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 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?

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.
>  
>  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.


>  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.

> +		return -EINVAL;
> +
> +	pid_ptr = &vec->pid[0];

All of the rest of the vector needs to be read in, in one go.

> +	do {
> +		if (!ns_capable(pid_ns->user_ns, CAP_SYS_ADMIN))
> +			return -EPERM;

Permission to change all of the namespaces should be checked before
writing to pid_ns->last_pid happens.
> +
> +		if (get_user(pid, pid_ptr))
> +			return -EFAULT;
> +		if (pid < 0 || pid > pid_max)
> +			return -EINVAL;
> +
> +		/* Write directly: see the comment in pid_ns_ctl_handler() */
> +		pid_ns->last_pid = pid;
> +
> +		pid_ns = pid_ns->parent;
> +		pid_ptr++;
> +	} while (--nr > 0 && pid_ns);
> +
> +	return nr ? -EINVAL : 0;
> +}
> +#endif	/* CONFIG_CHECKPOINT_RESTORE */
> +
>  static struct user_namespace *pidns_owner(struct ns_common *ns)
>  {
>  	return to_pid_ns(ns)->user_ns;