9 +2991,10 @@ static int packet_do_bind(struct sock *sk, struct net_device *dev, __be16 proto)
 	}
 
 out_unlock:
+	rcu_read_unlock();
 	spin_unlock(&po->bind_lock);
 	release_sock(sk);
-	return 0;
+	return ret;
 }
 
 /*
@@ -2972,8 +3006,6 @@ static int packet_bind_spkt(struct socket *sock, struct sockaddr *uaddr,
 {
 	struct sock *sk = sock->sk;
 	char name[15];
-	struct net_device *dev;
-	int err = -ENODEV;
 
 	/*
 	 *	Check legality
@@ -2983,19 +3015,13 @@ static int packet_bind_spkt(struct socket *sock, struct sockaddr *uaddr,
 		return -EINVAL;
 	strlcpy(name, uaddr->sa_data, sizeof(name));
 
-	dev = dev_get_by_name(sock_net(sk), name);
-	if (dev)
-		err = packet_do_bind(sk, dev, pkt_sk(sk)->num);
-	return err;
+	return packet_do_bind(sk, name, 0, pkt_sk(sk)->num);
 }
 
 static int packet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 {
 	struct sockaddr_ll *sll = (struct sockaddr_ll *)uaddr;
 	struct sock *sk = sock->sk;
-	struct net_device *dev = NULL;
-	int err;
-
 
 	/*
 	 *	Check legality
@@ -3006,16 +3032,8 @@ static int packet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len
 	if (sll->sll_family != AF_PACKET)
 		return -EINVAL;
 
-	if (sll->sll_ifindex) {
-		err = -ENODEV;
-		dev = dev_get_by_index(sock_net(sk), sll->sll_ifindex);
-		if (dev == NULL)
-			goto out;
-	}
-	err = packet_do_bind(sk, dev, sll->sll_protocol ? : pkt_sk(sk)->num);
-
-out:
-	return err;
+	return packet_do_bind(sk, NULL, sll->sll_ifindex,
+			      sll->sll_protocol ? : pkt_sk(sk)->num);
 }
 
 static struct proto packet_proto = {