[PATCH] Bluetooth: fix inconsistent lock state in sco

Desmond Cheong Zhi Xi desmondcheongzx at gmail.com
Mon Jun 28 07:48:34 UTC 2021


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:
https://syzkaller.appspot.com/bug?id=9089d89de0502e120f234ca0fc8a703f7368b31e

This inconsistent lock state can also happen between sco_conn_ready
and sco_sock_timeout.

The issue is that these three functions take a spin lock on the
socket, but sco_sock_timeout is called from an IRQ context. Since
bh_lock_sock calls spin_lock but does not disable softirqs, this could
lead to deadlocks:

       CPU0
       ----
  lock(slock-AF_BLUETOOTH-BTPROTO_SCO);
  <Interrupt>
    lock(slock-AF_BLUETOOTH-BTPROTO_SCO);

 *** DEADLOCK ***

We fix this by replacing bh_lock_sock with spin_lock_bh in
sco_conn_del and sco_conn_ready.

Additionally, to avoid regressions, we pull the clean-up code out from
sco_chan_del and use it directly in sco_conn_del. This is necessary
because sco_chan_del makes a call to sco_conn_lock which takes an
SOFTIRQ-unsafe lock. This means that calling sco_chan_del while
holding the socket lock would result in a SOFTIRQ-safe ->
SOFTIRQ-unsafe lock hierarchy between slock-AF_BLUETOOTH-BTPROTO_SCO
and &conn->lock#2. This could lead to a deadlock as well:

        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 ***

Pulling out the code from sco_chan_del allows us to avoid this lock
dependency by holding the two locks for only their required critical
sections.

Reported-and-tested-by: syzbot+2f6d7c28bb4bf7e82060 at syzkaller.appspotmail.com
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx at gmail.com>
---
 net/bluetooth/sco.c | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/net/bluetooth/sco.c b/net/bluetooth/sco.c
index 3bd41563f118..d05629d7cc55 100644
--- a/net/bluetooth/sco.c
+++ b/net/bluetooth/sco.c
@@ -173,10 +173,22 @@ static void sco_conn_del(struct hci_conn *hcon, int err)
 
 	if (sk) {
 		sock_hold(sk);
-		bh_lock_sock(sk);
+
+		spin_lock_bh(&sk->sk_lock.slock);
 		sco_sock_clear_timer(sk);
-		sco_chan_del(sk, err);
-		bh_unlock_sock(sk);
+		sco_pi(sk)->conn = NULL;
+		if (conn->hcon)
+			hci_conn_drop(conn->hcon);
+		sk->sk_state = BT_CLOSED;
+		sk->sk_err   = err;
+		sk->sk_state_change(sk);
+		sock_set_flag(sk, SOCK_ZAPPED);
+		spin_unlock_bh(&sk->sk_lock.slock);
+
+		sco_conn_lock(conn);
+		conn->sk = NULL;
+		sco_conn_unlock(conn);
+
 		sco_sock_kill(sk);
 		sock_put(sk);
 	}
@@ -1084,10 +1096,10 @@ static void sco_conn_ready(struct sco_conn *conn)
 
 	if (sk) {
 		sco_sock_clear_timer(sk);
-		bh_lock_sock(sk);
+		spin_lock_bh(&sk->sk_lock.slock);
 		sk->sk_state = BT_CONNECTED;
 		sk->sk_state_change(sk);
-		bh_unlock_sock(sk);
+		spin_unlock_bh(&sk->sk_lock.slock);
 	} else {
 		sco_conn_lock(conn);
 
@@ -1102,12 +1114,12 @@ static void sco_conn_ready(struct sco_conn *conn)
 			return;
 		}
 
-		bh_lock_sock(parent);
+		spin_lock_bh(&parent->sk_lock.slock);
 
 		sk = sco_sock_alloc(sock_net(parent), NULL,
 				    BTPROTO_SCO, GFP_ATOMIC, 0);
 		if (!sk) {
-			bh_unlock_sock(parent);
+			spin_unlock_bh(&parent->sk_lock.slock);
 			sco_conn_unlock(conn);
 			return;
 		}
@@ -1128,7 +1140,7 @@ static void sco_conn_ready(struct sco_conn *conn)
 		/* Wake up parent */
 		parent->sk_data_ready(parent);
 
-		bh_unlock_sock(parent);
+		spin_unlock_bh(&parent->sk_lock.slock);
 
 		sco_conn_unlock(conn);
 	}
-- 
2.25.1



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