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@xxxxxxxxxxxxxxx
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---
> 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
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