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

[ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)]

Richard Weinberger <richard-/L3Ra7n9ekc@public.gmane.org> writes:

> Am 08.12.2014 um 23:25 schrieb Andy Lutomirski:
>> On Mon, Dec 8, 2014 at 2:17 PM, Richard Weinberger <richard-/L3Ra7n9ekc@public.gmane.org> wrote:
>>> 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?
>>>
>> 
>> See my other comment -- the ordering will matter at the end of the series.
>
> But ns->gid_map.nr_extents is not atomic, it is a plain u32.
> This confuses me.

Read Documentation/atomic_ops.txt a plain u32 is atomic by definiton.

Which is a little bit convoluted.  However that is part of the of the
gid mapping path and I optimized that as far as I humanly could so that
calls like stat don't take a noticable slow donw.

On this path we don't particularly care except that I am using an the
existing data structure.

Eric