From: Yu Kuai <yukuai3@huawei.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@huawei.com>
---
block/blk-cgroup.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 4c94a6560f62..c6d7d1fce65a 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -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);
}
--
2.31.1