[Devel] Re: [PATCH] Make access to task's nsproxy liter

[Oleg Nesterov]

On 08/10, Pavel Emelyanov wrote:
>
> Oleg Nesterov wrote:
> >On 08/10, Serge E. Hallyn wrote:
> >>Quoting Pavel Emelyanov (xemul at openvz.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

Yes, but what I meant is that this code

        rcu_read_lock();
        nsproxy = task_nsproxy(tsk);
        if (nsproxy != NULL)
                get_nsproxy(nsproxy);
        rcu_read_unlock();

	if (nsproxy) {
		use_it(nsproxy);
		put_nsproxy(nsproxy);
	}

is not safe despite the fact we are _not_ changing tsk->nsproxy.

The patch itself is correct because we don't do that, and the comment
is right. Just it is not immediately obvious.

Oleg.