From: Serge E. Hallyn <serge@xxxxxxxxxx> 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. Oct 26: add changes suggested by David on Oct 19 Signed-off-by: Serge E. Hallyn <serge.hallyn@xxxxxxxxxxxxx> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx> Cc: David Howells <dhowells@xxxxxxxxxx> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxx> --- Documentation/namespaces/user_namespace.txt | 54 +++++++++++++++++++++++++++ 1 files changed, 54 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..805f0ef --- /dev/null +++ b/Documentation/namespaces/user_namespace.txt @@ -0,0 +1,54 @@ +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 fail, and +privilege will be denied. + +When a task belonging to (for example) UID 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. + +UID 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 over a +resource can be properly validated by checking for privilege targeted to the +user namespace which owns the resource. For instance, a check for the special +privilege to change a network interface address could be done by checking for +CAP_NET_ADMIN against the user namespace which created the network namespace +owning the network interface. + +( XXX TODO: add a list of capabilities corresponding to different namespaces) + +As an example, if a new task is cloned with a private user namespace but +not a private network namespace, then the task's network namespace is owned +by the parent user namespace. The new task has no special privilege over the +parent user namespace, so it will not be able to create or configure the +network devices therein. 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 over resources owned by the network namespace. It will +be able to create and configure network devices. -- 1.7.0.4 _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
