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

[Pablo Neira Ayuso]

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
> 
>