Re: [PATCH 1/1] cgroup: Fix race condition at rebind_subsystems()

[Mukesh Ojha]

On 8/23/2022 11:11 AM, Jing-Ting Wu wrote:
> Root cause:
> The rebind_subsystems() is no lock held when move css object from A
> list to B list,then let B's head be treated as css node at
> list_for_each_entry_rcu().
>
> Solution:
> Add grace period before invalidating the removed rstat_css_node.
>
> Reported-by: Jing-Ting Wu <jing-ting.wu@xxxxxxxxxxxx>
> Suggested-by: Michal Koutný <mkoutny@xxxxxxxx>
> Signed-off-by: Jing-Ting Wu <jing-ting.wu@xxxxxxxxxxxx>
> Tested-by: Jing-Ting Wu <jing-ting.wu@xxxxxxxxxxxx>
> Link: https://lore.kernel.org/linux-arm-kernel/d8f0bc5e2fb6ed259f9334c83279b4c011283c41.camel@xxxxxxxxxxxx/T/
> ---
>   kernel/cgroup/cgroup.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index ffaccd6373f1..0d0c959966ed 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -1820,6 +1820,7 @@ int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
>   
>   		if (ss->css_rstat_flush) {
>   			list_del_rcu(&css->rstat_css_node);
> +			synchronize_rcu();
>   			list_add_rcu(&css->rstat_css_node,
>   				     &dcgrp->rstat_css_list);
>   		}

Good catch.

Acked-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>

-Mukesh