Re: [RESEND PATCH v5 1/6] Bluetooth: schedule SCO timeouts with delayed_work

[Luiz Augusto von Dentz <luiz.dentz@gmail.com>]

Hi Desmond,

On Wed, Aug 4, 2021 at 8:48 AM Desmond Cheong Zhi Xi
<desmondcheongzx@gmail.com> wrote:
>
> struct sock.sk_timer should be used as a sock cleanup timer. However,
> SCO uses it to implement sock timeouts.
>
> This causes issues because struct sock.sk_timer's callback is run in
> an IRQ context, and the timer callback function sco_sock_timeout takes
> a spin lock on the socket. However, other functions such as
> sco_conn_del and sco_conn_ready take the spin lock with interrupts
> enabled.
>
> This inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} lock usage could
> lead to deadlocks as reported by Syzbot [1]:
>        CPU0
>        ----
>   lock(slock-AF_BLUETOOTH-BTPROTO_SCO);
>   <Interrupt>
>     lock(slock-AF_BLUETOOTH-BTPROTO_SCO);
>
> To fix this, we use delayed work to implement SCO sock timouts
> instead. This allows us to avoid taking the spin lock on the socket in
> an IRQ context, and corrects the misuse of struct sock.sk_timer.
>
> As a note, cancel_delayed_work is used instead of
> cancel_delayed_work_sync in sco_sock_set_timer and
> sco_sock_clear_timer to avoid a deadlock. In the future, the call to
> bh_lock_sock inside sco_sock_timeout should be changed to lock_sock to
> synchronize with other functions using lock_sock. However, since
> sco_sock_set_timer and sco_sock_clear_timer are sometimes called under
> the locked socket (in sco_connect and __sco_sock_close),
> cancel_delayed_work_sync might cause them to sleep until an
> sco_sock_timeout that has started finishes running. But
> sco_sock_timeout would also sleep until it can grab the lock_sock.
>
> Using cancel_delayed_work is fine because sco_sock_timeout does not
> change from run to run, hence there is no functional difference
> between:
> 1. waiting for a timeout to finish running before scheduling another
> timeout
> 2. scheduling another timeout while a timeout is running.
>
> Link: https://syzkaller.appspot.com/bug?id=9089d89de0502e120f234ca0fc8a703f7368b31e [1]
> Reported-by: syzbot+2f6d7c28bb4bf7e82060@syzkaller.appspotmail.com
> Tested-by: syzbot+2f6d7c28bb4bf7e82060@syzkaller.appspotmail.com
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> ---
>  net/bluetooth/sco.c | 41 +++++++++++++++++++++++++++++++++++------
>  1 file changed, 35 insertions(+), 6 deletions(-)
>
> diff --git a/net/bluetooth/sco.c b/net/bluetooth/sco.c
> index ffa2a77a3e4c..89cb987ca9eb 100644
> --- a/net/bluetooth/sco.c
> +++ b/net/bluetooth/sco.c
> @@ -48,6 +48,8 @@ struct sco_conn {
>         spinlock_t      lock;
>         struct sock     *sk;
>
> +       struct delayed_work     timeout_work;
> +
>         unsigned int    mtu;
>  };
>
> @@ -74,9 +76,20 @@ struct sco_pinfo {
>  #define SCO_CONN_TIMEOUT       (HZ * 40)
>  #define SCO_DISCONN_TIMEOUT    (HZ * 2)
>
> -static void sco_sock_timeout(struct timer_list *t)
> +static void sco_sock_timeout(struct work_struct *work)
>  {
> -       struct sock *sk = from_timer(sk, t, sk_timer);
> +       struct sco_conn *conn = container_of(work, struct sco_conn,
> +                                            timeout_work.work);
> +       struct sock *sk;
> +
> +       sco_conn_lock(conn);
> +       sk = conn->sk;
> +       if (sk)
> +               sock_hold(sk);
> +       sco_conn_unlock(conn);
> +
> +       if (!sk)
> +               return;
>
>         BT_DBG("sock %p state %d", sk, sk->sk_state);
>
> @@ -91,14 +104,27 @@ static void sco_sock_timeout(struct timer_list *t)
>
>  static void sco_sock_set_timer(struct sock *sk, long timeout)
>  {
> +       struct delayed_work *work;

Minor nitpick but I don't think using a dedicated variable here makes
much sense.

> +       if (!sco_pi(sk)->conn)
> +               return;
> +       work = &sco_pi(sk)->conn->timeout_work;
> +
>         BT_DBG("sock %p state %d timeout %ld", sk, sk->sk_state, timeout);
> -       sk_reset_timer(sk, &sk->sk_timer, jiffies + timeout);
> +       cancel_delayed_work(work);
> +       schedule_delayed_work(work, timeout);
>  }
>
>  static void sco_sock_clear_timer(struct sock *sk)
>  {
> +       struct delayed_work *work;

Ditto.

> +       if (!sco_pi(sk)->conn)
> +               return;
> +       work = &sco_pi(sk)->conn->timeout_work;
> +
>         BT_DBG("sock %p state %d", sk, sk->sk_state);
> -       sk_stop_timer(sk, &sk->sk_timer);
> +       cancel_delayed_work(work);
>  }
>
>  /* ---- SCO connections ---- */
> @@ -179,6 +205,9 @@ static void sco_conn_del(struct hci_conn *hcon, int err)
>                 bh_unlock_sock(sk);
>                 sco_sock_kill(sk);
>                 sock_put(sk);
> +
> +               /* Ensure no more work items will run before freeing conn. */
> +               cancel_delayed_work_sync(&conn->timeout_work);
>         }
>
>         hcon->sco_data = NULL;
> @@ -193,6 +222,8 @@ static void __sco_chan_add(struct sco_conn *conn, struct sock *sk,
>         sco_pi(sk)->conn = conn;
>         conn->sk = sk;
>
> +       INIT_DELAYED_WORK(&conn->timeout_work, sco_sock_timeout);
> +
>         if (parent)
>                 bt_accept_enqueue(parent, sk, true);
>  }
> @@ -500,8 +531,6 @@ static struct sock *sco_sock_alloc(struct net *net, struct socket *sock,
>
>         sco_pi(sk)->setting = BT_VOICE_CVSD_16BIT;
>
> -       timer_setup(&sk->sk_timer, sco_sock_timeout, 0);
> -
>         bt_sock_link(&sco_sk_list, sk);
>         return sk;
>  }
> --
> 2.25.1
>


-- 
Luiz Augusto von Dentz