Re: [PATCH v3 1/2] Bluetooth: fix inconsistent lock state in SCO

[Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>]

On 27/7/21 8:30 am, Luiz Augusto von Dentz wrote:
> Hi Desmond,
>
> On Wed, Jul 21, 2021 at 2:39 AM Desmond Cheong Zhi Xi
> <desmondcheongzx@xxxxxxxxx> wrote:
> > Syzbot reported an inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} lock
> > usage in sco_conn_del and sco_sock_timeout that could lead to
> > deadlocks.
> >
> > This inconsistent lock state can also happen in sco_conn_ready,
> > rfcomm_connect_ind, and bt_accept_enqueue.
> >
> > The issue is that these functions take a spin lock on the socket with
> > interrupts enabled, but sco_sock_timeout takes the lock in an IRQ
> > context. This could lead to deadlocks:
> >
> >         CPU0
> >         ----
> >    lock(slock-AF_BLUETOOTH-BTPROTO_SCO);
> >    <Interrupt>
> >      lock(slock-AF_BLUETOOTH-BTPROTO_SCO);
>
> Having a second look at this, it does seem this is due to use of
> sk->sk_timer which apparently run its callback on IRQ context, so I
> wonder if wouldn't be a better idea to switch to a delayed_work to
> avoid having to deal with the likes of local_bh_disable, in fact it
> seems we have been misusing it since the documentation says it is for
> sock cleanup not for handling things like SNDTIMEO, we don't really
> use it for other socket types so I wonder when we start using
> delayed_work we forgot about sco.c.

Hi Luiz,

That makes sense to me. I don't think there's a need for the timeout to 
be run in an IRQ context.

I'll prepare a patch for this, thanks for the suggestion.

