[PATCH v2] Bluetooth: Fix l2cap_disconnect_req deadlock

[Ying Hsu <yinghsu@xxxxxxxxxxxx>]

L2CAP assumes that the locks conn->chan_lock and chan->lock are
acquired in the order conn->chan_lock, chan->lock to avoid
potential deadlock.
For example, l2sock_shutdown acquires these locks in the order:
  mutex_lock(&conn->chan_lock)
  l2cap_chan_lock(chan)

However, l2cap_disconnect_req acquires chan->lock in
l2cap_get_chan_by_scid first and then acquires conn->chan_lock
before calling l2cap_chan_del. This means that these locks are
acquired in unexpected order, which leads to potential deadlock:
  l2cap_chan_lock(c)
  mutex_lock(&conn->chan_lock)

This patch uses __l2cap_get_chan_by_scid to replace
l2cap_get_chan_by_scid and adjusts the locking order to avoid the
potential deadlock.

Signed-off-by: Ying Hsu <yinghsu@xxxxxxxxxxxx>
---
This commit has been tested on a Chromebook device.

Changes in v2:
- Adding the prefix "Bluetooth:" to subject line.

 net/bluetooth/l2cap_core.c | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 376b523c7b26..8f08192b8fb1 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -4651,8 +4651,16 @@ static inline int l2cap_disconnect_req(struct l2cap_conn *conn,
 
 	BT_DBG("scid 0x%4.4x dcid 0x%4.4x", scid, dcid);
 
-	chan = l2cap_get_chan_by_scid(conn, dcid);
+	mutex_lock(&conn->chan_lock);
+	chan = __l2cap_get_chan_by_scid(conn, dcid);
+	if (chan) {
+		chan = l2cap_chan_hold_unless_zero(chan);
+		if (chan)
+			l2cap_chan_lock(chan);
+	}
+
 	if (!chan) {
+		mutex_unlock(&conn->chan_lock);
 		cmd_reject_invalid_cid(conn, cmd->ident, dcid, scid);
 		return 0;
 	}
@@ -4663,14 +4671,13 @@ static inline int l2cap_disconnect_req(struct l2cap_conn *conn,
 
 	chan->ops->set_shutdown(chan);
 
-	mutex_lock(&conn->chan_lock);
 	l2cap_chan_del(chan, ECONNRESET);
-	mutex_unlock(&conn->chan_lock);
 
 	chan->ops->close(chan);
 
 	l2cap_chan_unlock(chan);
 	l2cap_chan_put(chan);
+	mutex_unlock(&conn->chan_lock);
 
 	return 0;
 }
@@ -4691,25 +4698,32 @@ static inline int l2cap_disconnect_rsp(struct l2cap_conn *conn,
 
 	BT_DBG("dcid 0x%4.4x scid 0x%4.4x", dcid, scid);
 
-	chan = l2cap_get_chan_by_scid(conn, scid);
+	mutex_lock(&conn->chan_lock);
+	chan = __l2cap_get_chan_by_scid(conn, scid);
+	if (chan) {
+		chan = l2cap_chan_hold_unless_zero(chan);
+		if (chan)
+			l2cap_chan_lock(chan);
+	}
 	if (!chan) {
+		mutex_unlock(&conn->chan_lock);
 		return 0;
 	}
 
 	if (chan->state != BT_DISCONN) {
 		l2cap_chan_unlock(chan);
 		l2cap_chan_put(chan);
+		mutex_unlock(&conn->chan_lock);
 		return 0;
 	}
 
-	mutex_lock(&conn->chan_lock);
 	l2cap_chan_del(chan, 0);
-	mutex_unlock(&conn->chan_lock);
 
 	chan->ops->close(chan);
 
 	l2cap_chan_unlock(chan);
 	l2cap_chan_put(chan);
+	mutex_unlock(&conn->chan_lock);
 
 	return 0;
 }
-- 
2.40.1.698.g37aff9b760-goog