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

[ Upstream commit c7241babf0855d8a6180cd1743ff0ec34de40b4e ]

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@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230119110350.2287325-2-yukuai1@huaweicloud.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 7c91d91..8d1b775 100644 (file)
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -93,6 +93,8 @@ 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);
@@ -127,8 +129,6 @@ 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);
 }