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Subject: Re: [PATCH v4] Bluetooth: schedule SCO timeouts with delayed_work
From:   Marcel Holtmann <marcel@holtmann.org>
In-Reply-To: <20210728071721.411669-1-desmondcheongzx@gmail.com>
Date:   Thu, 29 Jul 2021 13:30:14 +0200
Cc:     Johan Hedberg <johan.hedberg@gmail.com>,
        Luiz Augusto von Dentz <luiz.dentz@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        linux-bluetooth <linux-bluetooth@vger.kernel.org>,
        "open list:NETWORKING [GENERAL]" <netdev@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        skhan@linuxfoundation.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel-mentees@lists.linuxfoundation.org,
        syzbot+2f6d7c28bb4bf7e82060@syzkaller.appspotmail.com
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References: <20210728071721.411669-1-desmondcheongzx@gmail.com>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
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Hi Desmond,

> 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, sco_conn_ready, rfcomm_connect_ind, and
> bt_accept_enqueue also 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.
> 
> Link: https://syzkaller.appspot.com/bug?id=9089d89de0502e120f234ca0fc8a703f7368b31e [1]
> Reported-by: syzbot+2f6d7c28bb4bf7e82060@syzkaller.appspotmail.com
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> ---
> 
> Hi,
> 
> As suggested, this patch addresses the inconsistent lock state while
> avoiding having to deal with local_bh_disable.
> 
> Now that sco_sock_timeout is no longer run in IRQ context, it might
> be the case that bh_lock_sock is no longer needed to sync between
> SOFTIRQ and user contexts, so we can switch to lock_sock.
> 
> I'm not too certain about this, or if there's any benefit to using
> lock_sock instead, so I've left that out of this patch.

I don’t see a reason why we can’t switch to lock_sock, but lets do that in a separate patch in case I missed something it is easier to revert.

> 
> v3 -> v4:
> - Switch to using delayed_work to schedule SCO sock timeouts instead
> of using local_bh_disable. As suggested by Luiz Augusto von Dentz.
> 
> v2 -> v3:
> - Split SCO and RFCOMM code changes, as suggested by Luiz Augusto von
> Dentz.
> - Simplify local bh disabling in SCO by using local_bh_disable/enable
> inside sco_chan_del since local_bh_disable/enable pairs are reentrant.
> 
> v1 -> v2:
> - Instead of pulling out the clean-up code out from sco_chan_del and
> using it directly in sco_conn_del, disable local softirqs for relevant
> sections.
> - Disable local softirqs more thoroughly for instances of
> bh_lock_sock/bh_lock_sock_nested in the bluetooth subsystem.
> Specifically, the calls in af_bluetooth.c and rfcomm/sock.c are now made
> with local softirqs disabled as well.
> 
> Best wishes,
> Desmond
> 
> net/bluetooth/sco.c | 39 ++++++++++++++++++++++++---------------
> 1 file changed, 24 insertions(+), 15 deletions(-)
> 
> diff --git a/net/bluetooth/sco.c b/net/bluetooth/sco.c
> index 3bd41563f118..b6dd16153d38 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	sk_timer;
> +

I don’t like the sk_timer name. That is confusing. Maybe better use timeout_work or to_work. The sk_* are really more struct sock fields (hence the sk->sk_xyz naming schema).

> 	unsigned int    mtu;
> };
> 
> @@ -74,9 +76,11 @@ 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,
> +					     sk_timer.work);
> +	struct sock *sk = conn->sk;
> 
> 	BT_DBG("sock %p state %d", sk, sk->sk_state);
> 
> @@ -89,16 +93,18 @@ static void sco_sock_timeout(struct timer_list *t)
> 	sock_put(sk);
> }
> 
> -static void sco_sock_set_timer(struct sock *sk, long timeout)
> +static void sco_sock_set_timer(struct sock *sk, struct delayed_work *work,
> +			       long timeout)
> {

I don’t get the extra variable here. Can we not just pass in struct hci_conn.


