packet: nlmon: virtual netlink monitoring device for packet sockets

Currently, there is no good possibility to debug netlink traffic that
is being exchanged between kernel and user space. Therefore, this patch
implements a netlink virtual device, so that netlink messages will be
made visible to PF_PACKET sockets. Once there was an approach with a
similar idea [1], but it got forgotten somehow.

I think it makes most sense to accept the "overhead" of an extra netlink
net device over implementing the same functionality from PF_PACKET
sockets once again into netlink sockets. We have BPF filters that can
already be easily applied which even have netlink extensions, we have
RX_RING zero-copy between kernel- and user space that can be reused,
and much more features. So instead of re-implementing all of this, we
simply pass the skb to a given PF_PACKET socket for further analysis.

Another nice benefit that comes from that is that no code needs to be
changed in user space packet analyzers (maybe adding a dissector, but
not more), thus out of the box, we can already capture pcap files of
netlink traffic to debug/troubleshoot netlink problems.

Also thanks goes to Thomas Graf, Flavio Leitner, Jesper Dangaard Brouer.

 [1] http://marc.info/?l=linux-netdev&m=113813401516110

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

3 files changed, 181 insertions, 0 deletions

diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index 00aba08f01a9..b45b240889f5 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -240,6 +240,16 @@ config VIRTIO_NET
           This is the virtual network driver for virtio.  It can be used with
           lguest or QEMU based VMMs (like KVM or Xen).  Say Y or M.
 
+config NLMON
+        tristate "Virtual netlink monitoring device"
+        ---help---
+          This option enables a monitoring net device for netlink skbs. The
+          purpose of this is to analyze netlink messages with packet sockets.
+          Thus applications like tcpdump will be able to see local netlink
+          messages if they tap into the netlink device, record pcaps for further
+          diagnostics, etc. This is mostly intended for developers or support
+          to debug netlink issues. If unsure, say N.
+
 endif # NET_CORE
 
 config SUNGEM_PHY
diff --git a/drivers/net/Makefile b/drivers/net/Makefile
index ef3d090efedf..3fef8a81c0f6 100644
--- a/drivers/net/Makefile
+++ b/drivers/net/Makefile
@@ -22,6 +22,7 @@ obj-$(CONFIG_TUN) += tun.o
 obj-$(CONFIG_VETH) += veth.o
 obj-$(CONFIG_VIRTIO_NET) += virtio_net.o
 obj-$(CONFIG_VXLAN) += vxlan.o
+obj-$(CONFIG_NLMON) += nlmon.o
 
