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

[Joel Fernandes]

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")

The issue happened when 5f3c8d620447 started looking at multiple
channels at the same time in the same work handler function.

I think a better fix might be to separate out the work handler
functions for each channel separately. That way we get more
parallelism.

but since this is urgent,
Acked-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

thanks,

 - Joel



> ---
>  kernel/rcu/tree.c | 27 +++++++++++++++++++--------
>  1 file changed, 19 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8e880c0..7b95ee9 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3024,6 +3024,18 @@ static void kfree_rcu_work(struct work_struct *work)
>         return !!READ_ONCE(krcp->head);
>  }
>
> +static bool
> +need_wait_for_krwp_work(struct kfree_rcu_cpu_work *krwp)
> +{
> +       int i;
> +
> +       for (i = 0; i < FREE_N_CHANNELS; i++)
> +               if (!list_empty(&krwp->bulk_head_free[i]))
> +                       return true;
> +
> +       return !!krwp->head_free;
> +}
> +
>  static int krc_count(struct kfree_rcu_cpu *krcp)
>  {
>         int sum = atomic_read(&krcp->head_count);
> @@ -3107,15 +3119,14 @@ static void kfree_rcu_monitor(struct work_struct *work)
>         for (i = 0; i < KFREE_N_BATCHES; i++) {
>                 struct kfree_rcu_cpu_work *krwp = &(krcp->krw_arr[i]);
>
> -               // Try to detach bulk_head or head and attach it over any
> -               // available corresponding free channel. It can be that
> -               // a previous RCU batch is in progress, it means that
> -               // immediately to queue another one is not possible so
> -               // in that case the monitor work is rearmed.
> -               if ((!list_empty(&krcp->bulk_head[0]) && list_empty(&krwp->bulk_head_free[0])) ||
> -                       (!list_empty(&krcp->bulk_head[1]) && list_empty(&krwp->bulk_head_free[1])) ||
> -                               (READ_ONCE(krcp->head) && !krwp->head_free)) {
> +               // Try to detach bulk_head or head and attach it, only when
> +               // all channels are free.  Any channel is not free means at krwp
> +               // there is on-going rcu work to handle krwp's free business.
> +               if (need_wait_for_krwp_work(krwp))
> +                       continue;
>
> +               // kvfree_rcu_drain_ready() might handle this krcp, if so give up.
> +               if (need_offload_krc(krcp)) {
>                         // Channel 1 corresponds to the SLAB-pointer bulk path.
>                         // Channel 2 corresponds to vmalloc-pointer bulk path.
>                         for (j = 0; j < FREE_N_CHANNELS; j++) {
> --
> 1.9.1
>