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

["Su Yanjun <suyj.fnst@cn.fujitsu.com>" <suyj.fnst@cn.fujitsu.com>]

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@cn.fujitsu.com>
>>
>> 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@cn.fujitsu.com>
>> ---
>>   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
>>
>>