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SubjectRe: [PATCH 17/19] sched: Inherit task cookie on fork()
On Mon, May 10, 2021 at 12:23 PM Chris Hyser <chris.hyser@oracle.com> 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@infradead.org> 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@infradead.org>
> >> ---
> >> 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

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