netdev - Re: [PATCH net] tun: fix use after free for ptr

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Date:   Fri, 11 May 2018 09:29:10 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "Michael S . Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH net] tun: fix use after free for ptr_ring



On 2018年05月11日 02:08, Cong Wang wrote:
> On Tue, May 8, 2018 at 11:59 PM, Jason Wang <jasowang@...hat.com> wrote:
>> We used to initialize ptr_ring during TUNSETIFF, this is because its
>> size depends on the tx_queue_len of netdevice. And we try to clean it
>> up when socket were detached from netdevice. A race were spotted when
>> trying to do uninit during a read which will lead a use after free for
>> pointer ring. Solving this by always initialize a zero size ptr_ring
>> in open() and do resizing during TUNSETIFF, and then we can safely do
>> cleanup during close(). With this, there's no need for the workaround
>> that was introduced by commit 4df0bfc79904 ("tun: fix a memory leak
>> for tfile->tx_array").
>>
> Ah, I didn't know ptr_ring_init(0) could work... Nice patch!
> Except one thing below.
>
>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index ef33950..298cb96 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -681,15 +681,6 @@ static void tun_queue_purge(struct tun_file *tfile)
>>          skb_queue_purge(&tfile->sk.sk_error_queue);
>>   }
>>
>> -static void tun_cleanup_tx_ring(struct tun_file *tfile)
>> -{
>> -       if (tfile->tx_ring.queue) {
>> -               ptr_ring_cleanup(&tfile->tx_ring, tun_ptr_free);
>> -               xdp_rxq_info_unreg(&tfile->xdp_rxq);
>> -               memset(&tfile->tx_ring, 0, sizeof(tfile->tx_ring));
>> -       }
>> -}
>
> I don't think you can totally remove ptr_ring_cleanup(), it should be
> called unconditionally with your ptr_ring_init(0) trick, right?

Right, my bad. Actually I do intend to cleanup it at close() like what 
commit log said.

Will send v2.

Thanks