Date:   Tue, 02 May 2017 16:26:12 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     <serge@...lyn.com>, <agruenba@...hat.com>,
        <gregkh@...uxfoundation.org>, <linux-kernel@...r.kernel.org>,
        <oleg@...hat.com>, <paul@...l-moore.com>,
        <viro@...iv.linux.org.uk>, <avagin@...nvz.org>,
        <linux-api@...r.kernel.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 v3] pid_ns: Introduce ioctl to set vector of ns_last_pid's on ns hierarhy

Kirill Tkhai <ktkhai@...tuozzo.com> writes:

> On 29.04.2017 22:12, Eric W. Biederman wrote:
>> Kirill Tkhai <ktkhai@...tuozzo.com> writes:
>> 
>>> On 27.04.2017 19:07, Eric W. Biederman wrote:
>>>> Kirill Tkhai <ktkhai@...tuozzo.com> writes:
>>>>
>>>>> On 27.04.2017 18:15, Eric W. Biederman wrote:
>>>>>> Kirill Tkhai <ktkhai@...tuozzo.com> writes:
>>>>>>
>>>>>>> On implementing of nested pid namespaces support in CRIU
>>>>>>> (checkpoint-restore in userspace tool) we run into
>>>>>>> the situation, that it's impossible to create a task with
>>>>>>> specific NSpid effectively. After commit 49f4d8b93ccf
>>>>>>> "pidns: Capture the user namespace and filter ns_last_pid"
>>>>>>> it is impossible to set ns_last_pid on any pid namespace,
>>>>>>> except task's active pid_ns (before the commit it was possible
>>>>>>> to write to pid_ns_for_children). Thus, if a restored task
>>>>>>> in a container has more than one pid_ns levels, the restorer
>>>>>>> code must have a task helper for every pid namespace
>>>>>>> of the task's pid_ns hierarhy.
>>>>>>>
>>>>>>> This is a big problem, because of communication with
>>>>>>> a helper for every pid_ns in the hierarchy is not cheap.
>>>>>>> It's not performance-good as it implies many helpers wakeups
>>>>>>> to create a single task (independently, how you communicate
>>>>>>> with the helpers). This patch tries to decide the problem.
>>>>>>
>>>>>> I see the problem and we definitely need to do something.
>>>>>> Your patch does appear better than what we have been doing.
>>>>>> So a tenative conceptual ack.
>>>>>>
>>>>>> At the same time it is legitimate to claim that the use of
>>>>>> task_active_pid_ns(current) rather than
>>>>>> current->nsproxy->pid_ns_for_children is a regression caused by the
>>>>>> above commit.  So we can fix the original issue as well.
>>>>>>
>>>>>> I do have to ask when was this problem discovered, and why did it take
>>>>>> so long to discover?  The regeression happened nearly 5 years ago.
>>>>>>
>>>>>> Was CRIU already using this?
>>>>>
>>>>> CRIU uses ns_last_pid, but we never had nested pid namespace hierarchy.
>>>>> When there is only one level of pid namespaces, then active pid namespace
>>>>> is the save as pid_ns_for_children, so we never faced with this
>>>>> problem.
>>>>
>>>> Ok.  So not a regression then.
>>>>
>>>>> Now we're working on Docker support, and its recent versions create nested
>>>>> pid namespaces (I have no information, when they begun to do that). So,
>>>>> we met this problem.
>>>>>  
>>>>>> It looks like the fix is a one line low danger change to
>>>>>> /proc/sys/kernel/ns_last_pid.  With a low danger as pid_ns_for_children
>>>>>> rarely differs from task_active_pid_ns().
>>>>>>
>>>>>>
>>>>>>> It introduces a new pid_ns ioctl(NS_SET_LAST_PID_VEC),
>>>>>>> which allows to write a vector of last pids on pid_ns hierarchy.
>>>>>>> The vector is passed as array of pids in struct ns_ioc_pid_vec,
>>>>>>> written in reverse order. The first number corresponds to
>>>>>>> the opened namespace ns_last_pid, the second is to its parent, etc.
>>>>>>> So, if you have the pid namespaces hierarchy like:
>>>>>>>
>>>>>>> pid_ns1 (grand father)
>>>>>>>   |
>>>>>>>   v
>>>>>>> pid_ns2 (father)
>>>>>>>   |
>>>>>>>   v
>>>>>>> pid_ns3 (child)
>>>>>>>
