Re: [Kernel Bug] KASAN: slab-use-after-free Read in __blkcg_rstat_flush

[Michal Koutný]

On Tue, Feb 03, 2026 at 10:08:07PM +0800, Ming Lei <ming.lei@xxxxxxxxxx> wrote:
> I can't parse your question, here blkg_release() simply needs to flush
> all stats. Why do you talk about preventing new flush? why is it related
> with this UAF?

What prevents this fix:

--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -169,14 +169,6 @@ static void __blkg_release(struct rcu_head *rcu)
 #ifdef CONFIG_BLK_CGROUP_PUNT_BIO
        WARN_ON(!bio_list_empty(&blkg->async_bios));
 #endif
-       /*
-        * Flush all the non-empty percpu lockless lists before releasing
-        * us, given these stat belongs to us.
-        *
-        * blkg_stat_lock is for serializing blkg stat update
-        */
-       for_each_possible_cpu(cpu)
-               __blkcg_rstat_flush(blkcg, cpu);

        /* release the blkcg and parent blkg refs this blkg has been holding */
        css_put(&blkg->blkcg->css);
@@ -195,6 +187,15 @@ static void blkg_release(struct percpu_ref *ref)
 {
        struct blkcg_gq *blkg = container_of(ref, struct blkcg_gq, refcnt);

+       /*
+        * Flush all the non-empty percpu lockless lists before releasing
+        * us, given these stat belongs to us.
+        *
+        * blkg_stat_lock is for serializing blkg stat update
+        */
+       for_each_possible_cpu(cpu)
+               __blkcg_rstat_flush(blkcg, cpu);
+
        call_rcu(&blkg->rcu_head, __blkg_release);
 }

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