> 	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)
> +static void sco_sock_clear_timer(struct sock *sk, struct delayed_work *work)
> {
> 	BT_DBG("sock %p state %d", sk, sk->sk_state);
> -	sk_stop_timer(sk, &sk->sk_timer);
> +	cancel_delayed_work(work);

Same as above, we pass in struct sock just for the debug message.

> }
> 
> /* ---- SCO connections ---- */
> @@ -174,7 +180,7 @@ static void sco_conn_del(struct hci_conn *hcon, int err)
> 	if (sk) {
> 		sock_hold(sk);
> 		bh_lock_sock(sk);
> -		sco_sock_clear_timer(sk);
> +		sco_sock_clear_timer(sk, &conn->sk_timer);
> 		sco_chan_del(sk, err);
> 		bh_unlock_sock(sk);
> 		sco_sock_kill(sk);
> @@ -193,6 +199,8 @@ static void __sco_chan_add(struct sco_conn *conn, struct sock *sk,
> 	sco_pi(sk)->conn = conn;
> 	conn->sk = sk;
> 
> +	INIT_DELAYED_WORK(&conn->sk_timer, sco_sock_timeout);
> +
> 	if (parent)
> 		bt_accept_enqueue(parent, sk, true);
> }
> @@ -260,11 +268,11 @@ static int sco_connect(struct sock *sk)
> 		goto done;
> 
> 	if (hcon->state == BT_CONNECTED) {
> -		sco_sock_clear_timer(sk);
> +		sco_sock_clear_timer(sk, &conn->sk_timer);
> 		sk->sk_state = BT_CONNECTED;
> 	} else {
> 		sk->sk_state = BT_CONNECT;
> -		sco_sock_set_timer(sk, sk->sk_sndtimeo);
> +		sco_sock_set_timer(sk, &conn->sk_timer, sk->sk_sndtimeo);
> 	}
> 
> done:
> @@ -419,7 +427,8 @@ static void __sco_sock_close(struct sock *sk)
> 	case BT_CONFIG:
> 		if (sco_pi(sk)->conn->hcon) {
> 			sk->sk_state = BT_DISCONN;
> -			sco_sock_set_timer(sk, SCO_DISCONN_TIMEOUT);
> +			sco_sock_set_timer(sk, &sco_pi(sk)->conn->sk_timer,
> +					   SCO_DISCONN_TIMEOUT);
> 			sco_conn_lock(sco_pi(sk)->conn);
> 			hci_conn_drop(sco_pi(sk)->conn->hcon);
> 			sco_pi(sk)->conn->hcon = NULL;
> @@ -443,7 +452,8 @@ static void __sco_sock_close(struct sock *sk)
> /* Must be called on unlocked socket. */
> static void sco_sock_close(struct sock *sk)
> {
> -	sco_sock_clear_timer(sk);
> +	if (sco_pi(sk)->conn)
> +		sco_sock_clear_timer(sk, &sco_pi(sk)->conn->sk_timer);
> 	lock_sock(sk);
> 	__sco_sock_close(sk);
> 	release_sock(sk);
> @@ -500,8 +510,6 @@ 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;
> }
> @@ -1036,7 +1044,8 @@ static int sco_sock_shutdown(struct socket *sock, int how)
> 
> 	if (!sk->sk_shutdown) {
> 		sk->sk_shutdown = SHUTDOWN_MASK;
> -		sco_sock_clear_timer(sk);
> +		if (sco_pi(sk)->conn)
> +			sco_sock_clear_timer(sk, &sco_pi(sk)->conn->sk_timer);
> 		__sco_sock_close(sk);
> 
> 		if (sock_flag(sk, SOCK_LINGER) && sk->sk_lingertime &&
> @@ -1083,7 +1092,7 @@ static void sco_conn_ready(struct sco_conn *conn)
> 	BT_DBG("conn %p", conn);
> 
> 	if (sk) {
> -		sco_sock_clear_timer(sk);
> +		sco_sock_clear_timer(sk, &conn->sk_timer);
> 		bh_lock_sock(sk);
> 		sk->sk_state = BT_CONNECTED;
> 		sk->sk_state_change(sk);

Other than these minor cleanups, this looks great.

Regards

Marcel