From: Zheng Qixing 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 checking hlist_unhashed(&blkg->blkcg_node) before getting a reference to the blkg. This is the same check used in blkg_destroy() to detect if a blkg has already been destroyed. If the blkg is already unhashed, skip processing it since it's being destroyed. Link: https://lore.kernel.org/all/20260108014416.3656493-4-zhengqixing@huaweicloud.com/ Fixes: f1c006f1c685 ("blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()") Signed-off-by: Zheng Qixing Signed-off-by: Yu Kuai --- block/blk-cgroup.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c index a6ac6ba9430d..8d9273f61dd5 100644 --- a/block/blk-cgroup.c +++ b/block/blk-cgroup.c @@ -1625,6 +1625,8 @@ int blkcg_activate_policy(struct gendisk *disk, const struct blkcg_policy *pol) * GFP_NOWAIT failed. Free the existing one and * prealloc for @blkg w/ GFP_KERNEL. */ + if (hlist_unhashed(&blkg->blkcg_node)) + continue; if (pinned_blkg) blkg_put(pinned_blkg); blkg_get(blkg); -- 2.51.0