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SubjectRe: [CFT][PATCH 6/7] userns: Add a knob to disable setgroups on a per user namespace basis
On Mon, Dec 8, 2014 at 2:44 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
> Andy Lutomirski <luto@amacapital.net> writes:
>
>> On Mon, Dec 8, 2014 at 2:11 PM, Eric W. Biederman <ebiederm@xmission.com> wrote:
>>>
>>> - Expose the knob to user space through a proc file /proc/<pid>/setgroups
>>>
>>> A value of 0 means the setgroups system call is disabled in the
>>
>> "deny"
>>
>>> current processes user namespace and can not be enabled in the
>>> future in this user namespace.
>>>
>>> A value of 1 means the segtoups system call is enabled.
>>>
>>
>> "allow"
>>
>>> - Descedent user namespaces inherit the value of setgroups from
>>
>> s/Descedent/Descendent/
>
> Bah. I updated everything but the changelog comment.
>
>>> --- a/kernel/groups.c
>>> +++ b/kernel/groups.c
>>> @@ -222,6 +222,7 @@ bool may_setgroups(void)
>>> * the user namespace has been established.
>>> */
>>> return userns_gid_mappings_established(user_ns) &&
>>> + userns_setgroups_allowed(user_ns) &&
>>> ns_capable(user_ns, CAP_SETGID);
>>> }
>>
>> Can you add a comment explaining the ordering? For example:
>
> I need to think on what I can say to make it clear.
> Perhaps: /* Careful the order of these checks is important. */
>
>> We need to check for a gid mapping before checking setgroups_allowed
>> because an unprivileged user can create a userns with setgroups
>> allowed, then disallow setgroups and add a mapping. If we check in
>> the opposite order, then we have a race: we could see that setgroups
>> is allowed before the user clears the bit and then see that there is a
>> gid mapping after the other thread is done.
>

This text was actually my suggested comment text.

If you put smp_rmb() in this function with a comment like that, then I
think it will all make sense and be obviously correct (even with most
of the other barriers removed).

--Andy

> Since these are independent atomic variables yes that ordering issue
> seems to be the case.
>
> For me it was the natural ordering of the checks so I didn't even bother
> to think about what happens when you reorder them.
>
> Eric



--
Andy Lutomirski
AMA Capital Management, LLC


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