netdev - Re: [PATCH v2 09/10] netns: Add a limit on the number of net namespaces

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Date:	Tue, 26 Jul 2016 15:00:05 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrei Vagin <avagin@...il.com>
Cc:	Linux Containers <containers@...ts.linux-foundation.org>,
	Kees Cook <keescook@...omium.org>, netdev@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Seth Forshee <seth.forshee@...onical.com>,
	Nikolay Borisov <kernel@...p.com>,
	Linux API <linux-api@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Jann Horn <jann@...jh.net>
Subject: Re: [PATCH v2 09/10] netns: Add a limit on the number of net namespaces

Andrei Vagin <avagin@...il.com> writes:

> On Thu, Jul 21, 2016 at 9:40 AM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> index 2c2eb1b629b1..a489f192d619 100644
>> --- a/net/core/net_namespace.c
>> +++ b/net/core/net_namespace.c
>> @@ -266,6 +266,16 @@ struct net *get_net_ns_by_id(struct net *net, int id)
>>         return peer;
>>  }
>>
>> +static bool inc_net_namespaces(struct user_namespace *ns)
>> +{
>> +       return inc_ucount(ns, UCOUNT_NET_NAMESPACES);
>> +}
>> +
>> +static void dec_net_namespaces(struct user_namespace *ns)
>> +{
>> +       dec_ucount(ns, UCOUNT_NET_NAMESPACES);
>> +}
>> +
>>  /*
>>   * setup_net runs the initializers for the network namespace object.
>>   */
>> @@ -276,6 +286,9 @@ static __net_init int setup_net(struct net *net, struct user_namespace *user_ns)
>>         int error = 0;
>>         LIST_HEAD(net_exit_list);
>>
>> +       if (!inc_net_namespaces(user_ns))
>> +               return -ENFILE;
>
> I think you need to move this check after initilizing  net->passive.
> When setup_net returns an error, net_drop_ns is called:
>
Good point.  Ouch!

> void net_drop_ns(void *p)
> {
>         struct net *ns = p;
>         if (ns && atomic_dec_and_test(&ns->passive))
>                 net_free(ns);
> }
>
> Actually, I think it would be better to make this check before
> net_alloc().

You are probably right.  I seem to be trying to be entirely too clever
putting that in setup_net so I can cover the initial network namespace.
Which really does not need to be counted.  As clearly I also goofed up
the decrement on error case as well.

Eric