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

From: Pablo Neira Ayuso
Date: Fri Mar 08 2019 - 10:59:42 EST


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!

>
> 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
>
>