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From:   Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Date:   Wed, 14 Jul 2021 14:50:07 -0700
Message-ID: <CABBYNZL37yLgj1LP7r=rbEcsPXCPy1y55ar816eZXka2W=7-Aw@mail.gmail.com>
Subject: Re: [PATCH] Bluetooth: fix use-after-free error in lock_sock_nested()
To:     Wang ShaoBo <bobo.shaobowang@huawei.com>
Cc:     cj.chengjian@huawei.com, Wei Yongjun <weiyongjun1@huawei.com>,
        yuehaibing@huawei.com, huawei.libin@huawei.com,
        Marcel Holtmann <marcel@holtmann.org>,
        Johan Hedberg <johan.hedberg@gmail.com>,
        "linux-bluetooth@vger.kernel.org" <linux-bluetooth@vger.kernel.org>,
        "open list:NETWORKING [GENERAL]" <netdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hi,

On Tue, Jul 13, 2021 at 8:20 PM Wang ShaoBo <bobo.shaobowang@huawei.com> wrote:
>
> use-after-free error in lock_sock_nested() is reported:
>
> [  179.140137][ T3731] =====================================================
> [  179.142675][ T3731] BUG: KMSAN: use-after-free in lock_sock_nested+0x280/0x2c0
> [  179.145494][ T3731] CPU: 4 PID: 3731 Comm: kworker/4:2 Not tainted 5.12.0-rc6+ #54
> [  179.148432][ T3731] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
> [  179.151806][ T3731] Workqueue: events l2cap_chan_timeout
> [  179.152730][ T3731] Call Trace:
> [  179.153301][ T3731]  dump_stack+0x24c/0x2e0
> [  179.154063][ T3731]  kmsan_report+0xfb/0x1e0
> [  179.154855][ T3731]  __msan_warning+0x5c/0xa0
> [  179.155579][ T3731]  lock_sock_nested+0x280/0x2c0
> [  179.156436][ T3731]  ? kmsan_get_metadata+0x116/0x180
> [  179.157257][ T3731]  l2cap_sock_teardown_cb+0xb8/0x890
> [  179.158154][ T3731]  ? __msan_metadata_ptr_for_load_8+0x10/0x20
> [  179.159141][ T3731]  ? kmsan_get_metadata+0x116/0x180
> [  179.159994][ T3731]  ? kmsan_get_shadow_origin_ptr+0x84/0xb0
> [  179.160959][ T3731]  ? l2cap_sock_recv_cb+0x420/0x420
> [  179.161834][ T3731]  l2cap_chan_del+0x3e1/0x1d50
> [  179.162608][ T3731]  ? kmsan_get_metadata+0x116/0x180
> [  179.163435][ T3731]  ? kmsan_get_shadow_origin_ptr+0x84/0xb0
> [  179.164406][ T3731]  l2cap_chan_close+0xeea/0x1050
> [  179.165189][ T3731]  ? kmsan_internal_unpoison_shadow+0x42/0x70
> [  179.166180][ T3731]  l2cap_chan_timeout+0x1da/0x590
> [  179.167066][ T3731]  ? __msan_metadata_ptr_for_load_8+0x10/0x20
> [  179.168023][ T3731]  ? l2cap_chan_create+0x560/0x560
> [  179.168818][ T3731]  process_one_work+0x121d/0x1ff0
> [  179.169598][ T3731]  worker_thread+0x121b/0x2370
> [  179.170346][ T3731]  kthread+0x4ef/0x610
> [  179.171010][ T3731]  ? process_one_work+0x1ff0/0x1ff0
> [  179.171828][ T3731]  ? kthread_blkcg+0x110/0x110
> [  179.172587][ T3731]  ret_from_fork+0x1f/0x30
> [  179.173348][ T3731]
> [  179.173752][ T3731] Uninit was created at:
> [  179.174409][ T3731]  kmsan_internal_poison_shadow+0x5c/0xf0
> [  179.175373][ T3731]  kmsan_slab_free+0x76/0xc0
> [  179.176060][ T3731]  kfree+0x3a5/0x1180
> [  179.176664][ T3731]  __sk_destruct+0x8af/0xb80
> [  179.177375][ T3731]  __sk_free+0x812/0x8c0
> [  179.178032][ T3731]  sk_free+0x97/0x130
> [  179.178686][ T3731]  l2cap_sock_release+0x3d5/0x4d0
> [  179.179457][ T3731]  sock_close+0x150/0x450
> [  179.180117][ T3731]  __fput+0x6bd/0xf00
> [  179.180787][ T3731]  ____fput+0x37/0x40
