lkml.org 
[lkml]   [2019]   [Mar]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
SubjectRe: [PATCH] netfilter: nf_ct_helper: Fix possible panic when nf_conntrack_helper_unregister is used in an unloadable module
Date

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
>>
>>


\
 
 \ /
  Last update: 2019-03-14 04:21    [W:0.088 / U:0.408 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site