[PATCH] bluetooth/l2cap: sync sock recv cb and release

[Edward Adam Davis]

The problem occurs between the system call to close the sock and hci_rx_work,
where the former releases the sock and the latter accesses it without lock protection.

           CPU0                       CPU1
           ----                       ----
           sock_close                 hci_rx_work
	   l2cap_sock_release         hci_acldata_packet
	   l2cap_sock_kill            l2cap_recv_frame
	   sk_free                    l2cap_conless_channel
	                              l2cap_sock_recv_cb

If hci_rx_work processes the data that needs to be received before the sock is
closed, then everything is normal; Otherwise, the work thread may access the
released sock when receiving data.

Add a chan mutex in the rx callback of the sock to achieve synchronization between
the sock release and recv cb.

Reported-and-tested-by: syzbot+b7f6f8c9303466e16c8a@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
---
 net/bluetooth/l2cap_sock.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index 6db60946c627..f3e9236293e1 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c
@@ -1413,6 +1413,8 @@ static int l2cap_sock_release(struct socket *sock)
 	l2cap_chan_hold(chan);
 	l2cap_chan_lock(chan);
 
+	if (refcount_read(&sk->sk_refcnt) == 1)
+		chan->data = NULL;
 	sock_orphan(sk);
 	l2cap_sock_kill(sk);
 
@@ -1481,12 +1483,22 @@ static struct l2cap_chan *l2cap_sock_new_connection_cb(struct l2cap_chan *chan)
 
 static int l2cap_sock_recv_cb(struct l2cap_chan *chan, struct sk_buff *skb)
 {
-	struct sock *sk = chan->data;
-	struct l2cap_pinfo *pi = l2cap_pi(sk);
+	struct sock *sk;
+	struct l2cap_pinfo *pi;
 	int err;
 
-	lock_sock(sk);
+	l2cap_chan_hold(chan);
+	l2cap_chan_lock(chan);
+	sk = chan->data;
+
+	if (!sk) {
+		l2cap_chan_unlock(chan);
+		l2cap_chan_put(chan);
+		return -ENXIO;
+	}
 
+	pi = l2cap_pi(sk);
+	lock_sock(sk);
 	if (chan->mode == L2CAP_MODE_ERTM && !list_empty(&pi->rx_busy)) {
 		err = -ENOMEM;
 		goto done;
@@ -1535,6 +1547,8 @@ static int l2cap_sock_recv_cb(struct l2cap_chan *chan, struct sk_buff *skb)
 
 done:
 	release_sock(sk);
+	l2cap_chan_unlock(chan);
+	l2cap_chan_put(chan);
 
 	return err;
 }
-- 
2.43.0