linux-kernel - Re: [PATCH] Make access to task's nsproxy liter

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Date:	Fri, 10 Aug 2007 19:09:32 +0400
From:	Pavel Emelyanov <xemul@...nvz.org>
To:	Oleg Nesterov <oleg@...sign.ru>
CC:	"Serge E. Hallyn" <serue@...ibm.com>,
	Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Containers <containers@...ts.osdl.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, devel@...nvz.org
Subject: Re: [PATCH] Make access to task's nsproxy liter

Oleg Nesterov wrote:
> On 08/10, Serge E. Hallyn wrote:
>> Quoting Pavel Emelyanov (xemul@...nvz.org):
>>> +/*
>>> + * the namespaces access rules are:
>>> + *
>>> + *  1. only current task is allowed to change tsk->nsproxy pointer or
>>> + *     any pointer on the nsproxy itself
>>> + *
>>> + *  2. when accessing (i.e. reading) current task's namespaces - no
>>> + *     precautions should be taken - just dereference the pointers
>>> + *
>>> + *  3. the access to other task namespaces is performed like this
>>> + *     rcu_read_lock();
>>> + *     nsproxy = task_nsproxy(tsk);
>>> + *     if (nsproxy != NULL) {
>>> + *             / *
>>> + *               * work with the namespaces here
>>> + *               * e.g. get the reference on one of them
>>> + *               * /
>>> + *     } / *
>>> + *         * NULL task_nsproxy() means that this task is
>>> + *         * almost dead (zombie)
>>> + *         * /
>>> + *     rcu_read_unlock();
>> And lastly, I guess that the caller to switch_task_namespaces() has
>> to ensure that new_nsproxy either (1) is the init namespace, (2) is a
>> brand-new namespace to which noone else has a reference, or (3) the
>> caller has to hold a reference to the new_nsproxy across the call to
>> switch_task_namespaces().
>>
>> As it happens the current calls fit (1) or (2).  Again if we happen to
>> jump into the game of switching a task into another task's nsproxy,
>> we'll need to be mindful of (3) so that new_nsproxy can't be tossed into
>> the bin between
>>
>> 	if (new)
>> 		get_nsproxy(new);
> 
> 4) Unless tsk == current, get_task_namespaces(tsk) and get_nsproxy(tsk)
>    are racy even if done under rcu_read_lock().

Yup :)

It is already written in comment that only the current is allowed
to change its nsproxy. I.e. when switch_task_nsproxy() is called
for tsk other than current it's a BUG

> Oleg.
> 
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