On 2019/3/8 23:59, Pablo Neira Ayuso wrote:
On Fri, Mar 01, 2019 at 01:56:06PM +0800, Su Yanjun wrote:
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();
Are you sure this is correct?
IIRC rcu_barrier() makes sure no pending callback is still waiting in
the queue to run. We have don't use call_rcu() in this code, which is
what rcu_barrier() is meant for.
Please correct me if I'm mistaken.
Thanks!
The issue is not whether your module invokes|call_rcu()|,
but rather whether the corresponding RCU callback invokes a function that is in a module.
For unloadable modules, **rcu_barrier** make the code more robust than synchronize_rcu.
Thanks
Su
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
