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

From: Su Yanjun
Date: Fri Mar 01 2019 - 00:59:15 EST

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

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

This patch uses rcu_barrier instead of synchronize_rcu will prevent this

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();