Re: [PATCH -next v3 3/3] blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()

[Tejun Heo]

On Thu, Jan 19, 2023 at 07:03:50PM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
> 
> Currently parent pd can be freed before child pd:
> 
> t1: remove cgroup C1
> blkcg_destroy_blkgs
>  blkg_destroy
>   list_del_init(&blkg->q_node)
>   // remove blkg from queue list
>   percpu_ref_kill(&blkg->refcnt)
>    blkg_release
>     call_rcu
> 
> t2: from t1
> __blkg_release
>  blkg_free
>   schedule_work
> 			t4: deactivate policy
> 			blkcg_deactivate_policy
> 			 pd_free_fn
> 			 // parent of C1 is freed first
> t3: from t2
>  blkg_free_workfn
>   pd_free_fn
> 
> If policy(for example, ioc_timer_fn() from iocost) access parent pd from
> child pd after pd_offline_fn(), then UAF can be triggered.
> 
> Fix the problem by delaying 'list_del_init(&blkg->q_node)' from
> blkg_destroy() to blkg_free_workfn(), and using a new disk level mutex to
> synchronize blkg_free_workfn() and blkcg_deactivate_policy().
> 
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun