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Sender: Tejun Heo <htejun@gmail.com>
Date:   Mon, 12 Jul 2021 07:12:23 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Yang Yingliang <yangyingliang@huawei.com>
Cc:     linux-kernel@vger.kernel.org, jiangshanlai@gmail.com,
        xuqiang36@huawei.com, paskripkin@gmail.com
Subject: Re: [PATCH v2] workqueue: fix UAF in pwq_unbound_release_workfn()
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References: <20210709071100.4057639-1-yangyingliang@huawei.com>
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In-Reply-To: <20210709071100.4057639-1-yangyingliang@huawei.com>
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Hello, Yang.

> +static void free_pwq(struct pool_workqueue *pwq)
> +{
> +	if (!pwq || --pwq->refcnt)
> +		return;
> +
> +	put_unbound_pool(pwq->pool);
> +	kmem_cache_free(pwq_cache, pwq);
> +}
> +
> +static void free_wqattrs_ctx(struct apply_wqattrs_ctx *ctx)
> +{
> +	int node;
> +
> +	if (!ctx)
> +		return;
> +
> +	for_each_node(node)
> +		free_pwq(ctx->pwq_tbl[node]);
> +	free_pwq(ctx->dfl_pwq);
> +
> +	free_workqueue_attrs(ctx->attrs);
> +
> +	kfree(ctx);
> +}

It bothers me that we're partially replicating the free path including pwq
refcnting. Does something like the following work?

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 104e3ef04e33..0c0ab363edeb 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -3693,7 +3693,7 @@ static void pwq_unbound_release_workfn(struct work_struct *work)
 	 * If we're the last pwq going away, @wq is already dead and no one
 	 * is gonna access it anymore.  Schedule RCU free.
 	 */
-	if (is_last) {
+	if (is_last && !list_empty(&wq->list)) {
 		wq_unregister_lockdep(wq);
 		call_rcu(&wq->rcu, rcu_free_wq);
 	}
@@ -4199,6 +4199,10 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)
 	}
 	put_online_cpus();
 
+	if (ret) {
+		flush_scheduled_work();
+	}
+
 	return ret;
 }
 
-- 
tejun