net: add notifier hooks for devmap bpf map

The BPF map devmap holds a refcnt on the net_device structure when
it is in the map. We need to do this to ensure on driver unload we
don't lose a dev reference.

However, its not very convenient to have to manually unload the map
when destroying a net device so add notifier handlers to do the cleanup
automatically. But this creates a race between update/destroy BPF
syscall and programs and the unregister netdev hook.

Unfortunately, the best I could come up with is either to live with
requiring manual removal of net devices from the map before removing
the net device OR to add a mutex in devmap to ensure the map is not
modified while we are removing a device. The fallout also requires
that BPF programs no longer update/delete the map from the BPF program
side because the mutex may sleep and this can not be done from inside
an rcu critical section.  This is not a real problem though because I
have not come up with any use cases where this is actually useful in
practice. If/when we come up with a compelling user for this we may
need to revisit this.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 73 insertions, 0 deletions

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index b2ef04a1c86a..899364d097f5 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -34,6 +34,17 @@
  * netdev_map consistent in this case. From the devmap side BPF programs
  * calling into these operations are the same as multiple user space threads
  * making system calls.
+ *
+ * Finally, any of the above may race with a netdev_unregister notifier. The
+ * unregister notifier must search for net devices in the map structure that
+ * contain a reference to the net device and remove them. This is a two step
+ * process (a) dereference the bpf_dtab_netdev object in netdev_map and (b)
+ * check to see if the ifindex is the same as the net_device being removed.
+ * Unfortunately, the xchg() operations do not protect against this. To avoid
+ * potentially removing incorrect objects the dev_map_list_mutex protects
+ * conflicting netdev unregister and BPF syscall operations. Updates and
+ * deletes from a BPF program (done in rcu critical section) are blocked
+ * because of this mutex.
  */
 #include <linux/bpf.h>
 #include <linux/jhash.h>
@@ -54,8 +65,12 @@ struct bpf_dtab {
         struct bpf_map map;
         struct bpf_dtab_netdev **netdev_map;
         unsigned long int __percpu *flush_needed;
+        struct list_head list;
 };
 
+static DEFINE_MUTEX(dev_map_list_mutex);
+static LIST_HEAD(dev_map_list);
+
 static struct bpf_map *dev_map_alloc(union bpf_attr *attr)
 {
         struct bpf_dtab *dtab;
@@ -112,6 +127,9 @@ static struct bpf_map *dev_map_alloc(union bpf_attr *attr)
         if (!dtab->netdev_map)
                 goto free_dtab;
 
+        mutex_lock(&dev_map_list_mutex);
+        list_add_tail(&dtab->list, &dev_map_list);
+        mutex_unlock(&dev_map_list_mutex);
         return &dtab->map;
 
 free_dtab:
@@ -146,6 +164,11 @@ static void dev_map_free(struct bpf_map *map)
                         cpu_relax();
         }
 
+        /* Although we should no longer have datapath or bpf syscall operations
+         * at this point we we can still race with netdev notifier, hence the
+         * lock.
+         */
+        mutex_lock(&dev_map_list_mutex);
         for (i = 0; i < dtab->map.max_entries; i++) {
                 struct bpf_dtab_netdev *dev;
 
@@ -160,6 +183,8 @@ static void dev_map_free(struct bpf_map *map)
         /* At this point bpf program is detached and all pending operations
          * _must_ be complete
          */
+        list_del(&dtab->list);
+        mutex_unlock(&dev_map_list_mutex);
         free_percpu(dtab->flush_needed);
         bpf_map_area_free(dtab->netdev_map);
         kfree(dtab);
@@ -296,9 +321,11 @@ static int dev_map_delete_elem(struct bpf_map *map, void *key)
          * the driver tear down ensures all soft irqs are complete before
          * removing the net device in the case of dev_put equals zero.
          */
+        mutex_lock(&dev_map_list_mutex);
         old_dev = xchg(&dtab->netdev_map[k], NULL);
         if (old_dev)
                 call_rcu(&old_dev->rcu, __dev_map_entry_free);
+        mutex_unlock(&dev_map_list_mutex);
         return 0;
 }
 
@@ -341,9 +368,11 @@ static int dev_map_update_elem(struct bpf_map *map, void *key, void *value,
          * Remembering the driver side flush operation will happen before the
          * net device is removed.
          */
+        mutex_lock(&dev_map_list_mutex);
         old_dev = xchg(&dtab->netdev_map[i], dev);
         if (old_dev)
                 call_rcu(&old_dev->rcu, __dev_map_entry_free);
+        mutex_unlock(&dev_map_list_mutex);
 
         return 0;
 }
@@ -356,3 +385,47 @@ const struct bpf_map_ops dev_map_ops = {
         .map_update_elem = dev_map_update_elem,
         .map_delete_elem = dev_map_delete_elem,
 };
+
+static int dev_map_notification(struct notifier_block *notifier,
+                                ulong event, void *ptr)
+{
+        struct net_device *netdev = netdev_notifier_info_to_dev(ptr);
+        struct bpf_dtab *dtab;
+        int i;
+
+        switch (event) {
+        case NETDEV_UNREGISTER:
+                mutex_lock(&dev_map_list_mutex);
+                list_for_each_entry(dtab, &dev_map_list, list) {
+                        for (i = 0; i < dtab->map.max_entries; i++) {
+                                struct bpf_dtab_netdev *dev;
+
+                                dev = dtab->netdev_map[i];
+                                if (!dev ||
+                                    dev->dev->ifindex != netdev->ifindex)
+                                        continue;
+                                dev = xchg(&dtab->netdev_map[i], NULL);
+                                if (dev)
+                                        call_rcu(&dev->rcu,
+                                                 __dev_map_entry_free);
+                        }
+                }
+                mutex_unlock(&dev_map_list_mutex);
+                break;
+        default:
+                break;
+        }
+        return NOTIFY_OK;
+}
+
+static struct notifier_block dev_map_notifier = {
+        .notifier_call = dev_map_notification,
+};
+
+static int __init dev_map_init(void)
+{
+        register_netdevice_notifier(&dev_map_notifier);
+        return 0;
+}
+
+subsys_initcall(dev_map_init);