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

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Mahesh Bandewar (महेश बंडेवार) (maheshb@google.com):
> On Tue, Nov 28, 2017 at 3:04 PM, Serge E. Hallyn <serge@hallyn.com> wrote:
> > Quoting Mahesh Bandewar (महेश बंडेवार) (maheshb@google.com):
> > ...
> >> >> 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?

I suppose if you did both (a) a kernel build and (b) a webserver
like https://github.com/m3ng9i/ran , being hit for a minute by a
heavy load of requests, those two together would be re-assuring.

thanks,
-serge