Re: [PATCHv2 2/2] userns: control capabilities of some user namespaces

[Mahesh Bandewar (àààà ààààààà)]

On Tue, Nov 28, 2017 at 3:04 PM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> Quoting Mahesh Bandewar (àààà ààààààà) (maheshb@xxxxxxxxxx):
> ...
>> >> diff --git a/security/commoncap.c b/security/commoncap.c
>> >> index fc46f5b85251..89103f16ac37 100644
>> >> --- a/security/commoncap.c
>> >> +++ b/security/commoncap.c
>> >> @@ -73,6 +73,14 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
>> >>  {
>> >>       struct user_namespace *ns = targ_ns;
>> >>
>> >> +     /* If the capability is controlled and user-ns that process
>> >> +      * belongs-to is 'controlled' then return EPERM and no need
>> >> +      * to check the user-ns hierarchy.
>> >> +      */
>> >> +     if (is_user_ns_controlled(cred->user_ns) &&
>> >> +         is_capability_controlled(cap))
>> >> +             return -EPERM;
>> >
>> > I'd be curious to see the performance impact on this on a regular
>> > workload (kernel build?) in a controlled ns.
>> >
>> Should it affect? If at all, it should be +ve since, the recursive
>> user-ns hierarchy lookup is avoided with the above check if the
>> capability is controlled.
>
> Yes but I expect that to be the rare case for normal lxc installs
> (which are of course what I am interested in)
>
>>  The additional cost otherwise is this check
>> per cap_capable() call.
>
> And pipeline refetching?
>
> Capability calls also shouldn't be all that frequent, but still I'm
> left wondering...

Correct, and capability checks are part of the control-path and not
the data-path so shouldn't matter but I guess it doesn't hurt to
find-out the number. Do you have any workload in mind, that we can use
for this test/benchmark?