Re: [PATCH 17/19] sched: Inherit task cookie on fork()

[Chris Hyser]

On 5/10/21 4:47 PM, Joel Fernandes wrote:
> On Mon, May 10, 2021 at 12:23 PM Chris Hyser <chris.hyser@xxxxxxxxxx> wrote:

> > > > +void sched_core_fork(struct task_struct *p)
> > > > +{
> > > > +       RB_CLEAR_NODE(&p->core_node);
> > > > +       p->core_cookie = sched_core_clone_cookie(current);
> > >
> > > Does this make sense also for !CLONE_THREAD forks?
> >
> > Yes. Given the absence of a cgroup interface, fork inheritance (clone the cookie) is the best way to create shared
> > cookie hierarchies. The security issue you mentioned was handled in my original code by setting a unique cookie on
> > 'exec', but Peter took that out for the reason mentioned above. It was part of the "lets get this in compromise" effort.
>
> Thanks for sharing the history of it. I guess one can argue that this
> policy is better to be hardcoded in userspace since core-scheduling
> can be used for non-security usecases as well. Maybe one could simply
> call the prctl(2) from userspace if they so desire, before calling
> exec() ?

I think the defining use case is a container's init. If the cookie is set for it by the container creator and without 
any other user code knowing about core_sched, every descendant spawned will have the same cookie and be in the same 
core_sched group much like the cgroup interface had provided. If we create a unique cookie in the kernel either on fork 
or exec, we are secure, but we will now have 1000's of core sched groups.

CLEAR was also removed (temporarily, I hope) because a core_sched knowledgeable program in the example core_sched 
container group should not be able to remove itself from _all_ core sched groups. It can modify it's cookie, but that is 
no different than the normal case.

Both of these beg for a kernel policy, but that discussion was TBD.

-chrish