netdev - Re: [RFC PATCH net-next 01/11] netns: Fix race between put_net() and netlink_kernel

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Message-ID: <CAHA+R7Mc==jHi-vtyNwboYW-ysacgbA-yAG+tetx8HJFUrrf9Q@mail.gmail.com>
Date:	Thu, 7 May 2015 10:19:12 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Ying Xue <ying.xue@...driver.com>, netdev <netdev@...r.kernel.org>,
	xemul@...nvz.org, David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>, maxk@....qualcomm.com,
	Stephen Hemminger <stephen@...workplumber.org>,
	Thomas Graf <tgraf@...g.ch>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	Tom Herbert <tom@...bertland.com>,
	James Chapman <jchapman@...alix.com>,
	Erik Hugne <erik.hugne@...csson.com>, jon.maloy@...csson.com,
	horms@...ge.net.au
Subject: Re: [RFC PATCH net-next 01/11] netns: Fix race between put_net() and netlink_kernel_create()

On Thu, May 7, 2015 at 2:04 AM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> On Thu, May 07, 2015 at 04:52:40PM +0800, Ying Xue wrote:
>>
>> @@ -409,12 +410,17 @@ void __put_net(struct net *net)
>>  {
>>       /* Cleanup the network namespace in process context */
>>       unsigned long flags;
>> +     bool added = false;
>>
>>       spin_lock_irqsave(&cleanup_list_lock, flags);
>> -     list_add(&net->cleanup_list, &cleanup_list);
>> +     if (list_empty(&net->cleanup_list)) {
>> +             list_add(&net->cleanup_list, &cleanup_list);
>> +             added = true;
>> +     }
>
> Stop right there.  If a ref count is hitting zero twice you've
> got big problems.  Because it means that after hitting zero the
> first time it'll become positive again (if only briefly) which
> opens you up to a race against the cleanup.

That is true.

>
> So this idea is a non-starter.
>
> The rules for ref counts are really simple, if you hit zero then
> you're dead.  Can someone explain to me in simple terms why this
> ref count is special and needs to hit zero twice?

Because after it hits zero, it queues the final cleanup to a workqueue,
which could be delayed (depends on when the worker will be scheduled),
therefore there is a small window that new ->init() could come in.

You can argue that netns->init() should not see a dead netns, but
unfortunately that seems even harder since removing netns from global
netns list requires rtnl lock which is not doable in __put_net().
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