Re: [PATCH 2/2] pid_ns: Introduce ioctl to set vector of ns_last_pid's on ns hierarhy

[Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>]

On 27.04.2017 19:39, Eric W. Biederman wrote:
> Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org> writes:
> 
>> On 27.04.2017 19:12, Oleg Nesterov wrote:
>>> On 04/26, Kirill Tkhai wrote:
>>>>
>>>> On 26.04.2017 18:53, Oleg Nesterov wrote:
>>>>>
>>>>>> +static long set_last_pid_vec(struct pid_namespace *pid_ns,
>>>>>> +			     struct pidns_ioc_req *req)
>>>>>> +{
>>>>>> +	char *str, *p;
>>>>>> +	int ret = 0;
>>>>>> +	pid_t pid;
>>>>>> +
>>>>>> +	read_lock(&tasklist_lock);
>>>>>> +	if (!pid_ns->child_reaper)
>>>>>> +		ret = -EINVAL;
>>>>>> +	read_unlock(&tasklist_lock);
>>>>>> +	if (ret)
>>>>>> +		return ret;
>>>>>
>>>>> why do you need to check ->child_reaper under tasklist_lock? this looks pointless.
>>>>>
>>>>> In fact I do not understand how it is possible to hit pid_ns->child_reaper == NULL,
>>>>> there must be at least one task in this namespace, otherwise you can't open a file
>>>>> which has f_op == ns_file_operations, no?
>>>>
>>>> Sure, it's impossible to pick a pid_ns, if there is no the pid_ns's tasks. I added
>>>> it under impression of
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=dfda351c729733a401981e8738ce497eaffcaa00
>>>> but here it's completely wrong. It will be removed in v2.
>>>
>>> Hmm. But if I read this commit correctly then we really need to check
>>> pid_ns->child_reaper != NULL ?
>>>
>>> Currently we can't pick an "empty" pid_ns. But after the commit above a task
>>> can do sys_unshare(CLONE_NEWPID), another (or the same) task can open its
>>> /proc/$pid/ns/pid_for_children and call ns_ioctl() before the 1st alloc_pid() ?
>>
>> Another task can't open /proc/$pid/ns/pid_for_children before the 1st alloc_pid(),
>> because pid_for_children is available to open only after the 1st alloc_pid().
>> So, it's impossible to call ioctl() on it.
> 
> That sounds reasonable.
> 
> There is definitely the chance of the child_reaper dying after we have
> joined a pid namespace.    So child_reaper can be stale if not NULL.
> 
> As long as we don't mess up the first pid allocation I don't
> see any reason why we should care about last_pid in a pid_namespace.
> And this ioctl can be used to set all of the other pids on the first
> pid allocation by calling it in the parent pid namespace.
> 
> There is still the chance of racing with a pid reaper dying.   Why do we
> care about child_reaper in this case?
> 
> Changing last_pid is completely pointless if child_reaper is dead or
> missing but why would we care?

I'm agree with you, there is no a reason we should care about died child_reaper.
The protection is already made in pidns_for_children_get(). It's only need to
prohibit creation of the first task with pid != 1, which leads to child_reaper-less
pid namespace.