Re: [PATCH v4 2/4] blk-cgroup: fix race between policy activation and blkg destruction

[Tao Cui]

于 2026年8月2日 GMT+08:00 19:25:18，Yu Kuai <yukuai@xxxxxxxxxx> 写道：
>From: Zheng Qixing <zhengqixing@xxxxxxxxxx>
>
>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: f1c006f1c685 ("blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()")
>Signed-off-by: Zheng Qixing <zhengqixing@xxxxxxxxxx>
>Reviewed-by: Tang Yizhou <yizhou.tang@xxxxxxxxxx>
>Signed-off-by: Yu Kuai <yukuai@xxxxxxx>
>---
> block/blk-cgroup.c | 3 +++
> 1 file changed, 3 insertions(+)
>
>diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
>index eb0cfb10b859..047bb42c282b 100644
>--- a/block/blk-cgroup.c
>+++ b/block/blk-cgroup.c
>@@ -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)
Reviewed-by: Tao Cui <cuitao@xxxxxxxxxx>