Re: [PATCH v2 2/3] blk-cgroup: skip dying blkg in blkcg_activate_policy()

[Michal Koutný]

On Tue, Jan 13, 2026 at 02:10:34PM +0800, Zheng Qixing <zhengqixing@xxxxxxxxxxxxxxx> wrote:
> From: Zheng Qixing <zhengqixing@xxxxxxxxxx>
> 
> When switching IO schedulers on a block device, blkcg_activate_policy()
> can race with concurrent blkcg deletion, leading to a use-after-free in
> rcu_accelerate_cbs.
> 
> T1:                               T2:
> 		                  blkg_destroy
>                  		  kill(&blkg->refcnt) // blkg->refcnt=1->0
> 				  blkg_release // call_rcu(__blkg_release)
>                                   ...
> 				  blkg_free_workfn
>                                   ->pd_free_fn(pd)
> elv_iosched_store
> elevator_switch
> ...
> iterate blkg list
> blkg_get(blkg) // blkg->refcnt=0->1
>                                   list_del_init(&blkg->q_node)
> blkg_put(pinned_blkg) // blkg->refcnt=1->0
> blkg_release // call_rcu again
> rcu_accelerate_cbs // uaf
> 
> Fix this by replacing blkg_get() with blkg_tryget(), which fails if
> the blkg's refcount has already reached zero. If blkg_tryget() fails,
> skip processing this blkg since it's already being destroyed.
> 
> Link: https://lore.kernel.org/all/20260108014416.3656493-4-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>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> ---
>  block/blk-cgroup.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

Reviewed-by: Michal Koutný <mkoutny@xxxxxxxx>

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