From:   Sasha Levin <sashal@kernel.org>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
Cc:     Yu Kuai <yukuai3@huawei.com>, Tejun Heo <tj@kernel.org>,
        Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        Sasha Levin <sashal@kernel.org>, josef@toxicpanda.com,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org
Subject: [PATCH AUTOSEL 6.1 09/21] blk-cgroup: dropping parent refcount after pd_free_fn() is done
Date:   Sat, 25 Feb 2023 22:42:44 -0500
Message-Id: <20230226034256.771769-9-sashal@kernel.org>
In-Reply-To: <20230226034256.771769-1-sashal@kernel.org>
References: <20230226034256.771769-1-sashal@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

From: Yu Kuai <yukuai3@huawei.com>

[ 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>
---
 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 7c91d9195da8d..8d1b7757f1e4f 100644
--- 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);
 }
 
-- 
2.39.0