linux-kernel - Re: [PATCH -next v2 1/3] blk-cgroup: dropping parent refcount after pd_free

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Date:   Wed, 18 Jan 2023 06:54:41 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     hch@....de, josef@...icpanda.com, axboe@...nel.dk,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yukuai3@...wei.com,
        yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH -next v2 1/3] blk-cgroup: dropping parent refcount after
 pd_free_fn() is done

On Wed, Jan 18, 2023 at 08:31:50PM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@...wei.com>
> 
> Some cgroup policies will access parent pd through child pd even
> after pd_offline_fn() is done. If pd_free_fn() for parent is called
> before child, then UAF can be triggered. Hence it's better to guarantee
> the order of pd_free_fn().
> 
> Currently refcount of parent blkg is dropped in __blkg_release(), which
> is before pd_free_fn() is called in blkg_free_work_fn() while
> blkg_free_work_fn() is called asynchronously.
> 
> This patch make sure pd_free_fn() called from removing cgroup is ordered
> by delaying dropping parent refcount after calling pd_free_fn() for
> child.
> 
> BTW, pd_free_fn() will also be called from blkcg_deactivate_policy()
> from deleting device, and following patches will guarantee the order.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun