[PATCH 08/11] pidns: Deny strange cases when creating pid namespaces.

["Eric W. Biederman" <ebiederm@xmission.com>]

From: "Eric W. Biederman" <ebiederm@xmission.com>

task_active_pid_ns(current) != current->ns_proxy->pid_ns will
soon be allowed to support unshare and setns.

The definition of creating a child pid namespace when
task_active_pid_ns(current) != current->ns_proxy->pid_ns could be that
we create a child pid namespace of current->ns_proxy->pid_ns.  However
that leads to strange cases like trying to have a single process be
init in multiple pid namespaces, which is racy and hard to think
about.

The definition of creating a child pid namespace when
task_active_pid_ns(current) != current->ns_proxy->pid_ns could be that
we create a child pid namespace of task_active_pid_ns(current).  While
that seems less racy it does not provide any utility.

Therefore define the semantics of creating a child pid namespace when
task_active_pid_ns(current) != current->ns_proxy->pid_ns to be that the
pid namespace creation fails.  That is easy to implement and easy
to think about.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/pid_namespace.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index e106b18..066c058 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -145,6 +145,8 @@ struct pid_namespace *copy_pid_ns(unsigned long flags,
 		return get_pid_ns(old_ns);
 	if (flags & (CLONE_THREAD|CLONE_PARENT))
 		return ERR_PTR(-EINVAL);
+	if (task_active_pid_ns(current) != old_ns)
+		return ERR_PTR(-EINVAL);
 	return create_pid_namespace(user_ns, old_ns);
 }
 
-- 
1.7.5.4