>>>>>>> and the pid_ns3 is open, then the corresponding vector will be
>>>>>>> {last_ns_pid3, last_ns_pid2, last_ns_pid1}. This vector may be
>>>>>>> short and it may contain less levels. For example,
>>>>>>> {last_ns_pid3, last_ns_pid2} or even {last_ns_pid3}, in dependence
>>>>>>> of which levels you want to populate.
>>>>>>>
>>>>>>> v3: Use __u32 in uapi instead of unsigned int.
>>>>>>>
>>>>>>> v2: Kill pid_ns->child_reaper check as it's impossible to have
>>>>>>> such a pid namespace file open.
>>>>>>> Use generic namespaces ioctl() number.
>>>>>>> Pass pids as array, not as a string.
>>>>>>>
>>>>>>> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
>>>>>>> ---
>>>>>>>  fs/nsfs.c                     |    5 +++++
>>>>>>>  include/linux/pid_namespace.h |   12 ++++++++++++
>>>>>>>  include/uapi/linux/nsfs.h     |    7 +++++++
>>>>>>>  kernel/pid_namespace.c        |   35 +++++++++++++++++++++++++++++++++++
>>>>>>>  4 files changed, 59 insertions(+)
>>>>>>>
>>>>>>> diff --git a/fs/nsfs.c b/fs/nsfs.c
>>>>>>> index 323f492e0822..f669a1552003 100644
>>>>>>> --- a/fs/nsfs.c
>>>>>>> +++ b/fs/nsfs.c
>>>>>>> @@ -6,6 +6,7 @@
>>>>>>>  #include <linux/ktime.h>
>>>>>>>  #include <linux/seq_file.h>
>>>>>>>  #include <linux/user_namespace.h>
>>>>>>> +#include <linux/pid_namespace.h>
>>>>>>>  #include <linux/nsfs.h>
>>>>>>>  #include <linux/uaccess.h>
>>>>>>>  
>>>>>>> @@ -186,6 +187,10 @@ static long ns_ioctl(struct file *filp, unsigned int ioctl,
>>>>>>>  		argp = (uid_t __user *) arg;
>>>>>>>  		uid = from_kuid_munged(current_user_ns(), user_ns->owner);
>>>>>>>  		return put_user(uid, argp);
>>>>>>> +	case NS_SET_LAST_PID_VEC:
>>>>>>> +		if (ns->ops->type != CLONE_NEWPID)
>>>>>>> +			return -EINVAL;
>>>>>>> +		return pidns_set_last_pid_vec(ns, (void *)arg);
>>>>>>>  	default:
>>>>>>>  		return -ENOTTY;
>>>>>>>  	}
>>>>>>> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
>>>>>>> index c2a989dee876..c8dc4173a4e8 100644
>>>>>>> --- a/include/linux/pid_namespace.h
>>>>>>> +++ b/include/linux/pid_namespace.h
>>>>>>> @@ -9,6 +9,7 @@
>>>>>>>  #include <linux/nsproxy.h>
>>>>>>>  #include <linux/kref.h>
>>>>>>>  #include <linux/ns_common.h>
>>>>>>> +#include <uapi/linux/nsfs.h>
>>>>>>
>>>>>> No need for the extra include and slowing down the build. Just
>>>>>> declare the relevant structures.
>>>>>
>>>>> So, I'll write just:
>>>>>
>>>>> struct ns_ioc_pid_vec;
>>>>>
>>>>> instead of that.
>>>>>
>>>>>>>  
>>>>>>>  struct pidmap {
>>>>>>>         atomic_t nr_free;
>>>>>>> @@ -103,6 +104,17 @@ static inline int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd)
>>>>>>>  }
>>>>>>>  #endif /* CONFIG_PID_NS */
>>>>>>>  
>>>>>>> +#if defined(CONFIG_PID_NS) && defined(CONFIG_CHECKPOINT_RESTORE)
>>>>>>> +extern long pidns_set_last_pid_vec(struct ns_common *ns,
>>>>>>> +				   struct ns_ioc_pid_vec __user *vec);
>>>>>>> +#else /* CONFIG_PID_NS && CONFIG_CHECKPOINT_RESTORE */
>>>>>>> +static inline long pidns_set_last_pid_vec(struct ns_common *ns,
>>>>>>> +					  struct ns_ioc_pid_vec __user *vec)
>>>>>>> +{
>>>>>>> +	return -ENOTTY;
>>>>>>> +}
>>>>>>> +#endif /* CONFIG_PID_NS && CONFIG_CHECKPOINT_RESTORE */
>>>>>>
>>>>>> Just CONFIG_PID_NS please.  Either this is good enough for everyone who
>>>>>> has pid namespace support enabled or it isn't.
>>>>>
>>>>> Great, if it's so. For me it looks better too.
>>>>>  
>>>>>>>  extern struct pid_namespace *task_active_pid_ns(struct task_struct *tsk);
>>>>>>>  void pidhash_init(void);
>>>>>>>  void pidmap_init(void);
>>>>>>> diff --git a/include/uapi/linux/nsfs.h b/include/uapi/linux/nsfs.h
>>>>>>> index 1a3ca79f466b..1254b02a47fa 100644
>>>>>>> --- a/include/uapi/linux/nsfs.h
>>>>>>> +++ b/include/uapi/linux/nsfs.h
>>>>>>> @@ -14,5 +14,12 @@
>>>>>>>  #define NS_GET_NSTYPE		_IO(NSIO, 0x3)
