Re: [PATCH v3] pid_ns: Introduce ioctl to set vector of ns_last_pid's on ns hierarhy

[Kirill Tkhai]

On 29.04.2017 22:12, Eric W. Biederman wrote:
> Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> writes:
> 
>> On 27.04.2017 19:07, Eric W. Biederman wrote:
>>> Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> writes:
>>>
>>>> On 27.04.2017 18:15, Eric W. Biederman wrote:
>>>>> Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>>>>>> ---
>>>>>>  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.