Re: [PATCH 1/2] user namespaces: let user_ns be cloned with fairsched

James Morris
2008-12-08T21:15:34Z

On Mon, 8 Dec 2008, Serge E. Hallyn wrote:

> > Applied to
> > git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/security-testing-2.6#next
>
> Thanks, James. I talked about patch 1 with Dhaval, and while he's ok
> with the patch he (rightfully) thought there should be some extra
> documentation. If it's not too much trouble would you mind swapping
> out patch 1 for the following? (Otherwise I can send a new patch on
> top of the original)

It was applied to a public tree, so an update patch is needed.

>
> thanks,
> -serge
>
> >From 047b66fff5e014ac0eb995b8a60ff396abe2e8b2 Mon Sep 17 00:00:00 2001
> From: Serge E. Hallyn <serue@us.ibm.com>
> Date: Mon, 8 Dec 2008 07:24:33 -0800
> Subject: [PATCH 1/1] user namespaces: let user_ns be cloned with fairsched
>
> fairsched creates a per-uid directory under /sys/kernel/uids/.
> So when you clone(CLONE_NEWUSER), it tries to create
> /sys/kernel/uids/0, which already exists, and you get back
> -ENOMEM.
>
> This was supposed to be fixed by sysfs tagging, but that
> was postponed (ok, rejected until sysfs locking is fixed).
> So, just as with network namespaces, we just don't create
> those directories for user namespaces other than the init.
>
> Changelog:
> Dec 8 2008: Documented the currently bogus state of
> support for user groups with user namespaces. In
> particular, all users in a user namespace should be
> children of the user which created the user namespace.
> This is yet to be unimplemented.
>
> Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
> Acked-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
> ---
> Documentation/scheduler/sched-design-CFS.txt | 21 +++++++++++++++++++++
> kernel/user.c | 12 +++++++++++-
> 2 files changed, 32 insertions(+), 1 deletions(-)
>
> diff --git a/Documentation/scheduler/sched-design-CFS.txt b/Documentation/scheduler/sched-design-CFS.txt
> index eb471c7..8398ca4 100644
> --- a/Documentation/scheduler/sched-design-CFS.txt
> +++ b/Documentation/scheduler/sched-design-CFS.txt
> @@ -273,3 +273,24 @@ task groups and modify their CPU share using the "cgroups" pseudo filesystem.
>
> # #Launch gmplayer (or your favourite movie player)
> # echo <movie_player_pid> > multimedia/tasks
> +
> +8. Implementation note: user namespaces
> +
> +User namespaces are intended to be hierarchical. But they are currently
> +only partially implemented. Each of those has ramifications for CFS.
> +
> +First, since user namespaces are hierarchical, the /sys/kernel/uids
> +presentation is inadequate. Eventually we will likely want to use sysfs
> +tagging to provide private views of /sys/kernel/uids within each user
> +namespace.
> +
> +Second, the hierarchical nature is intended to support completely
> +unprivileged use of user namespaces. So if using user groups, then
> +we want the users in a user namespace to be children of the user
> +who created it.
> +
> +That is currently unimplemented. So instead, every user in a new
> +user namespace will receive 1024 shares just like any user in the
> +initial user namespace. Note that at the moment creation of a new
> +user namespace requires each of CAP_SYS_ADMIN, CAP_SETUID, and
> +CAP_SETGID.
> diff --git a/kernel/user.c b/kernel/user.c
> index 97202cb..6608a3d 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -239,13 +239,21 @@ static struct kobj_type uids_ktype = {
> .release = uids_release,
> };
>
> -/* create /sys/kernel/uids/<uid>/cpu_share file for this user */
> +/*
> + * Create /sys/kernel/uids/<uid>/cpu_share file for this user
> + * We do not create this file for users in a user namespace (until
> + * sysfs tagging is implemented).
> + *
> + * See Documentation/scheduler/sched-design-CFS.txt for ramifications.
> + */
> static int uids_user_create(struct user_struct *up)
> {
> struct kobject *kobj = &up->kobj;
> int error;
>
> memset(kobj, 0, sizeof(struct kobject));
> + if (up->user_ns != &init_user_ns)
> + return 0;
> kobj->kset = uids_kset;
> error = kobject_init_and_add(kobj, &uids_ktype, NULL, "%d", up->uid);
> if (error) {
> @@ -281,6 +289,8 @@ static void remove_user_sysfs_dir(struct work_struct *w)
> unsigned long flags;
> int remove_user = 0;
>
> + if (up->user_ns != &init_user_ns)
> + return;
> /* Make uid_hash_remove() + sysfs_remove_file() + kobject_del()
> * atomic.
> */
> --
> 1.5.4.3
>
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--
James Morris
<jmorris@namei.org>