[PATCH net] net: fix NULL pointer dereference in l3mdev_l3_rcv

[Wang Liang <wangliang74@huawei.com>]

When delete l3s ipvlan:

    ip link del link eth0 ipvlan1 type ipvlan mode l3s

This may cause a null pointer dereference:

    Call trace:
     ip_rcv_finish+0x48/0xd0
     ip_rcv+0x5c/0x100
     __netif_receive_skb_one_core+0x64/0xb0
     __netif_receive_skb+0x20/0x80
     process_backlog+0xb4/0x204
     napi_poll+0xe8/0x294
     net_rx_action+0xd8/0x22c
     __do_softirq+0x12c/0x354

This is because l3mdev_l3_rcv() visit dev->l3mdev_ops after
ipvlan_l3s_unregister() assign the dev->l3mdev_ops to NULL. The process
like this:

    (CPU1)                     | (CPU2)
    l3mdev_l3_rcv()            |
      check dev->priv_flags:   |
        master = skb->dev;     |
                               |
                               | ipvlan_l3s_unregister()
                               |   set dev->priv_flags
                               |   dev->l3mdev_ops = NULL;
                               |
      visit master->l3mdev_ops |

Add lock for dev->priv_flags and dev->l3mdev_ops is too expensive. Resolve
this issue by add check for master->l3mdev_ops.

Fixes: c675e06a98a4 ("ipvlan: decouple l3s mode dependencies from other modes")
Signed-off-by: Wang Liang <wangliang74@huawei.com>
---
 include/net/l3mdev.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/net/l3mdev.h b/include/net/l3mdev.h
index f7fe796e8429..b5af87a35a9f 100644
--- a/include/net/l3mdev.h
+++ b/include/net/l3mdev.h
@@ -165,6 +165,7 @@ static inline
 struct sk_buff *l3mdev_l3_rcv(struct sk_buff *skb, u16 proto)
 {
 	struct net_device *master = NULL;
+	const struct l3mdev_ops *l3mdev_ops;
 
 	if (netif_is_l3_slave(skb->dev))
 		master = netdev_master_upper_dev_get_rcu(skb->dev);
@@ -172,8 +173,12 @@ struct sk_buff *l3mdev_l3_rcv(struct sk_buff *skb, u16 proto)
 		 netif_has_l3_rx_handler(skb->dev))
 		master = skb->dev;
 
-	if (master && master->l3mdev_ops->l3mdev_l3_rcv)
-		skb = master->l3mdev_ops->l3mdev_l3_rcv(master, skb, proto);
+	if (!master)
+		return skb;
+
+	l3mdev_ops = master->l3mdev_ops;
+	if (l3mdev_ops && l3mdev_ops->l3mdev_l3_rcv)
+		skb = l3mdev_ops->l3mdev_l3_rcv(master, skb, proto);
 
 	return skb;
 }
-- 
2.34.1