net: netlink: virtual tap device management

Similarly to the networking receive path with ptype_all taps, we add
the possibility to register netdevices that are for ARPHRD_NETLINK to
the netlink subsystem, so that those can be used for netlink analyzers
resp. debuggers. We do not offer a direct callback function as out-of-tree
modules could do crap with it. Instead, a netdevice must be registered
properly and only receives a clone, managed by the netlink layer. Symbols
are exported as GPL-only.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index f78b430..86fde81 100644 (file)
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -145,4 +145,14 @@ static inline int netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
        return __netlink_dump_start(ssk, skb, nlh, control);
 }
 
+struct netlink_tap {
+       struct net_device *dev;
+       struct module *module;
+       struct list_head list;
+};
+
+extern int netlink_add_tap(struct netlink_tap *nt);
+extern int __netlink_remove_tap(struct netlink_tap *nt);
+extern int netlink_remove_tap(struct netlink_tap *nt);
+
 #endif /* __LINUX_NETLINK_H */

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 275d901..6967fbc 100644 (file)
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -57,6 +57,7 @@
 #include <linux/audit.h>
 #include <linux/mutex.h>
 #include <linux/vmalloc.h>
+#include <linux/if_arp.h>
 #include <asm/cacheflush.h>
 
 #include <net/net_namespace.h>
@@ -101,6 +102,9 @@ static atomic_t nl_table_users = ATOMIC_INIT(0);
 
 static ATOMIC_NOTIFIER_HEAD(netlink_chain);
 
+static DEFINE_SPINLOCK(netlink_tap_lock);
+static struct list_head netlink_tap_all __read_mostly;
+
 static inline u32 netlink_group_mask(u32 group)
 {
        return group ? 1 << (group - 1) : 0;
@@ -111,6 +115,100 @@ static inline struct hlist_head *nl_portid_hashfn(struct nl_portid_hash *hash, u
        return &hash->table[jhash_1word(portid, hash->rnd) & hash->mask];
 }
 
+int netlink_add_tap(struct netlink_tap *nt)
+{
+       if (unlikely(nt->dev->type != ARPHRD_NETLINK))
+               return -EINVAL;
+
+       spin_lock(&netlink_tap_lock);
+       list_add_rcu(&nt->list, &netlink_tap_all);
+       spin_unlock(&netlink_tap_lock);
+
+       if (nt->module)
+               __module_get(nt->module);
+
+       return 0;
+}
+EXPORT_SYMBOL_GPL(netlink_add_tap);
+
+int __netlink_remove_tap(struct netlink_tap *nt)
+{
+       bool found = false;
+       struct netlink_tap *tmp;
+
+       spin_lock(&netlink_tap_lock);
+
+       list_for_each_entry(tmp, &netlink_tap_all, list) {
+               if (nt == tmp) {
+                       list_del_rcu(&nt->list);
+                       found = true;
+                       goto out;
+               }
+       }
+
+       pr_warn("__netlink_remove_tap: %p not found\n", nt);
+out:
+       spin_unlock(&netlink_tap_lock);
+
+       if (found && nt->module)
+               module_put(nt->module);
+
+       return found ? 0 : -ENODEV;
+}
+EXPORT_SYMBOL_GPL(__netlink_remove_tap);
+
+int netlink_remove_tap(struct netlink_tap *nt)
+{
+       int ret;
+
+       ret = __netlink_remove_tap(nt);
+       synchronize_net();
+
+       return ret;
+}
+EXPORT_SYMBOL_GPL(netlink_remove_tap);
+
+static int __netlink_deliver_tap_skb(struct sk_buff *skb,
+                                    struct net_device *dev)
+{
+       struct sk_buff *nskb;
+       int ret = -ENOMEM;
+
+       dev_hold(dev);
+       nskb = skb_clone(skb, GFP_ATOMIC);
+       if (nskb) {
+               nskb->dev = dev;
+               ret = dev_queue_xmit(nskb);
+               if (unlikely(ret > 0))
+                       ret = net_xmit_errno(ret);
+       }
+
+       dev_put(dev);
+       return ret;
+}
+
+static void __netlink_deliver_tap(struct sk_buff *skb)
+{
+       int ret;
+       struct netlink_tap *tmp;
+
+       list_for_each_entry_rcu(tmp, &netlink_tap_all, list) {
+               ret = __netlink_deliver_tap_skb(skb, tmp->dev);
+               if (unlikely(ret))
+                       break;
+       }
+}
+
+static void netlink_deliver_tap(struct sk_buff *skb)
+{
+       rcu_read_lock();
+
+       if (unlikely(!list_empty(&netlink_tap_all)))
+               __netlink_deliver_tap(skb);
+
+       rcu_read_unlock();
+}
+
 static void netlink_overrun(struct sock *sk)
 {
        struct netlink_sock *nlk = nlk_sk(sk);
@@ -1518,6 +1616,8 @@ static int __netlink_sendskb(struct sock *sk, struct sk_buff *skb)
 {
        int len = skb->len;
 
+       netlink_deliver_tap(skb);
+
 #ifdef CONFIG_NETLINK_MMAP
        if (netlink_skb_is_mmaped(skb))
                netlink_queue_mmaped_skb(sk, skb);
@@ -1578,6 +1678,11 @@ static int netlink_unicast_kernel(struct sock *sk, struct sk_buff *skb,
 
        ret = -ECONNREFUSED;
        if (nlk->netlink_rcv != NULL) {
+               /* We could do a netlink_deliver_tap(skb) here as well
+                * but since this is intended for the kernel only, we
+                * should rather let it stay under the hood.
+                */
+
                ret = skb->len;
                netlink_skb_set_owner_r(skb, sk);
                NETLINK_CB(skb).sk = ssk;
@@ -2975,6 +3080,8 @@ static int __init netlink_proto_init(void)
                nl_table[i].compare = netlink_compare;
        }
 
+       INIT_LIST_HEAD(&netlink_tap_all);
+
        netlink_add_usersock_entry();
 
        sock_register(&netlink_family_ops);