Re: [PATCH v2 1/3] blk-cgroup: fix race between policy activation and blkg destruction

[Michal Koutný]

On Tue, Jan 13, 2026 at 02:10:33PM +0800, Zheng Qixing <zhengqixing@xxxxxxxxxxxxxxx> wrote:
> 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

The rollback path is under q->queue_lock same like the list removal in
blkg_free_workfn().
Why is queue_lock not enough for synchronization in this case?

(BTW have you observed this case "naturally" or have you injected the
memory allocation failure?)


>   - Triggers use-after-free
> 
> In addition, blkg_free_workfn() frees pd before removing the blkg from
> q->blkg_list.

Yeah, this looks weirdly reversed.

> 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.
> 
> Link: https://lore.kernel.org/all/20260108014416.3656493-3-zhengqixing@xxxxxxxxxxxxxxx/
> Fixes: f1c006f1c685 ("blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()")
> Signed-off-by: Zheng Qixing <zhengqixing@xxxxxxxxxx>

Thanks,
Michal

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