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

[Chris Hyser]

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.

-chrish