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

[Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>]

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@gmail.com> 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@syzkaller.appspotmail.com
>> Tested-by: syzbot+2f6d7c28bb4bf7e82060@syzkaller.appspotmail.com
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
>> ---
>>   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
>>
> 
>