netdev - Re: [PATCH v2] Bluetooth: call lock_sock() outside of spinlock section

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Date:   Wed, 7 Jul 2021 11:20:54 -0700
From:   Luiz Augusto von Dentz <luiz.dentz@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        "linux-bluetooth@...r.kernel.org" <linux-bluetooth@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Lin Ma <linma@....edu.cn>,
        "open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>
Subject: Re: [PATCH v2] Bluetooth: call lock_sock() outside of spinlock section

Hi Tetsuo,

On Wed, Jul 7, 2021 at 2:43 AM Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
>
> syzbot is hitting might_sleep() warning at hci_sock_dev_event() due to
> calling lock_sock() with rw spinlock held [1]. Defer calling lock_sock()
> via sock_hold().
>
> Link: https://syzkaller.appspot.com/bug?extid=a5df189917e79d5e59c9 [1]
> Reported-by: syzbot <syzbot+a5df189917e79d5e59c9@...kaller.appspotmail.com>
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Tested-by: syzbot <syzbot+a5df189917e79d5e59c9@...kaller.appspotmail.com>
> Fixes: e305509e678b3a4a ("Bluetooth: use correct lock to prevent UAF of hdev object")
> ---
> Changes in v2:
>   Take hci_sk_list.lock for write in case bt_sock_unlink() is called after
>   sk_hashed(sk) test, and defer hci_dev_put(hdev) till schedulable context.
>
>  net/bluetooth/hci_sock.c | 32 +++++++++++++++++++++++++++++---
>  1 file changed, 29 insertions(+), 3 deletions(-)
>
> diff --git a/net/bluetooth/hci_sock.c b/net/bluetooth/hci_sock.c
> index b04a5a02ecf3..d8e1ac1ae10d 100644
> --- a/net/bluetooth/hci_sock.c
> +++ b/net/bluetooth/hci_sock.c
> @@ -758,20 +758,46 @@ void hci_sock_dev_event(struct hci_dev *hdev, int event)
>
>         if (event == HCI_DEV_UNREG) {
>                 struct sock *sk;
> +               bool put_dev;
>
> +restart:
> +               put_dev = false;
>                 /* Detach sockets from device */
>                 read_lock(&hci_sk_list.lock);
>                 sk_for_each(sk, &hci_sk_list.head) {
> +                       /* hci_sk_list.lock is preventing hci_sock_release()
> +                        * from calling bt_sock_unlink().
> +                        */
> +                       if (hci_pi(sk)->hdev != hdev || sk_unhashed(sk))
> +                               continue;
> +                       /* Take a ref because we can't call lock_sock() with
> +                        * hci_sk_list.lock held.
> +                        */
> +                       sock_hold(sk);
> +                       read_unlock(&hci_sk_list.lock);
>                         lock_sock(sk);
> -                       if (hci_pi(sk)->hdev == hdev) {
> +                       /* Since hci_sock_release() might have already called
> +                        * bt_sock_unlink() while waiting for lock_sock(),
> +                        * use sk_hashed(sk) for checking that bt_sock_unlink()
> +                        * is not yet called.
> +                        */
> +                       write_lock(&hci_sk_list.lock);
> +                       if (sk_hashed(sk) && hci_pi(sk)->hdev == hdev) {
>                                 hci_pi(sk)->hdev = NULL;
>                                 sk->sk_err = EPIPE;
>                                 sk->sk_state = BT_OPEN;
>                                 sk->sk_state_change(sk);
> -
> -                               hci_dev_put(hdev);
> +                               put_dev = true;
>                         }
> +                       write_unlock(&hci_sk_list.lock);
>                         release_sock(sk);
> +                       sock_put(sk);
> +                       if (put_dev)
> +                               hci_dev_put(hdev);
> +                       /* Restarting is safe, for hci_pi(sk)->hdev != hdev if
> +                        * condition met and sk_unhashed(sk) == true otherwise.
> +                        */
> +                       goto restart;

This sounds a little too complicated, afaik backward goto is not even
consider a good practice either, since it appears we don't unlink the
sockets here we could perhaps don't release the reference to hdev
either and leave hci_sock_release to deal with it and then perhaps we
can take away the backward goto, actually why are you restarting to
begin with? It is also weird that this only manifests in the Bluetooth
HCI sockets or other subsystems don't use such locking mechanism
anymore?


>                 }
>                 read_unlock(&hci_sk_list.lock);
>         }
> --
> 2.18.4
>
>


-- 
Luiz Augusto von Dentz