[Devel] Re: [PATCH] allow a task to join a pid namespace

[Eric W. Biederman]

Glauber Costa <glommer at parallels.com> writes:

> Currently, it is possible for a process  to join existing
> net, uts and ipc namespaces. This patch allows a process to join an
> existing pid namespace as well.
>
> For that to remain sane, some restrictions are made in the calling process:
>
> * It needs to be in the parent namespace of the namespace it wants to jump to
> * It needs to sit in its own session and group as a leader.
>
> The rationale for that, is that people want to trigger actions in a Container
> from the outside. For instance, mainstream linux recently gained the ability
> to safely reboot a container. It would be desirable, however, that this
> action is triggered from an admin in the outside world, very much like a
> power switch in a physical box.
>
> This would also allow us to connect a console to the container, provide a
> repair mode for setups without networking (or with a broken one), etc.
>
> Signed-off-by: Glauber Costa <glommer at parallels.com>
> CC: Serge Hallyn <serge.hallyn at canonical.com>
> CC: Oleg Nesterov <oleg at redhat.com>
> CC: Michael Kerrisk <mtk.manpages at gmail.com>
> CC: "Eric W. Biederman" <ebiederm at xmission.com>
> CC: Tejun Heo <tj at kernel.org>
> CC: Daniel Lezcano <daniel.lezcano at linaro.org>
> ---
>  fs/proc/namespaces.c    |    3 ++
>  include/linux/proc_fs.h |    1 +
>  kernel/pid_namespace.c  |   76 +++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 80 insertions(+)
>
> diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
> index 0d9e23a..6b52af5 100644
> --- a/fs/proc/namespaces.c
> +++ b/fs/proc/namespaces.c
> @@ -24,6 +24,9 @@ static const struct proc_ns_operations *ns_entries[] = {
>  #ifdef CONFIG_IPC_NS
>  	&ipcns_operations,
>  #endif
> +#ifdef CONFIG_PID_NS
> +	&pidns_operations,
> +#endif
>  };
>  
>  static const struct file_operations ns_file_operations = {
> diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
> index 3fd2e87..acaafcd 100644
> --- a/include/linux/proc_fs.h
> +++ b/include/linux/proc_fs.h
> @@ -251,6 +251,7 @@ struct proc_ns_operations {
>  extern const struct proc_ns_operations netns_operations;
>  extern const struct proc_ns_operations utsns_operations;
>  extern const struct proc_ns_operations ipcns_operations;
> +extern const struct proc_ns_operations pidns_operations;
>  
>  union proc_op {
>  	int (*proc_get_link)(struct dentry *, struct path *);
> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
> index 57bc1fd..c4555b9d 100644
> --- a/kernel/pid_namespace.c
> +++ b/kernel/pid_namespace.c
> @@ -258,3 +258,79 @@ static __init int pid_namespaces_init(void)
>  }
>  
>  __initcall(pid_namespaces_init);
> +
> +static void *pidns_get(struct task_struct *task)
> +{
> +	struct pid_namespace *pid = NULL;
> +	struct nsproxy *nsproxy;
> +
> +	rcu_read_lock();
> +	nsproxy = task_nsproxy(task);
> +	if (nsproxy)
> +		pid = get_pid_ns(nsproxy->pid_ns);
> +	rcu_read_unlock();
> +
> +	return pid;
> +}
> +
> +static void pidns_put(void *ns)
> +{
> +	put_pid_ns(ns);
> +}
> +
> +/*
> + * pid_ns' callback for setns
> + *
> + * this call switches current's pid_ns from nsproxy to ns.
> + * In order to do that successfully, we need to create a new pid living
> + * in the new namespace, and attach_pid() it.
> + *
> + * Because we don't want to deal with processes leaving their current
> + * namespace or being duplicate, it is mandatory that the namespace
> + * we're switching from is the parent of the namespace we are switching to.
> + * This is because in this scenario, a view of the pid exists there anyway.
> + *
> + * Caller must be group and session leader. This restriction guarantees
> + * that we won't mess with more than we should, like the controlling terminal
> + * in our host namespace, and ambiguities about who is the child reaper.
> + */
> +static int pidns_install(struct nsproxy *nsproxy, void *_ns)
> +{
> +	struct pid *newpid;
> +	struct pid_namespace *ns = _ns;
> +
> +	if (is_container_init(current))
> +		return -EINVAL;
> +
> +	if (nsproxy->pid_ns != ns->parent)
> +		return -EPERM;
> +
> +	if (task_pgrp(current) !=  task_pid(current))
> +		return -EPERM;
> +
> +	if (task_session(current) !=  task_pid(current))
> +		return -EPERM;
> +
> +	newpid = alloc_pid(ns);
> +	if (!newpid)
> +		return -ENOMEM;
> +
> +	put_pid_ns(nsproxy->pid_ns);
> +	nsproxy->pid_ns = get_pid_ns(ns);
> +
> +	write_lock_irq(&tasklist_lock);
> +	change_pid(current, PIDTYPE_PID, newpid);
> +	change_pid(current, PIDTYPE_PGID, newpid);
> +	change_pid(current, PIDTYPE_SID, newpid);
> +	write_unlock_irq(&tasklist_lock);

This part where you are changing the pid on an existing process
to something different is flat out wrong.

The practical issues include processes sending signals will no longer
find this process, not handling multithreaded processes and glibc
caching getpid and getting totally confusedl, and those are just
off the top of my head.

We do need to figure out a practical way of entering a pid namespace.

Eric