mpls: fix possible use after free of device

The mpls device is used in an RCU read context without a lock being
held. As the memory is freed without waiting for the RCU grace period
to elapse, the freed memory could still be in use.

Address this by using kfree_rcu to free the memory for the mpls device
after the RCU grace period has elapsed.

Fixes: 03c57747a702 ("mpls: Per-device MPLS state")
Signed-off-by: Robert Shearman <rshearma@brocade.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mpls/af_mpls.c

@@ -541,7 +541,7 @@ static void mpls_ifdown(struct net_device *dev)
 
 	RCU_INIT_POINTER(dev->mpls_ptr, NULL);
 
-	kfree(mdev);
+	kfree_rcu(mdev, rcu);
 }
 
 static int mpls_dev_notify(struct notifier_block *this, unsigned long event,

net/mpls/internal.h

@@ -16,6 +16,7 @@ struct mpls_dev {
 	int			input_enabled;
 
 	struct ctl_table_header *sysctl;
+	struct rcu_head		rcu;
 };
 
 struct sk_buff;