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([2400:79e0:1203:64b0:8460:77be:58ac:5bd9]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-8434ac9c4c0sm12310480b3a.3.2026.06.16.09.50.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 16 Jun 2026 09:50:04 -0700 (PDT) Message-ID: <774f145f-cec0-4408-9557-5db196302ebd@gmail.com> Date: Wed, 17 Jun 2026 00:50:01 +0800 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V3] blk-cgroup: defer blkcg css_put until blkg is unlinked from queue To: Zizhi Wo , axboe@kernel.dk, tj@kernel.org, josef@toxicpanda.com, linux-block@vger.kernel.org Cc: cgroups@vger.kernel.org, yangerkun@huawei.com, chengzhihao1@huawei.com, houtao1@huawei.com, yukuai@fygo.io References: <20260616011746.2451461-1-wozizhi@huaweicloud.com> Content-Language: en-US From: Tang Yizhou In-Reply-To: <20260616011746.2451461-1-wozizhi@huaweicloud.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 16/6/26 9:17 am, Zizhi Wo wrote: > From: Zizhi Wo > > [BUG] > Our fuzz testing triggered a blkcg use-after-free issue: > > BUG: KASAN: slab-use-after-free in _raw_spin_lock+0x75/0xe0 > Call Trace: > ... > blkcg_deactivate_policy+0x244/0x4d0 > ioc_rqos_exit+0x44/0xe0 > rq_qos_exit+0xba/0x120 > __del_gendisk+0x50b/0x800 > del_gendisk+0xff/0x190 > ... > > [CAUSE] > process1 process2 > cgroup_rmdir > ... > css_killed_work_fn > offline_css > ... > blkcg_destroy_blkgs > ... > __blkg_release > css_put(&blkg->blkcg->css) > blkg_free > INIT_WORK(xxx, blkg_free_workfn) > schedule_work > css_put > ... > blkcg_css_free > kfree(blkcg)--------blkcg has been freed!!! > ====================================schedule_work > blkg_free_workfn > __del_gendisk > rq_qos_exit > ioc_rqos_exit > blkcg_deactivate_policy > mutex_lock(&q->blkcg_mutex) > spin_lock_irq(&q->queue_lock) > list_for_each_entry(blkg, xxx) > blkcg = blkg->blkcg > spin_lock(&blkcg->lock)-------UAF!!! > mutex_lock(&q->blkcg_mutex) > spin_lock_irq(&q->queue_lock) > /* Only then is the blkg removed from the list */ > list_del_init(&blkg->q_node) > > As a result, a blkg can still be reachable through q->blkg_list while > its ->blkcg has already been freed. > > [Fix] > Fix this by deferring the blkcg css_put() until after the blkg has been > unlinked from q->blkg_list in blkg_free_workfn(). This ensures that the > blkcg outlives every blkg still reachable through q->blkg_list, so any > iterator holding q->queue_lock is guaranteed to observe a valid > blkg->blkcg. > > While at it, move css_tryget_online() from blkg_create() into blkg_alloc() > so that the css reference is owned by the alloc/free pair rather than > straddling layers: > blkg_alloc() <-> blkg_free() > blkg_create() <-> blkg_destroy() > > Fixes: f1c006f1c685 ("blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()") > Suggested-by: Hou Tao > Signed-off-by: Zizhi Wo > Reviewed-by: Yu Kuai > --- > v3: > - move css_put() after mutex_unlock() in blkg_free_workfn(). > > v2: > - Move css_tryget_online() from blkg_create() into blkg_alloc() so the > css reference follows the blkg's own lifetime, making the put in > blkg_free_workfn() symmetric with the get in blkg_alloc(). > > v1: https://lore.kernel.org/all/20260518010932.633707-1-wozizhi@huaweicloud.com/ > block/blk-cgroup.c | 24 ++++++++++++------------ > 1 file changed, 12 insertions(+), 12 deletions(-) > > diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c > index bc63bd220865..3ac41f766caf 100644 > --- a/block/blk-cgroup.c > +++ b/block/blk-cgroup.c > @@ -136,6 +136,11 @@ static void blkg_free_workfn(struct work_struct *work) > spin_unlock_irq(&q->queue_lock); > mutex_unlock(&q->blkcg_mutex); > > + /* > + * Release blkcg css ref only after blkg is removed from q->blkg_list, > + * so concurrent iterators won't see a blkg with a freed blkcg. > + */ > + css_put(&blkg->blkcg->css); > blk_put_queue(q); > free_percpu(blkg->iostat_cpu); > percpu_ref_exit(&blkg->refcnt); > @@ -179,8 +184,6 @@ static void __blkg_release(struct rcu_head *rcu) > 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); > blkg_free(blkg); > } > > @@ -313,6 +316,9 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk, > goto out_exit_refcnt; > if (!blk_get_queue(disk->queue)) > goto out_free_iostat; > + /* blkg holds a reference to blkcg */ > + if (!css_tryget_online(&blkcg->css)) > + goto out_put_queue; > > blkg->q = disk->queue; > INIT_LIST_HEAD(&blkg->q_node); > @@ -353,6 +359,8 @@ static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk, > while (--i >= 0) > if (blkg->pd[i]) > blkcg_policy[i]->pd_free_fn(blkg->pd[i]); > + css_put(&blkcg->css); > +out_put_queue: > blk_put_queue(disk->queue); > out_free_iostat: > free_percpu(blkg->iostat_cpu); > @@ -381,18 +389,12 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg, struct gendisk *disk, > goto err_free_blkg; > } > > - /* blkg holds a reference to blkcg */ > - if (!css_tryget_online(&blkcg->css)) { > - ret = -ENODEV; > - goto err_free_blkg; > - } > - > /* allocate */ > if (!new_blkg) { > new_blkg = blkg_alloc(blkcg, disk, GFP_NOWAIT); > if (unlikely(!new_blkg)) { > ret = -ENOMEM; > - goto err_put_css; > + goto err_free_blkg; > } > } > blkg = new_blkg; > @@ -402,7 +404,7 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg, struct gendisk *disk, > blkg->parent = blkg_lookup(blkcg_parent(blkcg), disk->queue); > if (WARN_ON_ONCE(!blkg->parent)) { > ret = -ENODEV; > - goto err_put_css; > + goto err_free_blkg; > } > blkg_get(blkg->parent); > } > @@ -442,8 +444,6 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg, struct gendisk *disk, > blkg_put(blkg); > return ERR_PTR(ret); > > -err_put_css: > - css_put(&blkcg->css); > err_free_blkg: > if (new_blkg) > blkg_free(new_blkg); LGTM. Reviewed-by: Tang Yizhou -- Best Regards, Yi