Re: [CFT][PATCH 2/7] userns: Don't allow setgroups until a gid mapping has been setablished

[Richard Weinberger <richard-/L3Ra7n9ekc@public.gmane.org>]

Am 08.12.2014 um 23:07 schrieb Eric W. Biederman:
> 
> setgroups is unique in not needing a valid mapping before it can be called,
> in the case of setgroups(0, NULL) which drops all supplemental groups.
> 
> The design of the user namespace assumes that CAP_SETGID can not actually
> be used until a gid mapping is established.  Therefore add a helper function
> to see if the user namespace gid mapping has been established and call
> that function in the setgroups permission check.
> 
> This is part of the fix for CVE-2014-8989, being able to drop groups
> without privilege using user namespaces.
> 
> Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> Signed-off-by: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
> ---
>  include/linux/user_namespace.h | 9 +++++++++
>  kernel/groups.c                | 7 ++++++-
>  2 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index e95372654f09..41cc26e5a350 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -37,6 +37,15 @@ struct user_namespace {
>  
>  extern struct user_namespace init_user_ns;
>  
> +static inline bool userns_gid_mappings_established(const struct user_namespace *ns)
> +{
> +	bool established;
> +	smp_mb__before_atomic();
> +	established = ACCESS_ONCE(ns->gid_map.nr_extents) != 0;
> +	smp_mb__after_atomic();
> +	return established;
> +}
> +

Maybe this is a stupid question, but why do we need all this magic
around established =  ... ?
The purpose of this code is to check whether ns->gid_map.nr_extents != 0
in a lock-free manner?

Thanks,
//richard