> [  179.181481][ T3731]  task_work_run+0x140/0x280
> [  179.182219][ T3731]  do_exit+0xe51/0x3e60
> [  179.182930][ T3731]  do_group_exit+0x20e/0x450
> [  179.183656][ T3731]  get_signal+0x2dfb/0x38f0
> [  179.184344][ T3731]  arch_do_signal_or_restart+0xaa/0xe10
> [  179.185266][ T3731]  exit_to_user_mode_prepare+0x2d2/0x560
> [  179.186136][ T3731]  syscall_exit_to_user_mode+0x35/0x60
> [  179.186984][ T3731]  do_syscall_64+0xc5/0x140
> [  179.187681][ T3731]  entry_SYSCALL_64_after_hwframe+0x44/0xae
> [  179.188604][ T3731] =====================================================
>
> In our case, there are two Thread A and B:
>
> Context: Thread A:              Context: Thread B:
>
> l2cap_chan_timeout()            __se_sys_shutdown()
>   l2cap_chan_close()              l2cap_sock_shutdown()
>     l2cap_chan_del()                l2cap_chan_close()
>       l2cap_sock_teardown_cb()        l2cap_sock_teardown_cb()
>
> Once l2cap_sock_teardown_cb() excuted, this sock will be marked as SOCK_ZAPPED,
> and can be treated as killable in l2cap_sock_kill() if sock_orphan() has
> excuted, at this time we close sock through sock_close() which end to call
> l2cap_sock_kill() like Thread C:
>
> Context: Thread C:
>
> sock_close()
>   l2cap_sock_release()
>     sock_orphan()
>     l2cap_sock_kill()  #free sock if refcnt is 1
>
> If C completed, Once A or B reaches l2cap_sock_teardown_cb() again,
> use-after-free happened.
>
> We should set chan->data to NULL if sock is freed, for telling teardown
> operation is not allowed in l2cap_sock_teardown_cb(), and also we should
> avoid killing an already killed socket in l2cap_sock_close_cb().
>
> Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
> ---
>  net/bluetooth/l2cap_sock.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
> index c99d65ef13b1..ddc6a692b237 100644
> --- a/net/bluetooth/l2cap_sock.c
> +++ b/net/bluetooth/l2cap_sock.c
> @@ -1215,14 +1215,18 @@ static int l2cap_sock_recvmsg(struct socket *sock, struct msghdr *msg,
>   */
>  static void l2cap_sock_kill(struct sock *sk)
>  {
> +       struct l2cap_chan *chan;
> +
>         if (!sock_flag(sk, SOCK_ZAPPED) || sk->sk_socket)
>                 return;
>
>         BT_DBG("sk %p state %s", sk, state_to_string(sk->sk_state));
>
>         /* Kill poor orphan */
> -
> -       l2cap_chan_put(l2cap_pi(sk)->chan);
> +       chan = l2cap_pi(sk)->chan;
> +       l2cap_chan_put(chan);
> +       if (refcount_read(&sk->sk_refcnt) == 1)
> +               chan->data = NULL;

Instead of checking if it is the last reference here, wouldn't it be
better to reset the chan->data to NULL on l2cap_sock_destruct?

>         sock_set_flag(sk, SOCK_DEAD);
>         sock_put(sk);
>  }
> @@ -1508,6 +1512,9 @@ static void l2cap_sock_close_cb(struct l2cap_chan *chan)
>  {
>         struct sock *sk = chan->data;
>
> +       if (!sk)
> +               return;
> +
>         l2cap_sock_kill(sk);
>  }
>
> @@ -1516,6 +1523,9 @@ static void l2cap_sock_teardown_cb(struct l2cap_chan *chan, int err)
>         struct sock *sk = chan->data;
>         struct sock *parent;
>
> +       if (!sk)
> +               return;
> +
>         BT_DBG("chan %p state %s", chan, state_to_string(chan->state));
>
>         /* This callback can be called both for server (BT_LISTEN)
> --
> 2.27.0
>


-- 
Luiz Augusto von Dentz