linux-kernel - Re: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of ns_last

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <de392430-18b8-d296-b868-82fdedcd0c0e@virtuozzo.com>
Date:   Tue, 2 May 2017 20:33:00 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     <serge@...lyn.com>, <ebiederm@...ssion.com>, <agruenba@...hat.com>,
        <linux-api@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <paul@...l-moore.com>, <viro@...iv.linux.org.uk>,
        <avagin@...nvz.org>, <linux-fsdevel@...r.kernel.org>,
        <mtk.manpages@...il.com>, <akpm@...ux-foundation.org>,
        <luto@...capital.net>, <gorcunov@...nvz.org>, <mingo@...nel.org>,
        <keescook@...omium.org>
Subject: Re: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of
 ns_last_pid's on ns hierarhy

On 02.05.2017 19:33, Oleg Nesterov wrote:
> sorry for delay, vacation...
> 
> On 04/28, Kirill Tkhai wrote:
>>
>> On 27.04.2017 19:22, Oleg Nesterov wrote:
>>>
>>> Ah, OK, I didn't notice the ns->child_reaper check in pidns_for_children_get().
>>>
>>> But note that it doesn't need tasklist_lock too.
>>
>> Hm, are there possible strange situations with memory ordering, when we see
>> ns->child_reaper of already died ns, which was placed in the same memory?
>> Do we have to use some memory barriers here?
> 
> Could you spell please? I don't understand your concerns...
> 
> I don't see how, say,
> 
> 	static struct ns_common *pidns_for_children_get(struct task_struct *task)
> 	{
> 		struct ns_common *ns = NULL;
> 		struct pid_namespace *pid_ns;
> 
> 		task_lock(task);
> 		if (task->nsproxy) {
> 			pid_ns = task->nsproxy->pid_ns_for_children;
> 			if (pid_ns->child_reaper) {
> 				ns = &pid_ns->ns;
> 				get_pid_ns(ns);
> 			}
> 		}
> 		task_unlock(task);
> 
> 		return ns;
> 	}
> 
> can be wrong. It also looks more clean to me.
> 
> ->child_reaper is not stable without tasklist, it can be dead/etc, but
> we do not care?

I mean the following. We had a pid_ns1 with a child_reaper set. Then
it became dead, and a new pid_ns2 were allocated in the same memory.

A task on another cpu opens the pid_for_children file, and because
of there is no memory ordering, it sees pid_ns1->child_reaper,
when it opens pid_ns2.

I forgot, what guarantees this situation is impossible? What guarantees,
the renewed content of pid_ns2 on another cpu is seen not later, than
we can't open it?