linux-kernel - Re: [PATCH 1/1] introduce user

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Date:	Thu, 19 Mar 2009 16:55:03 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>, mingo@...e.hu,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	peterz@...radead.org, Linux Containers <containers@...ts.osdl.org>
Subject: Re: [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@...ibm.com>
> Signed-off-by: Dhaval Giani <dhaval@...ux.vnet.ibm.com>
> Cc: mingo@...e.hu
> Cc: Bharata B Rao <bharata@...ux.vnet.ibm.com>
> Cc: peterz@...radead.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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