 #
 # Networking Drivers
diff --git a/drivers/net/nlmon.c b/drivers/net/nlmon.c
new file mode 100644
index 000000000000..dc364be6e61e
--- /dev/null
+++ b/drivers/net/nlmon.c
@@ -0,0 +1,170 @@
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/netdevice.h>
+#include <linux/netlink.h>
+#include <net/net_namespace.h>
+#include <linux/if_arp.h>
+
+struct pcpu_lstats {
+        u64 packets;
+        u64 bytes;
+        struct u64_stats_sync syncp;
+};
+
+static netdev_tx_t nlmon_xmit(struct sk_buff *skb, struct net_device *dev)
+{
+        int len = skb->len;
+        struct pcpu_lstats *stats = this_cpu_ptr(dev->lstats);
+
+        u64_stats_update_begin(&stats->syncp);
+        stats->bytes += len;
+        stats->packets++;
+        u64_stats_update_end(&stats->syncp);
+
+        dev_kfree_skb(skb);
+
+        return NETDEV_TX_OK;
+}
+
+static int nlmon_is_valid_mtu(int new_mtu)
+{
+        return new_mtu >= sizeof(struct nlmsghdr) && new_mtu <= INT_MAX;
+}
+
+static int nlmon_change_mtu(struct net_device *dev, int new_mtu)
+{
+        if (!nlmon_is_valid_mtu(new_mtu))
+                return -EINVAL;
+
+        dev->mtu = new_mtu;
+        return 0;
+}
+
+static int nlmon_dev_init(struct net_device *dev)
+{
+        dev->lstats = alloc_percpu(struct pcpu_lstats);
+
+        return dev->lstats == NULL ? -ENOMEM : 0;
+}
+
+static void nlmon_dev_uninit(struct net_device *dev)
+{
+        free_percpu(dev->lstats);
+}
+
+static struct netlink_tap nlmon_tap;
+
+static int nlmon_open(struct net_device *dev)
+{
+        return netlink_add_tap(&nlmon_tap);
+}
+
+static int nlmon_close(struct net_device *dev)
+{
+        return netlink_remove_tap(&nlmon_tap);
+}
+
+static struct rtnl_link_stats64 *
+nlmon_get_stats64(struct net_device *dev, struct rtnl_link_stats64 *stats)
+{
+        int i;
+        u64 bytes = 0, packets = 0;
+
+        for_each_possible_cpu(i) {
+                const struct pcpu_lstats *nl_stats;
+                u64 tbytes, tpackets;
+                unsigned int start;
+
+                nl_stats = per_cpu_ptr(dev->lstats, i);
+
+                do {
+                        start = u64_stats_fetch_begin_bh(&nl_stats->syncp);
+                        tbytes = nl_stats->bytes;
+                        tpackets = nl_stats->packets;
+                } while (u64_stats_fetch_retry_bh(&nl_stats->syncp, start));
+
+                packets += tpackets;
+                bytes += tbytes;
+        }
+
+        stats->rx_packets = packets;
+        stats->tx_packets = 0;
+
+        stats->rx_bytes = bytes;
+        stats->tx_bytes = 0;
+
+        return stats;
+}
+
+static u32 always_on(struct net_device *dev)
+{
+        return 1;
+}
+
+static const struct ethtool_ops nlmon_ethtool_ops = {
+        .get_link = always_on,
+};
+
+static const struct net_device_ops nlmon_ops = {
+        .ndo_init = nlmon_dev_init,
+        .ndo_uninit = nlmon_dev_uninit,
+        .ndo_open = nlmon_open,
+        .ndo_stop = nlmon_close,
+        .ndo_start_xmit = nlmon_xmit,
+        .ndo_get_stats64 = nlmon_get_stats64,
+        .ndo_change_mtu = nlmon_change_mtu,
+};
+
+static struct netlink_tap nlmon_tap __read_mostly = {
+        .module = THIS_MODULE,
+};
+
+static void nlmon_setup(struct net_device *dev)
+{
+        dev->type = ARPHRD_NETLINK;
+        dev->tx_queue_len = 0;
+
+        dev->netdev_ops = &nlmon_ops;
+        dev->ethtool_ops = &nlmon_ethtool_ops;
+        dev->destructor = free_netdev;
+
+        dev->features = NETIF_F_FRAGLIST | NETIF_F_HIGHDMA;
+        dev->flags = IFF_NOARP;
+
+        /* That's rather a softlimit here, which, of course,
+         * can be altered. Not a real MTU, but what is to be
+         * expected in most cases.
+         */
+        dev->mtu = NLMSG_GOODSIZE;
+}
+
+static __init int nlmon_register(void)
+{
+        int err;
+        struct net_device *nldev;
+
+        nldev = nlmon_tap.dev = alloc_netdev(0, "netlink", nlmon_setup);
+        if (unlikely(nldev == NULL))
+                return -ENOMEM;
+
+        err = register_netdev(nldev);
+        if (unlikely(err))
+                free_netdev(nldev);
+
+        return err;
+}
+
+static __exit void nlmon_unregister(void)
+{
+        struct net_device *nldev = nlmon_tap.dev;
+
+        unregister_netdev(nldev);
+}
+
+module_init(nlmon_register);
+module_exit(nlmon_unregister);
+