[PATCH] netfilter: nf_ct_helper: Fix possible panic when nf_conntrack_helper_unregister is used in an unloadable module

[Su Yanjun]

From: Su Yanjun <suyj.fnst@xxxxxxxxxxxxxx>

Because nf_conntrack_helper_unregister maybe used in an unloadable module,
it uses 'synchronize_rcu' which may cause kernel panic.

According to the artical:
RCU and Unloadable Modules
https://lwn.net/Articles/217484/

When we have a heavy rcu callback load, then some of the callbacks might be
deferred in order to allow other processing to proceed. sychnorize_rcu does
not wait rcu callback complete and module may be unloaded before callback
done.

This patch uses rcu_barrier instead of synchronize_rcu will prevent this
situation.

Signed-off-by: Su Yanjun <suyj.fnst@xxxxxxxxxxxxxx>
---
 net/netfilter/nf_conntrack_helper.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nf_conntrack_helper.c b/net/netfilter/nf_conntrack_helper.c
index 274baf1..0ee9378 100644
--- a/net/netfilter/nf_conntrack_helper.c
+++ b/net/netfilter/nf_conntrack_helper.c
@@ -397,8 +397,15 @@ void nf_conntrack_helper_unregister(struct nf_conntrack_helper *me)
 
 	/* Make sure every nothing is still using the helper unless its a
 	 * connection in the hash.
+	 *
+	 * 'synchronize_rcu' may have problem when rcu callback function
+	 * is used in unloadable modules. Use rcu_barrier instead, so that
+	 * it will wait until rcu callback completes before modules are
+	 * unloaded.
+	 * More detail about rcu_barrier please see:
+	 * https://lwn.net/Articles/217484/
 	 */
-	synchronize_rcu();
+	rcu_barrier();
 
 	nf_ct_expect_iterate_destroy(expect_iter_me, NULL);
 	nf_ct_iterate_destroy(unhelp, me);
@@ -406,7 +413,7 @@ void nf_conntrack_helper_unregister(struct nf_conntrack_helper *me)
 	/* Maybe someone has gotten the helper already when unhelp above.
 	 * So need to wait it.
 	 */
-	synchronize_rcu();
+	rcu_barrier();
 }
 EXPORT_SYMBOL_GPL(nf_conntrack_helper_unregister);
 
-- 
2.7.4