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

[Andy Lutomirski]

On Mon, Dec 8, 2014 at 2:39 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
> Richard Weinberger <richard@xxxxxx> writes:
>
>> Am 08.12.2014 um 23:25 schrieb Andy Lutomirski:
>>> On Mon, Dec 8, 2014 at 2:17 PM, Richard Weinberger <richard@xxxxxx> 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@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?
>>>>
>>>
>>> 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.
>

I still don't understand why the helper changed to smp_mb__before_atomic.

> 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.

As an example, arm64 defines both smp_mb__before_atomic and
smp_mb__after_atomic as smp_mb(), which is heavier then smp_rmb(), and
there are two of them.  So I still like the explicit smp_rmb() better.

--Andy

>
> Eric
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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