[PATCH RESEND nf] netfilter: avoid a race between nf_register_hook() and cleanup_net()

[Michal Kubecek]

There is a race condition between nf_{,un}register_hook() and
cleanup_net() which can either trigger WARN check or cause a memory
leak. The scenario is like this (2a and 2b are alternatives):

1.  cleanup_net() removes one or more struct net from net_namespace_list
2a. nf_register_hook() adds per-netns hooks to all netns (but not those
    removed in step 1) and adds the hook to global nf_hook_list
2b. nf_unregister_hook() deletes per-netns hooks from all netns (but not
    those removed in step 1) and removes the hook from nf_hook_list
3.  cleanup_net() calls pernet subsystem exit functions for netns being
    removed; one of them is netfilter_net_exit() which (among others)
    calls nf_unregister_net_hook() to unregister per-netns hooks for all
    hooks in nf_hook_list.

In case (a), per-netns hooks are never added as the namespace was
already invisible to for_each_net() in step 2a but an attempt to remove
them in step 3 (the hook is already in nf_hook_list) triggers a WARN
check in nf_unregister_net_hook() (no real harm done, however). In case
(b), the per-netns hook is removed neither in step 2b (netns is already
invisible to for_each_net()) nor in step 3 (the hook is already removed
from nf_hook_list), causing a memory leak.

Prevent the race by protecting the for_each_net() loop in
nf_{,un}register_hook() (also) by net_mutex. There is already a
precendens for this in rtnl_link_unregister() which addresses similar
race.

Fixes: 085db2c04557 ("netfilter: Per network namespace netfilter hooks.")
Signed-off-by: Michal Kubecek <mkubecek@xxxxxxx>
---
 net/netfilter/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/netfilter/core.c b/net/netfilter/core.c
index f39276d1c2d7..860978c9f82e 100644
--- a/net/netfilter/core.c
+++ b/net/netfilter/core.c
@@ -193,6 +193,8 @@ int nf_register_hook(struct nf_hook_ops *reg)
 	struct net *net, *last;
 	int ret;
 
+	/* prevent race with cleanup_net() */
+	mutex_lock(&net_mutex);
 	rtnl_lock();
 	for_each_net(net) {
 		ret = nf_register_net_hook(net, reg);
@@ -201,6 +203,7 @@ int nf_register_hook(struct nf_hook_ops *reg)
 	}
 	list_add_tail(&reg->list, &nf_hook_list);
 	rtnl_unlock();
+	mutex_unlock(&net_mutex);
 
 	return 0;
 rollback:
@@ -211,6 +214,7 @@ rollback:
 		nf_unregister_net_hook(net, reg);
 	}
 	rtnl_unlock();
+	mutex_unlock(&net_mutex);
 	return ret;
 }
 EXPORT_SYMBOL(nf_register_hook);
@@ -219,11 +223,14 @@ void nf_unregister_hook(struct nf_hook_ops *reg)
 {
 	struct net *net;
 
+	/* prevent race with cleanup_net() */
+	mutex_lock(&net_mutex);
 	rtnl_lock();
 	list_del(&reg->list);
 	for_each_net(net)
 		nf_unregister_net_hook(net, reg);
 	rtnl_unlock();
+	mutex_unlock(&net_mutex);
 }
 EXPORT_SYMBOL(nf_unregister_hook);
 
-- 
2.9.2