> >   *** DEADLOCK ***
> >
> > We fix this by ensuring that local bh is disabled before calling
> > bh_lock_sock.
> >
> > After doing this, we additionally need to protect sco_conn_lock by
> > disabling local bh.
> >
> > This is necessary because sco_conn_del makes a call to sco_chan_del
> > while holding on to the sock lock, and sco_chan_del itself makes a
> > call to sco_conn_lock. If sco_conn_lock is held elsewhere with
> > interrupts enabled, there could still be a
> > slock-AF_BLUETOOTH-BTPROTO_SCO --> &conn->lock#2 lock inversion as
> > follows:
> >
> >          CPU0                    CPU1
> >          ----                    ----
> >     lock(&conn->lock#2);
> >                                  local_irq_disable();
> >                                  lock(slock-AF_BLUETOOTH-BTPROTO_SCO);
> >                                  lock(&conn->lock#2);
> >     <Interrupt>
> >       lock(slock-AF_BLUETOOTH-BTPROTO_SCO);
> >
> >    *** DEADLOCK ***
> >
> > Although sco_conn_del disables local bh before calling sco_chan_del,
> > we can still wrap the calls to sco_conn_lock in sco_chan_del, with
> > local_bh_disable/enable as this pair of functions are reentrant.
> >
> > Reported-by: syzbot+2f6d7c28bb4bf7e82060@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Tested-by: syzbot+2f6d7c28bb4bf7e82060@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> > ---
> >   net/bluetooth/sco.c | 21 +++++++++++++++++++++
> >   1 file changed, 21 insertions(+)
> >
> > diff --git a/net/bluetooth/sco.c b/net/bluetooth/sco.c
> > index 3bd41563f118..34f3419c3330 100644
> > --- a/net/bluetooth/sco.c
> > +++ b/net/bluetooth/sco.c
> > @@ -140,10 +140,12 @@ static void sco_chan_del(struct sock *sk, int err)
> >          BT_DBG("sk %p, conn %p, err %d", sk, conn, err);
> >
> >          if (conn) {
> > +               local_bh_disable();
> >                  sco_conn_lock(conn);
> >                  conn->sk = NULL;
> >                  sco_pi(sk)->conn = NULL;
> >                  sco_conn_unlock(conn);
> > +               local_bh_enable();
> >
> >                  if (conn->hcon)
> >                          hci_conn_drop(conn->hcon);
> > @@ -167,16 +169,22 @@ static void sco_conn_del(struct hci_conn *hcon, int err)
> >          BT_DBG("hcon %p conn %p, err %d", hcon, conn, err);
> >
> >          /* Kill socket */
> > +       local_bh_disable();
> >          sco_conn_lock(conn);
> >          sk = conn->sk;
> >          sco_conn_unlock(conn);
> > +       local_bh_enable();
> >
> >          if (sk) {
> >                  sock_hold(sk);
> > +
> > +               local_bh_disable();
> >                  bh_lock_sock(sk);
> >                  sco_sock_clear_timer(sk);
> >                  sco_chan_del(sk, err);
> >                  bh_unlock_sock(sk);
> > +               local_bh_enable();
> > +
> >                  sco_sock_kill(sk);
> >                  sock_put(sk);
> >          }
> > @@ -202,6 +210,7 @@ static int sco_chan_add(struct sco_conn *conn, struct sock *sk,
> >   {
> >          int err = 0;
> >
> > +       local_bh_disable();
> >          sco_conn_lock(conn);
> >          if (conn->sk)
> >                  err = -EBUSY;
> > @@ -209,6 +218,7 @@ static int sco_chan_add(struct sco_conn *conn, struct sock *sk,
> >                  __sco_chan_add(conn, sk, parent);
> >
> >          sco_conn_unlock(conn);
> > +       local_bh_enable();
> >          return err;
> >   }
> >
> > @@ -303,9 +313,11 @@ static void sco_recv_frame(struct sco_conn *conn, struct sk_buff *skb)
> >   {
> >          struct sock *sk;
> >
> > +       local_bh_disable();
> >          sco_conn_lock(conn);
> >          sk = conn->sk;
> >          sco_conn_unlock(conn);
> > +       local_bh_enable();
> >
> >          if (!sk)
> >                  goto drop;
> > @@ -420,10 +432,12 @@ static void __sco_sock_close(struct sock *sk)
> >                  if (sco_pi(sk)->conn->hcon) {
> >                          sk->sk_state = BT_DISCONN;
> >                          sco_sock_set_timer(sk, SCO_DISCONN_TIMEOUT);
> > +                       local_bh_disable();
> >                          sco_conn_lock(sco_pi(sk)->conn);
> >                          hci_conn_drop(sco_pi(sk)->conn->hcon);
> >                          sco_pi(sk)->conn->hcon = NULL;
> >                          sco_conn_unlock(sco_pi(sk)->conn);
> > +                       local_bh_enable();
> >                  } else
> >                          sco_chan_del(sk, ECONNRESET);
> >                  break;
> > @@ -1084,21 +1098,26 @@ static void sco_conn_ready(struct sco_conn *conn)
> >
> >          if (sk) {
> >                  sco_sock_clear_timer(sk);
> > +               local_bh_disable();
> >                  bh_lock_sock(sk);
> >                  sk->sk_state = BT_CONNECTED;
> >                  sk->sk_state_change(sk);
> >                  bh_unlock_sock(sk);
> > +               local_bh_enable();
> >          } else {
> > +               local_bh_disable();
> >                  sco_conn_lock(conn);
> >
> >                  if (!conn->hcon) {
> >                          sco_conn_unlock(conn);
> > +                       local_bh_enable();
> >                          return;
> >                  }
> >
> >                  parent = sco_get_sock_listen(&conn->hcon->src);
> >                  if (!parent) {
> >                          sco_conn_unlock(conn);
> > +                       local_bh_enable();
> >                          return;
> >                  }
> >
> > @@ -1109,6 +1128,7 @@ static void sco_conn_ready(struct sco_conn *conn)
> >                  if (!sk) {
> >                          bh_unlock_sock(parent);
> >                          sco_conn_unlock(conn);
> > +                       local_bh_enable();
> >                          return;
> >                  }
> >
> > @@ -1131,6 +1151,7 @@ static void sco_conn_ready(struct sco_conn *conn)
> >                  bh_unlock_sock(parent);
> >
> >                  sco_conn_unlock(conn);
> > +               local_bh_enable();
> >          }
> >   }
> >
> > --
> > 2.25.1