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

[Su Yanjun <suyj.fnst@xxxxxxxxxxxxxx>]

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