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

[Eric W. Biederman]

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.

Eric