linux-kernel - Re: [PATCH net V2] tun: fix use after free for ptr

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Date:   Fri, 11 May 2018 10:39:28 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Jason Wang <jasowang@...hat.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 V2] tun: fix use after free for ptr_ring

On Thu, May 10, 2018 at 7:49 PM, Jason Wang <jasowang@...hat.com> wrote:
>  static void __tun_detach(struct tun_file *tfile, bool clean)
>  {
>         struct tun_file *ntfile;
> @@ -736,7 +727,8 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
>                             tun->dev->reg_state == NETREG_REGISTERED)
>                                 unregister_netdevice(tun->dev);
>                 }
> -               tun_cleanup_tx_ring(tfile);
> +               if (tun)
> +                       xdp_rxq_info_unreg(&tfile->xdp_rxq);
>                 sock_put(&tfile->sk);
>         }
>  }
> @@ -783,14 +775,14 @@ static void tun_detach_all(struct net_device *dev)
>                 tun_napi_del(tun, tfile);
>                 /* Drop read queue */
>                 tun_queue_purge(tfile);
> +               xdp_rxq_info_unreg(&tfile->xdp_rxq);
>                 sock_put(&tfile->sk);
> -               tun_cleanup_tx_ring(tfile);
>         }
>         list_for_each_entry_safe(tfile, tmp, &tun->disabled, next) {
>                 tun_enable_queue(tfile);
>                 tun_queue_purge(tfile);
> +               xdp_rxq_info_unreg(&tfile->xdp_rxq);
>                 sock_put(&tfile->sk);
> -               tun_cleanup_tx_ring(tfile);

Are you sure this is safe?

xdp_rxq_info_unreg() can't be called more than once either,
please make sure the warning that commit c13da21cdb80
("tun: avoid calling xdp_rxq_info_unreg() twice") fixed will not
show up again.