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blk-cgroup: fix race between policy activation and blkg destruction

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When switching an IO scheduler on a block device, blkcg_activate_policy()
allocates blkg_policy_data (pd) for all blkgs attached to the queue.
However, blkcg_activate_policy() may race with concurrent blkcg deletion,
leading to use-after-free and memory leak issues.

The use-after-free occurs in the following race:

T1 (blkcg_activate_policy):
  - Successfully allocates pd for blkg1 (loop0->queue, blkcgA)
  - Fails to allocate pd for blkg2 (loop0->queue, blkcgB)
  - Enters the enomem rollback path to release blkg1 resources

T2 (blkcg deletion):
  - blkcgA is deleted concurrently
  - blkg1 is freed via blkg_free_workfn()
  - blkg1->pd is freed

T1 (continued):
  - Rollback path accesses blkg1->pd->online after pd is freed
  - Triggers use-after-free

In addition, blkg_free_workfn() frees pd before removing the blkg from
q->blkg_list. This allows blkcg_activate_policy() to allocate a new pd
for a blkg that is being destroyed, leaving the newly allocated pd
unreachable when the blkg is finally freed.

Fix these races by extending blkcg_mutex coverage to serialize
blkcg_activate_policy() rollback and blkg destruction, ensuring pd
lifecycle is synchronized with blkg list visibility.

Fixes: f1c006f1c6 ("blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()")
Signed-off-by: Zheng Qixing <zhengqixing@huawei.com>
Reviewed-by: Tang Yizhou <yizhou.tang@shopee.com>
Signed-off-by: Yu Kuai <yukuai@fygo.io>
Reviewed-by: Tao Cui <cuitao@kylinos.cn>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Link: https://patch.msgid.link/20260802112525.3933753-3-yukuai@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Zheng Qixing
2026-08-02 19:25:18 +08:00
committed by Jens Axboe
parent 9a91679894
commit 5313d4d417
1 changed files with 3 additions and 0 deletions
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3
block/blk-cgroup.c
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@@ -1566,6 +1566,8 @@ int blkcg_activate_policy(struct gendisk *disk, const struct blkcg_policy *pol)
if (queue_is_mq(q))
memflags = blk_mq_freeze_queue(q);
mutex_lock(&q->blkcg_mutex);
retry:
spin_lock_irq(&q->queue_lock);
@@ -1628,6 +1630,7 @@ int blkcg_activate_policy(struct gendisk *disk, const struct blkcg_policy *pol)
spin_unlock_irq(&q->queue_lock);
out:
mutex_unlock(&q->blkcg_mutex);
if (queue_is_mq(q))
blk_mq_unfreeze_queue(q, memflags);
if (pinned_blkg)