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Date: Thu, 5 Aug 2021 12:06:47 -0700
From: Luiz Augusto von Dentz <luiz.dentz@...il.com>
To: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
Cc: Marcel Holtmann <marcel@...tmann.org>,
Johan Hedberg <johan.hedberg@...il.com>,
David Miller <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>, sudipm.mukherjee@...il.com,
"linux-bluetooth@...r.kernel.org" <linux-bluetooth@...r.kernel.org>,
"open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
skhan@...uxfoundation.org,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
linux-kernel-mentees@...ts.linuxfoundation.org,
syzbot+2f6d7c28bb4bf7e82060@...kaller.appspotmail.com
Subject: Re: [RESEND PATCH v5 1/6] Bluetooth: schedule SCO timeouts with delayed_work
Hi Desmond,
On Wed, Aug 4, 2021 at 8:48 AM Desmond Cheong Zhi Xi
<desmondcheongzx@...il.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@...kaller.appspotmail.com
> Tested-by: syzbot+2f6d7c28bb4bf7e82060@...kaller.appspotmail.com
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.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
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