blk-cgroup: Fix class @block_class's subsystem refcount leakage

JIRA: https://issues.redhat.com/browse/RHEL-81398
CVE: CVE-2025-21745

commit d124843
Author: Zijun Hu <quic_zijuhu@quicinc.com>
Date:   Sun Jan 5 16:34:03 2025 +0800

    blk-cgroup: Fix class @block_class's subsystem refcount leakage

    blkcg_fill_root_iostats() iterates over @block_class's devices by
    class_dev_iter_(init|next)(), but does not end iterating with
    class_dev_iter_exit(), so causes the class's subsystem refcount leakage.

    Fix by ending the iterating with class_dev_iter_exit().

    Fixes: ef45fe4 ("blk-cgroup: show global disk stats in root cgroup io.stat")
    Reviewed-by: Michal Koutný <mkoutny@suse.com>
    Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Cc: stable@vger.kernel.org
    Acked-by: Tejun Heo <tj@kernel.org>
    Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
    Link: https://lore.kernel.org/r/20250105-class_fix-v6-2-3a2f1768d4d4@quicinc.com
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

block/blk-cgroup.c

@@ -1139,6 +1139,7 @@ static void blkcg_fill_root_iostats(void)
 		blkg_iostat_set(&blkg->iostat.cur, &tmp);
 		u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);
 	}
+	class_dev_iter_exit(&iter);
 }
 
 static void blkcg_print_one_stat(struct blkcg_gq *blkg, struct seq_file *s)