>>>>>>>  /* Get owner UID (in the caller's user namespace) for a user namespace */
>>>>>>>  #define NS_GET_OWNER_UID	_IO(NSIO, 0x4)
>>>>>>> +/* Set a vector of ns_last_pid for a pid namespace stack */
>>>>>>> +#define NS_SET_LAST_PID_VEC	_IO(NSIO, 0x5)
>>>>>>> +
>>>>>>> +struct ns_ioc_pid_vec {
>>>>>>> +	__u32	nr;
>>>>>>> +	pid_t	pid[0];
>>>>>>> +};
>>>>>>>  
>>>>>>>  #endif /* __LINUX_NSFS_H */
>>>>>>> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
>>>>>>> index de461aa0bf9a..08b5fef23534 100644
>>>>>>> --- a/kernel/pid_namespace.c
>>>>>>> +++ b/kernel/pid_namespace.c
>>>>>>> @@ -21,6 +21,7 @@
>>>>>>>  #include <linux/export.h>
>>>>>>>  #include <linux/sched/task.h>
>>>>>>>  #include <linux/sched/signal.h>
>>>>>>> +#include <uapi/linux/nsfs.h>
>>>>>>>  
>>>>>>>  struct pid_cache {
>>>>>>>  	int nr_ids;
>>>>>>> @@ -428,6 +429,40 @@ static struct ns_common *pidns_get_parent(struct ns_common *ns)
>>>>>>>  	return &get_pid_ns(pid_ns)->ns;
>>>>>>>  }
>>>>>>>  
>>>>>>> +#ifdef CONFIG_CHECKPOINT_RESTORE
>>>>>>> +long pidns_set_last_pid_vec(struct ns_common *ns,
>>>>>>> +			    struct ns_ioc_pid_vec __user *vec)
>>>>>>> +{
>>>>>>> +	struct pid_namespace *pid_ns = to_pid_ns(ns);
>>>>>>> +	pid_t pid, __user *pid_ptr;
>>>>>>> +	u32 nr;
>>>>>>> +
>>>>>>> +	if (get_user(nr, &vec->nr))
>>>>>>> +		return -EFAULT;
>>>>>>> +	if (nr > 32 || nr < 1)
>>>>>>
>>>>>> The maximum needs not to be hard coded.
>>>>>
>>>>> Ah, I've missed MAX_PID_NS_LEVEL, thanks. 
>>>>>
>>>>>>> +		return -EINVAL;
>>>>>>> +
>>>>>>> +	pid_ptr = &vec->pid[0];
>>>>>>
>>>>>> All of the rest of the vector needs to be read in, in one go.
>>>>>
>>>>> Hm, Oleg said we shouldn't allocate a memory for that. Should
>>>>> I create array of MAX_PID_NS_LEVEL pids on stack?
>>>>
>>>> *scratches head*
>>>>
>>>> The really important part is that we perform all of the permission
>>>> checks before we perform the rest of the work.
>>>>
>>>> I would like to be able to say that the permission checks and
>>>> the rest of it all happen atomically.  Which requires copying the
>>>> data into kernel memory and sanitizing it (aka all checks) before
>>>> we apply the changes.
>>>
>>> This way, we better check the permissions on the top pid namespace
>>> of the passed vector, because every children's pid_ns->user_ns is
>>> the same as its parent's, or it's descendant.
>> 
>> In practice this makes sense and is a useful simplification.
>> 
>> Looking at your suggesting I am noticing we don't actually enforce this
>> constraint, and that with careful usage of setns I can get around that.
>> 
>> This seems like a hazard for kernel developers and not at all useful
>> for userspace developers.  So it looks like we need a patch to enforce
>> this constraint.  Patch to fix this issue in a moment.
>> 
>> 
>>>> "BUILD_BUG_ON(sizeof(u32) * MAX_PID_NS_LEVEL < 64);" if we are
>>>
>>> What does this check mean? Why do we have to limit minimal MAX_PID_NS_LEVEL?
>> 
>> That should have been paranenthesized as:
>> BUILD_BUG_ON((sizeof(u32) * MAX_PID_NS_LEVEL) < 128);
>> or possibly writen as:
>> BUILD_BUG_ON(sizeof(on_stack_array) < 128);
>> 
>> The point being that if someone changes MAX_PID_NS_LEVEL and the stack
>> usage goes up noticably we have a warning, and then someone can
>> determine if the array is still small enough to fit on the stack
>> or if it needs to be kmalloced.
>> 
>> The goal is not to leave a trap for maintainers in the future.
>
> Thanks for the explanation, Eric, but it's not the question I asked you :)
> You limit *minimal* MAX_PID_NS_LEVEL, while you write about *maximal* MAX_PID_NS_LEVEL.
> That's what about I wanted to know.
>
> So, the "<" is just a mistake, OK.

Yes.  Sigh.  Replying when tired.  A good way to generate confusion.

Eric

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