Re: [PATCH V2] rcu: Make sure new krcp free business is handled after the wanted rcu grace period.

[Joel Fernandes]

On Wed, Apr 5, 2023 at 1:39 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> On Fri, Mar 31, 2023 at 8:43 AM Ziwei Dai <ziwei.dai@xxxxxxxxxx> wrote:
> >
> > In kfree_rcu_monitor(), new free business at krcp is attached to any free
> > channel at krwp. kfree_rcu_monitor() is responsible to make sure new free
> > business is handled after the rcu grace period. But if there is any none-free
> > channel at krwp already, that means there is an on-going rcu work,
> > which will cause the kvfree_call_rcu()-triggered free business is done
> > before the wanted rcu grace period ends.
> >
> > This commit ignore krwp which has non-free channel at kfree_rcu_monitor(),
> > to fix the issue that kvfree_call_rcu() loses effectiveness.
> >
> > Below is the css_set obj "from_cset" use-after-free case caused by
> > kvfree_call_rcu() losing effectiveness.
> > CPU 0 calls rcu_read_lock(), then use "from_cset", then hard irq comes,
> > the task is schedule out.
> > CPU 1 calls kfree_rcu(cset, rcu_head), willing to free "from_cset" after new gp.
> > But "from_cset" is freed right after current gp end. "from_cset" is reallocated.
> > CPU 0 's task arrives back, references "from_cset"'s member, which causes crash.
> >
> > CPU 0                                   CPU 1
> > count_memcg_event_mm()
> > |rcu_read_lock()  <---
> > |mem_cgroup_from_task()
> >  |// css_set_ptr is the "from_cset" mentioned on CPU 1
> >  |css_set_ptr = rcu_dereference((task)->cgroups)
> >  |// Hard irq comes, current task is scheduled out.
> >
> >                                         cgroup_attach_task()
> >                                         |cgroup_migrate()
> >                                         |cgroup_migrate_execute()
> >                                         |css_set_move_task(task, from_cset, to_cset, true)
> >                                         |cgroup_move_task(task, to_cset)
> >                                         |rcu_assign_pointer(.., to_cset)
> >                                         |...
> >                                         |cgroup_migrate_finish()
> >                                         |put_css_set_locked(from_cset)
> >                                         |from_cset->refcount return 0
> >                                         |kfree_rcu(cset, rcu_head) // means to free from_cset after new gp
> >                                         |add_ptr_to_bulk_krc_lock()
> >                                         |schedule_delayed_work(&krcp->monitor_work, ..)
> >
> >                                         kfree_rcu_monitor()
> >                                         |krcp->bulk_head[0]'s work attached to krwp->bulk_head_free[]
> >                                         |queue_rcu_work(system_wq, &krwp->rcu_work)
> >                                         |if rwork->rcu.work is not in WORK_STRUCT_PENDING_BIT state,
> >                                         |call_rcu(&rwork->rcu, rcu_work_rcufn) <--- request a new gp
> >
> >                                         // There is a perious call_rcu(.., rcu_work_rcufn)
> >                                         // gp end, rcu_work_rcufn() is called.
> >                                         rcu_work_rcufn()
> >                                         |__queue_work(.., rwork->wq, &rwork->work);
> >
> >                                         |kfree_rcu_work()
> >                                         |krwp->bulk_head_free[0] bulk is freed before new gp end!!!
> >                                         |The "from_cset" is freed before new gp end.
> >
> > // the task is scheduled in after many ms.
> >  |css_set_ptr->subsys[(subsys_id) <--- Caused kernel crash, because css_set_ptr is freed.
> >
> > v2: Use helper function instead of inserted code block at kfree_rcu_monitor().
> >
> > Fixes: c014efeef76a ("rcu: Add multiple in-flight batches of kfree_rcu() work")
> > Signed-off-by: Ziwei Dai <ziwei.dai@xxxxxxxxxx>
>
> Please update the fixes tag to:
> 5f3c8d620447 ("rcu/tree: Maintain separate array for vmalloc ptrs")

Vlad pointed out in another thread that the fix is actually to 34c881745549.

So just to be sure, it could be updated to:
Fixes: 34c881745549 ("rcu: Support kfree_bulk() interface in kfree_rcu()")
Fixes: 5f3c8d620447 ("rcu/tree: Maintain separate array for vmalloc ptrs")

thanks,

 - Joel