netdev - Re: [PATCH] net: Fix potential memory leak in proto

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Date:   Wed, 12 Aug 2020 09:21:36 +0000
From:   linmiaohe <linmiaohe@...wei.com>
To:     David Miller <davem@...emloft.net>,
        "xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>
CC:     "kuba@...nel.org" <kuba@...nel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kafai@...com" <kafai@...com>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "jakub@...udflare.com" <jakub@...udflare.com>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "zhang.lin16@....com.cn" <zhang.lin16@....com.cn>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: Fix potential memory leak in proto_register()

Hi all:
David Miller <davem@...emloft.net> wrote:
>From: Cong Wang <xiyou.wangcong@...il.com>
>Date: Tue, 11 Aug 2020 16:02:51 -0700
>
>>> @@ -3406,6 +3406,16 @@ static void sock_inuse_add(struct net *net, 
>>> int val)  }  #endif
>>>
>>> +static void tw_prot_cleanup(struct timewait_sock_ops *twsk_prot) {
>>> +       if (!twsk_prot)
>>> +               return;
>>> +       kfree(twsk_prot->twsk_slab_name);
>>> +       twsk_prot->twsk_slab_name = NULL;
>>> +       kmem_cache_destroy(twsk_prot->twsk_slab);
>> 
>> Hmm, are you sure you can free the kmem cache name before 
>> kmem_cache_destroy()? To me, it seems kmem_cache_destroy() frees the 
>> name via slab_kmem_cache_release() via kfree_const().
>> With your patch, we have a double-free on the name?
>> 
>> Or am I missing anything?
>
>Yep, there is a double free here.
>
>Please fix this.

Many thanks for both of you to point this issue out. But I'am not really understand, could you please explain it more?
As far as I can see, the double free path is:
1. kfree(twsk_prot->twsk_slab_name)
2. kmem_cache_destroy 
	--> shutdown_memcg_caches
		--> shutdown_cache
			--> slab_kmem_cache_release
				--> kfree_const(s->name)
But twsk_prot->twsk_slab_name is allocated from kasprintf via kmalloc_track_caller while twsk_prot->twsk_slab->name is allocated 
via kstrdup_const. So I think twsk_prot->twsk_slab_name and twsk_prot->twsk_slab->name point to different memory, and there is no double free.

Or am I missing anything?

By the way, req_prot_cleanup() do the same things, i.e. free the slab_name before involve kmem_cache_destroy(). If there is a double
free, so as here?

Thanks.