veth: avoid a NULL deref in veth_stats_one

commit 2681128f0ced8a (veth: extend device features) added a NULL deref
in veth_stats_one(), as veth_get_stats64() was not testing if the peer
device was setup or not.

At init time, we call dev_get_stats() before veth pair is fully setup.

[  178.854758]  [<ffffffffa00f5677>] veth_get_stats64+0x47/0x70 [veth]
[  178.861013]  [<ffffffff814f0a2d>] dev_get_stats+0x6d/0x130
[  178.866486]  [<ffffffff81504efc>] rtnl_fill_ifinfo+0x47c/0x930
[  178.872299]  [<ffffffff81505b93>] rtmsg_ifinfo+0x83/0x100
[  178.877678]  [<ffffffff81505cc6>] rtnl_configure_link+0x76/0xa0
[  178.883580]  [<ffffffffa00f52fa>] veth_newlink+0x16a/0x350 [veth]
[  178.889654]  [<ffffffff815061cc>] rtnl_newlink+0x4dc/0x5e0
[  178.895128]  [<ffffffff81505e1e>] ? rtnl_newlink+0x12e/0x5e0
[  178.900769]  [<ffffffff8150587d>] rtnetlink_rcv_msg+0x11d/0x310
[  178.906669]  [<ffffffff81505760>] ? __rtnl_unlock+0x20/0x20
[  178.912225]  [<ffffffff81521f89>] netlink_rcv_skb+0xa9/0xd0
[  178.917779]  [<ffffffff81502d55>] rtnetlink_rcv+0x25/0x40
[  178.923159]  [<ffffffff815218d1>] netlink_unicast+0x1b1/0x230
[  178.928887]  [<ffffffff81521c4e>] netlink_sendmsg+0x2fe/0x3b0
[  178.934615]  [<ffffffff814dbe22>] sock_sendmsg+0xd2/0xf0

So we must check if peer was setup in veth_get_stats64()

As pointed out by Ben Hutchings, priv->peer is missing proper
synchronization. Adding RCU protection is a safe and well documented
way to make sure we don't access about to be freed or already
freed data.

Reported-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
CC: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 8b2e112..0f71a4f 100644 (file)
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -32,7 +32,7 @@ struct pcpu_vstats {
 };
 
 struct veth_priv {
-       struct net_device       *peer;
+       struct net_device __rcu *peer;
        atomic64_t              dropped;
 };
 
@@ -89,10 +89,10 @@ static int veth_get_sset_count(struct net_device *dev, int sset)
 static void veth_get_ethtool_stats(struct net_device *dev,
                struct ethtool_stats *stats, u64 *data)
 {
-       struct veth_priv *priv;
+       struct veth_priv *priv = netdev_priv(dev);
+       struct net_device *peer = rtnl_dereference(priv->peer);
 
-       priv = netdev_priv(dev);
-       data[0] = priv->peer->ifindex;
+       data[0] = peer ? peer->ifindex : 0;
 }
 
 static const struct ethtool_ops veth_ethtool_ops = {
@@ -107,9 +107,15 @@ static const struct ethtool_ops veth_ethtool_ops = {
 static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
 {
        struct veth_priv *priv = netdev_priv(dev);
-       struct net_device *rcv = priv->peer;
+       struct net_device *rcv;
        int length = skb->len;
 
+       rcu_read_lock();
+       rcv = rcu_dereference(priv->peer);
+       if (unlikely(!rcv)) {
+               kfree_skb(skb);
+               goto drop;
+       }
        /* don't change ip_summed == CHECKSUM_PARTIAL, as that
         * will cause bad checksum on forwarded packets
         */
@@ -125,9 +131,10 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
                stats->packets++;
                u64_stats_update_end(&stats->syncp);
        } else {
+drop:
                atomic64_inc(&priv->dropped);
        }
-
+       rcu_read_unlock();
        return NETDEV_TX_OK;
 }
 
@@ -162,30 +169,36 @@ static struct rtnl_link_stats64 *veth_get_stats64(struct net_device *dev,
                                                  struct rtnl_link_stats64 *tot)
 {
        struct veth_priv *priv = netdev_priv(dev);
+       struct net_device *peer;
        struct pcpu_vstats one;
 
        tot->tx_dropped = veth_stats_one(&one, dev);
        tot->tx_bytes = one.bytes;
        tot->tx_packets = one.packets;
 
-       tot->rx_dropped = veth_stats_one(&one, priv->peer);
-       tot->rx_bytes = one.bytes;
-       tot->rx_packets = one.packets;
+       rcu_read_lock();
+       peer = rcu_dereference(priv->peer);
+       if (peer) {
+               tot->rx_dropped = veth_stats_one(&one, peer);
+               tot->rx_bytes = one.bytes;
+               tot->rx_packets = one.packets;
+       }
+       rcu_read_unlock();
 
        return tot;
 }
 
 static int veth_open(struct net_device *dev)
 {
-       struct veth_priv *priv;
+       struct veth_priv *priv = netdev_priv(dev);
+       struct net_device *peer = rtnl_dereference(priv->peer);
 
-       priv = netdev_priv(dev);
-       if (priv->peer == NULL)
+       if (!peer)
                return -ENOTCONN;
 
-       if (priv->peer->flags & IFF_UP) {
+       if (peer->flags & IFF_UP) {
                netif_carrier_on(dev);
-               netif_carrier_on(priv->peer);
+               netif_carrier_on(peer);
        }
        return 0;
 }
@@ -195,7 +208,7 @@ static int veth_close(struct net_device *dev)
        struct veth_priv *priv = netdev_priv(dev);
 
        netif_carrier_off(dev);
-       netif_carrier_off(priv->peer);
+       netif_carrier_off(rtnl_dereference(priv->peer));
 
        return 0;
 }
@@ -380,10 +393,10 @@ static int veth_newlink(struct net *src_net, struct net_device *dev,
         */
 
        priv = netdev_priv(dev);
-       priv->peer = peer;
+       rcu_assign_pointer(priv->peer, peer);
 
        priv = netdev_priv(peer);
-       priv->peer = dev;
+       rcu_assign_pointer(priv->peer, dev);
        return 0;
 
 err_register_dev:
@@ -404,7 +417,16 @@ static void veth_dellink(struct net_device *dev, struct list_head *head)
        struct net_device *peer;
 
        priv = netdev_priv(dev);
-       peer = priv->peer;
+       peer = rtnl_dereference(priv->peer);
+
+       /* Note : dellink() is called from default_device_exit_batch(),
+        * before a rcu_synchronize() point. The devices are guaranteed
+        * not being freed before one RCU grace period.
+        */
+       RCU_INIT_POINTER(priv->peer, NULL);
+
+       priv = netdev_priv(peer);
+       RCU_INIT_POINTER(priv->peer, NULL);
 
        unregister_netdevice_queue(dev, head);
        unregister_netdevice_queue(peer, head);