[PATCH v6] 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 releases chan->lock before acquiring the conn_chan_lock
to avoid the potential deadlock.

Fixes: a2a9339e1c9d ("Bluetooth: L2CAP: Fix use-after-free in l2cap_disconnect_{req,rsp}")
Signed-off-by: Ying Hsu <yinghsu@xxxxxxxxxxxx>
---
This commit has been tested on a Chromebook device.

Changes in v6:
- Fixing format of the fixes tag.

Changes in v5:
- Fixing the merge conflict by removing l2cap_del_chan_by_scid.

Changes in v4:
- Using l2cap_get_chan_by_scid to avoid repeated code.
- Releasing chan->lock before acquiring conn->chan_lock.

Changes in v3:
- Adding the fixes tag.

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

 net/bluetooth/l2cap_core.c | 37 +++++++++++++++----------------------
 1 file changed, 15 insertions(+), 22 deletions(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 036bc147f4de..16ac4aac0638 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -4634,26 +4634,6 @@ static inline int l2cap_config_rsp(struct l2cap_conn *conn,
 	return err;
 }
 
-static struct l2cap_chan *l2cap_del_chan_by_scid(struct l2cap_conn *conn,
-						 u16 cid, int err)
-{
-	struct l2cap_chan *c;
-
-	mutex_lock(&conn->chan_lock);
-	c = __l2cap_get_chan_by_scid(conn, cid);
-	if (c) {
-		/* Only lock if chan reference is not 0 */
-		c = l2cap_chan_hold_unless_zero(c);
-		if (c) {
-			l2cap_chan_lock(c);
-			l2cap_chan_del(c, err);
-		}
-	}
-	mutex_unlock(&conn->chan_lock);
-
-	return c;
-}
-
 static inline int l2cap_disconnect_req(struct l2cap_conn *conn,
 				       struct l2cap_cmd_hdr *cmd, u16 cmd_len,
 				       u8 *data)
@@ -4671,7 +4651,7 @@ static inline int l2cap_disconnect_req(struct l2cap_conn *conn,
 
 	BT_DBG("scid 0x%4.4x dcid 0x%4.4x", scid, dcid);
 
-	chan = l2cap_del_chan_by_scid(conn, dcid, ECONNRESET);
+	chan = l2cap_get_chan_by_scid(conn, dcid);
 	if (!chan) {
 		cmd_reject_invalid_cid(conn, cmd->ident, dcid, scid);
 		return 0;
@@ -4682,6 +4662,13 @@ static inline int l2cap_disconnect_req(struct l2cap_conn *conn,
 	l2cap_send_cmd(conn, cmd->ident, L2CAP_DISCONN_RSP, sizeof(rsp), &rsp);
 
 	chan->ops->set_shutdown(chan);
+
+	l2cap_chan_unlock(chan);
+	mutex_lock(&conn->chan_lock);
+	l2cap_chan_lock(chan);
+	l2cap_chan_del(chan, ECONNRESET);
+	mutex_unlock(&conn->chan_lock);
+
 	chan->ops->close(chan);
 
 	l2cap_chan_unlock(chan);
@@ -4706,7 +4693,7 @@ static inline int l2cap_disconnect_rsp(struct l2cap_conn *conn,
 
 	BT_DBG("dcid 0x%4.4x scid 0x%4.4x", dcid, scid);
 
-	chan = l2cap_del_chan_by_scid(conn, scid, 0);
+	chan = l2cap_get_chan_by_scid(conn, scid);
 	if (!chan)
 		return 0;
 
@@ -4716,6 +4703,12 @@ static inline int l2cap_disconnect_rsp(struct l2cap_conn *conn,
 		return 0;
 	}
 
+	l2cap_chan_unlock(chan);
+	mutex_lock(&conn->chan_lock);
+	l2cap_chan_lock(chan);
+	l2cap_chan_del(chan, 0);
+	mutex_unlock(&conn->chan_lock);
+
 	chan->ops->close(chan);
 
 	l2cap_chan_unlock(chan);
-- 
2.41.0.rc0.172.g3f132b7071-goog