blk-cgroup: skip dying blkg in blkcg_activate_policy()

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.

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/20260802112525.3933753-4-yukuai@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -1577,6 +1577,8 @@ int blkcg_activate_policy(struct gendisk *disk, const struct blkcg_policy *pol)
 
 		if (blkg->pd[pol->plid])
 			continue;
+		if (hlist_unhashed(&blkg->blkcg_node))
+			continue;
 
 		/* If prealloc matches, use it; otherwise try GFP_NOWAIT */
 		if (blkg == pinned_blkg) {