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SubjectRe: [PATCH 1/1] introduce user_ns inheritance in user-sched
On Thu, Mar 19, 2009 at 04:16:15PM -0500, Serge E. Hallyn wrote:
> In a kernel compiled with CONFIG_USER_SCHED=y, cpu shares are
> allocated according to uid. Shares are specifiable under
> /sys/kernel/uids/<uid>/
>
> In a kernel compiled with CONFIG_USER_NS=y, clone(2) with the
> CLONE_NEWUSER flag creates a new user namespace, and the newly
> cloned task will belong to uid 0 in the new user namespace.
>
> Without this patch, if uid 500 calls clone(CLONE_NEWUSER) (which
> is possible using a program with the cap_sys_admin,cap_setuid,cap_setgid=pe
> file capabilities), then the new task will get the cpu shares of
> uid 0.
>
> After this patch, if uid 500 calls clone(CLONE_NEWUSER), then even
> though it is uid 0 in the new user namespace, it will be restricted to
> the cpu shares of uid 500.
>
> Currently there is no way to set shares for uids in user namespaces
> other than the initial one. That will be trivial to add when
> sysfs tagging (or its functional equivalent, also needed to
> expose network devices in network namespaces other than init)
> becomes available.
>
> Until cross-user-namespace file accesses are enforced, nothing
> stops uid 0 in a child namespace from simply writing new values
> into /sys/kernel/uids/500.
>
> Here are results of some testing with and without the patch.
>
> Cpu shares are initialized as follows::
> user root: 2048
> user hallyn: 1024
> user serge: 512
>
> Results are the 'real' part of time make -j4 > o 2>&1,
> each time after a make clean.
>
> =================================================================
> UNPATCHED
> User 1: user serge creates a child user_ns and runs as user root
> User 2: hallyn runs as user hallyn
> =================================================================
> User 1 User 2
> run 1: 2m58.834s 3m0.609s
> run 2: 2m59.248s 2m59.457s
>
> =============================================================
> PATCHED
> User 1: user serge
> User 2: user hallyn
> =============================================================
>
> User 1 User 2
> run 1: 3m6.337s 2m22.681s
> run 2: 3m6.323s 2m21.855s
>
> =============================================================
> PATCHED
> User 1: user serge setuid to user root
> User 2: hallyn
> =============================================================
>
> User 1 User 2
> run 1: 2m17.782s 3m3.947s
> run 2: 2m18.497s 3m7.961s
>
> ==========================================================
> PATCHED
> User 1: user root inside userns created by userid serge
> User 2: hallyn
> ==========================================================
>
> User 1 User 2
> run 1: 3m9.876s 2m8.428s
> run 2: 3m8.539s 2m6.356s
>
> Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
> Signed-off-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
> Cc: mingo@elte.hu
> Cc: Bharata B Rao <bharata@linux.vnet.ibm.com>
> Cc: peterz@infradead.org
> ---
> kernel/user.c | 12 +++++++++---
> kernel/user_namespace.c | 2 +-
> 2 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/user.c b/kernel/user.c
> index 850e0ba..53aeea2 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -101,7 +101,12 @@ static int sched_create_user(struct user_struct *up)
> {
> int rc = 0;
>
> - up->tg = sched_create_group(&root_task_group);
> + struct task_group *parent = &root_task_group;
> +
> + if (up->user_ns != &init_user_ns)
> + parent = up->user_ns->creator->tg;
> +
> + up->tg = sched_create_group(parent);
> if (IS_ERR(up->tg))
> rc = -ENOMEM;
>
> @@ -434,11 +439,11 @@ struct user_struct *alloc_uid(struct user_namespace *ns, uid_t uid)
> new->uid = uid;
> atomic_set(&new->__count, 1);
>
> + new->user_ns = get_user_ns(ns);
> +
> if (sched_create_user(new) < 0)
> goto out_free_user;
>
> - new->user_ns = get_user_ns(ns);
> -
> if (uids_user_create(new))
> goto out_destoy_sched;
>
> @@ -472,6 +477,7 @@ out_destoy_sched:
> sched_destroy_user(new);
> put_user_ns(new->user_ns);

Shouldn't this put_user_ns(new->user_ns) be removed? It looks like two
references to new->user_ns are being dropped if anything fails
after sched_create_user(new) succeeds yet as far as I can tell the
patch does not introduce any new references to new->user_ns.

Otherwise looks good to me.

Cheers,
-Matt Helsley


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