linux-kernel - [PATCH 6/9] Add Documentation/namespaces/user

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Date:	Tue, 18 Oct 2011 21:54:55 +0000
From:	Serge Hallyn <serge@...lyn.com>
To:	linux-kernel@...r.kernel.org
Cc:	ebiederm@...ssion.com, akpm@...ux-foundation.org, oleg@...hat.com,
	richard@....at, mikevs@...all.net, segoon@...nwall.com,
	gregkh@...e.de, dhowells@...hat.com, eparis@...hat.com,
	"Serge E. Hallyn" <serge@...lyn.com>,
	"Serge E. Hallyn" <serge.hallyn@...onical.com>,
	Randy Dunlap <rdunlap@...otime.net>
Subject: [PATCH 6/9] Add Documentation/namespaces/user_namespace.txt (v3)

From: "Serge E. Hallyn" <serge@...lyn.com>

Provide a description of user namespaces

Changelog:
   jul 26: incorporate feedback from David Howells.
   jul 29: incorporate feedback from Randy Dunlap.
   sep 15: remove information which is not yet certain.

Signed-off-by: Serge E. Hallyn <serge.hallyn@...onical.com>
Cc: Eric W. Biederman <ebiederm@...ssion.com>
Cc: David Howells <dhowells@...hat.com>
Cc: Randy Dunlap <rdunlap@...otime.net>
---
 Documentation/namespaces/user_namespace.txt |   51 +++++++++++++++++++++++++++
 1 files changed, 51 insertions(+), 0 deletions(-)
 create mode 100644 Documentation/namespaces/user_namespace.txt

diff --git a/Documentation/namespaces/user_namespace.txt b/Documentation/namespaces/user_namespace.txt
new file mode 100644
index 0000000..0ca6564
--- /dev/null
+++ b/Documentation/namespaces/user_namespace.txt
@@ -0,0 +1,51 @@
+Description
+===========
+
+Traditionally, each task is owned by a user ID (UID) and belongs to one or more
+groups (GID).  Both are simple numeric IDs, though userspace usually translates
+them to names.  The user namespace allows tasks to have different views of the
+UIDs and GIDs associated with tasks and other resources.  (See 'UID mapping'
+below for more.)
+
+The user namespace is a simple hierarchical one.  The system starts with all
+tasks belonging to the initial user namespace.  A task creates a new user
+namespace by passing the CLONE_NEWUSER flag to clone(2).  This requires the
+creating task to have the CAP_SETUID, CAP_SETGID, and CAP_CHOWN capabilities,
+but it does not need to be running as root.  The clone(2) call will result in a
+new task which to itself appears to be running as UID and GID 0, but to its
+creator seems to have the creator's credentials.
+
+To this new task, any resource belonging to the initial user namespace will
+appear to belong to user and group 'nobody', which are UID and GID -1.
+Permission to open such files will be granted according to world access
+permissions.  UID comparisons and group membership checks will return false,
+and privilege will be denied.
+
+When a task belonging to (for example) userid 500 in the initial user namespace
+creates a new user namespace, even though the new task will see itself as
+belonging to UID 0, any task in the initial user namespace will see it as
+belonging to UID 500.  Therefore, UID 500 in the initial user namespace will be
+able to kill the new task.
+
+Userid mapping for the VFS is not yet implemented, though prototypes exist.
+
+Relationship between the User namespace and other namespaces
+============================================================
+
+Other namespaces, such as UTS and network, are owned by a user namespace.  When
+such a namespace is created, it is assigned to the user namespace of the task
+by which it was created.  Therefore, attempts to exercise privilege to
+resources in, for instance, a particular network namespace, can be properly
+validated by checking whether the caller has the needed privilege (i.e.
+CAP_NET_ADMIN) targeted to the user namespace which owns the network namespace.
+This is done using the ns_capable() function.
+
+As an example, if a new task is cloned with a private user namespace but
+no private network namespace, then the task's network namespace is owned
+by the parent user namespace.  The new task has no privilege to the
+parent user namespace, so it will not be able to create or configure
+network devices.  If, instead, the task were cloned with both private
+user and network namespaces, then the private network namespace is owned
+by the private user namespace, and so root in the new user namespace
+will have privilege targeted to the network namespace.  It will be able
+to create and configure network devices.
-- 
1.7.5.4

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