Commit c7241bab authored by Yu Kuai's avatar Yu Kuai Committed by Jens Axboe
Browse files

blk-cgroup: dropping parent refcount after pd_free_fn() is done 



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: default avatarYu Kuai <yukuai3@huawei.com>
Acked-by: default avatarTejun Heo <tj@kernel.org>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230119110350.2287325-2-yukuai1@huaweicloud.com


Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent b3678103

block/blk-cgroup.c

+2 −2
Original line number Diff line number Diff line
@@ -124,6 +124,8 @@ static void blkg_free_workfn(struct work_struct *work) 
		if (blkg->pd[i])

			blkcg_policy[i]->pd_free_fn(blkg->pd[i]);



	if (blkg->parent)

		blkg_put(blkg->parent);

	if (blkg->q)

		blk_put_queue(blkg->q);

	free_percpu(blkg->iostat_cpu);
@@ -158,8 +160,6 @@ static void __blkg_release(struct rcu_head *rcu) 


	/* release the blkcg and parent blkg refs this blkg has been holding */

	css_put(&blkg->blkcg->css);

	if (blkg->parent)

		blkg_put(blkg->parent);

	blkg_free(blkg);

}