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

[Joel Fernandes]

On Mon, May 10, 2021 at 12:23 PM Chris Hyser <chris.hyser@xxxxxxxxxx> wrote:
>
> On 5/10/21 12:06 PM, Joel Fernandes wrote:
> > Hi Peter,
> >
> > On Thu, Apr 22, 2021 at 8:36 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >>
> >> Note that sched_core_fork() is called from under tasklist_lock, and
> >> not from sched_fork() earlier. This avoids a few races later.
> >>
> >> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> >> ---
> >>   include/linux/sched.h     |    2 ++
> >>   kernel/fork.c             |    3 +++
> >>   kernel/sched/core_sched.c |    6 ++++++
> >>   3 files changed, 11 insertions(+)
> >>
> >> --- a/include/linux/sched.h
> >> +++ b/include/linux/sched.h
> >> @@ -2172,8 +2172,10 @@ const struct cpumask *sched_trace_rd_spa
> >>
> >>   #ifdef CONFIG_SCHED_CORE
> >>   extern void sched_core_free(struct task_struct *tsk);
> >> +extern void sched_core_fork(struct task_struct *p);
> >>   #else
> >>   static inline void sched_core_free(struct task_struct *tsk) { }
> >> +static inline void sched_core_fork(struct task_struct *p) { }
> >>   #endif
> >>
> >>   #endif
> >> --- a/kernel/fork.c
> >> +++ b/kernel/fork.c
> >> @@ -2249,6 +2249,8 @@ static __latent_entropy struct task_stru
> >>
> >>          klp_copy_process(p);
> >>
> >> +       sched_core_fork(p);
> >> +
> >>          spin_lock(&current->sighand->siglock);
> >>
> >>          /*
> >> @@ -2336,6 +2338,7 @@ static __latent_entropy struct task_stru
> >>          return p;
> >>
> >>   bad_fork_cancel_cgroup:
> >> +       sched_core_free(p);
> >>          spin_unlock(&current->sighand->siglock);
> >>          write_unlock_irq(&tasklist_lock);
> >>          cgroup_cancel_fork(p, args);
> >> --- a/kernel/sched/core_sched.c
> >> +++ b/kernel/sched/core_sched.c
> >> @@ -100,6 +100,12 @@ static unsigned long sched_core_clone_co
> >>          return cookie;
> >>   }
> >>
> >> +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() ?

- Joel