linux-kernel - Re: [PATCH] allow a task to join a pid namespace

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Date:	Mon, 4 Jun 2012 18:51:17 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	devel@...nvz.org, kir@...allels.com,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tejun Heo <tj@...nel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH] allow a task to join a pid namespace

On 06/04, Glauber Costa wrote:
>
> Currently, it is possible for a process  to join existing
> net, uts and ipc namespaces. This patch allows a process to join an
> existing pid namespace as well.

I can't understand this patch... but probably I missed something,
I never really understood setns.

> +static int pidns_install(struct nsproxy *nsproxy, void *_ns)
> +{
> +	struct pid *newpid;
> +	struct pid_namespace *ns = _ns;
> +
> +	if (is_container_init(current))
> +		return -EINVAL;
> +
> +	if (nsproxy->pid_ns != ns->parent)
> +		return -EPERM;

At least you should also check that current is single-threaded,
I guess.

> +
> +	if (task_pgrp(current) !=  task_pid(current))
> +		return -EPERM;
> +
> +	if (task_session(current) !=  task_pid(current))
> +		return -EPERM;

Both checks are obviously racy without tasklist.

> +	newpid = alloc_pid(ns);
> +	if (!newpid)
> +		return -ENOMEM;

Hmm. Doesn't this mean that pid_nr of this task (as it seen
in its current namespace) will be changed? This doesn't look
sane.

> +	put_pid_ns(nsproxy->pid_ns);
> +	nsproxy->pid_ns = get_pid_ns(ns);
> +
> +	write_lock_irq(&tasklist_lock);
> +	change_pid(current, PIDTYPE_PID, newpid);
> +	change_pid(current, PIDTYPE_PGID, newpid);
> +	change_pid(current, PIDTYPE_SID, newpid);
> +	write_unlock_irq(&tasklist_lock);

Hmm. So, until the caller does switch_task_namespaces()
task_active_pid_ns(current) != current->nsproxy->pid_ns,
doesn't look very nice too.

I don't think this can be right. If nothing else, this breaks
it_real_fn().

Oleg.

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