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

[Richard Weinberger]

Am 08.12.2014 um 23:25 schrieb Andy Lutomirski:
> On Mon, Dec 8, 2014 at 2:17 PM, Richard Weinberger <richard at nod.at> 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 at vger.kernel.org
>>> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
>>> ---
>>>  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.

Thanks,
//richard