Message-ID: <20170804212132.7678.11425.stgit@john-Precision-Tower-5810>
Date:   Fri, 04 Aug 2017 14:21:32 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     davem@...emloft.net, daniel@...earbox.net,
        alexander.levin@...izon.com
Cc:     netdev@...r.kernel.org, john.fastabend@...il.com
Subject: [net-next PATCH] net: devmap fix mutex in rcu critical section

Originally we used a mutex to protect concurrent devmap update
and delete operations from racing with netdev unregister notifier
callbacks.

The notifier hook is needed because we increment the netdev ref
count when a dev is added to the devmap. This ensures the netdev
reference is valid in the datapath. However, we don't want to block
unregister events, hence the initial mutex and notifier handler.

The concern was in the notifier hook we search the map for dev
entries that hold a refcnt on the net device being torn down. But,
in order to do this we require two steps,

  (i) dereference the netdev:  dev = rcu_dereference(map[i])
 (ii) test ifindex:   dev->ifindex == removing_ifindex

and then finally we can swap in the NULL dev in the map via an
xchg operation,

  xchg(map[i], NULL)

The danger here is a concurrent update could run a different
xchg op concurrently leading us to replace the new dev with a
NULL dev incorrectly.

      CPU 1                        CPU 2

   notifier hook                   bpf devmap update

   dev = rcu_dereference(map[i])
                                   dev = rcu_dereference(map[i])
                                   xchg(map[i]), new_dev);
                                   rcu_call(dev,...)
   xchg(map[i], NULL)

The above flow would create the incorrect state with the dev
reference in the update path being lost. To resolve this the
original code used a mutex around the above block. However,
updates, deletes, and lookups occur inside rcu critical sections
so we can't use a mutex in this context safely.

Fortunately, by writing slightly better code we can avoid the
mutex altogether. If CPU 1 in the above example uses a cmpxchg
and _only_ replaces the dev reference in the map when it is in
fact the expected dev the race is removed completely. The two
cases being illustrated here, first the racy condition,

      CPU 1                          CPU 2

   notifier hook                     bpf devmap update

   dev = rcu_dereference(map[i])
                                     dev = rcu_dereference(map[i])
                                     xchg(map[i]), new_dev);
                                     rcu_call(dev,...)
   odev = cmpxchg(map[i], dev, NULL)

Now we can test the cmpxchg return value, detect odev != dev and
abort. Or in the good case,

      CPU 1                          CPU 2

   notifier hook                     bpf devmap update
   dev = rcu_dereference(map[i])
   odev = cmpxchg(map[i], dev, NULL)
                                     [...]

Now 'odev == dev' and we can do proper cleanup.

And viola the original race we tried to solve with a mutex is
corrected and the trace noted by Sasha below is resolved due
to removal of the mutex.

BUG: sleeping function called from invalid context at kernel/locking/mutex.c:747
in_atomic(): 1, irqs_disabled(): 0, pid: 16315, name: syz-executor1
1 lock held by syz-executor1/16315:
 #0:  (rcu_read_lock){......}, at: [<ffffffff8c363bc2>] map_delete_elem kernel/bpf/syscall.c:577 [inline]
 #0:  (rcu_read_lock){......}, at: [<ffffffff8c363bc2>] SYSC_bpf kernel/bpf/syscall.c:1427 [inline]
 #0:  (rcu_read_lock){......}, at: [<ffffffff8c363bc2>] SyS_bpf+0x1d32/0x4ba0 kernel/bpf/syscall.c:1388

Fixes: 2ddf71e23cc2 ("net: add notifier hooks for devmap bpf map")
Reported-by: Sasha Levin <alexander.levin@...izon.com>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Signed-off-by: John Fastabend <john.fastabend@...il.com>
---
 kernel/bpf/devmap.c |   27 +++++++--------------------
 1 file changed, 7 insertions(+), 20 deletions(-)

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index d439ee0..087f7230 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -40,11 +40,9 @@
  * 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.
+ * When removing the dev a cmpxchg() is used to ensure the correct dev is
+ * removed, in the case of a concurrent update or delete operation it is
+ * possible that the initially referenced dev is no longer in the map.
  */
 #include <linux/bpf.h>
 #include <linux/jhash.h>
@@ -68,7 +66,6 @@ struct bpf_dtab {
 	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)
@@ -128,9 +125,7 @@ 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:
@@ -169,7 +164,6 @@ static void dev_map_free(struct bpf_map *map)
 	 * 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;
 
@@ -185,7 +179,6 @@ static void dev_map_free(struct bpf_map *map)
 	 * _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);
@@ -322,11 +315,9 @@ 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;
 }
 
@@ -369,11 +360,9 @@ 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;
 }
@@ -396,22 +385,20 @@ static int dev_map_notification(struct notifier_block *notifier,
 
 	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;
+				struct bpf_dtab_netdev *dev, *odev;
 
-				dev = dtab->netdev_map[i];
+				dev = READ_ONCE(dtab->netdev_map[i]);
 				if (!dev ||
 				    dev->dev->ifindex != netdev->ifindex)
 					continue;
-				dev = xchg(&dtab->netdev_map[i], NULL);
-				if (dev)
+				odev = cmpxchg(&dtab->netdev_map[i], dev, NULL);
+				if (dev == odev)
 					call_rcu(&dev->rcu,
 						 __dev_map_entry_free);
 			}
 		}
-		mutex_unlock(&dev_map_list_mutex);
 		break;
 	default:
 		break;

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