[PATCH] Bluetooth: L2CAP: Fix use-after-free

[Sungwoo Kim]

Due to the race condition between l2cap_sock_cleanup_listen and
l2cap_sock_close_cb, l2cap_sock_kill can receive already freed sk,
resulting in use-after-free inside l2cap_sock_kill.
This patch prevent this by adding a null check in l2cap_sock_kill.

Context 1:
l2cap_sock_cleanup_listen();
  // context switched
  l2cap_chan_lock(chan);
  l2cap_sock_kill(sk); // <-- sk is already freed below

Context 2:
l2cap_chan_timeout();
  l2cap_chan_lock(chan);
  chan->ops->close(chan);
    l2cap_sock_close_cb()
    l2cap_sock_kill(sk); // <-- sk is freed here
  l2cap_chan_unlock(chan);

Signed-off-by: Sungwoo Kim <iam@xxxxxxxxxxxx>
---
 net/bluetooth/l2cap_sock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index ca8f07f35..657704059 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c
@@ -1245,7 +1245,7 @@ static int l2cap_sock_recvmsg(struct socket *sock, struct msghdr *msg,
  */
 static void l2cap_sock_kill(struct sock *sk)
 {
-	if (!sock_flag(sk, SOCK_ZAPPED) || sk->sk_socket)
+	if (!sk || !sock_flag(sk, SOCK_ZAPPED) || sk->sk_socket)
 		return;
 
 	BT_DBG("sk %p state %s", sk, state_to_string(sk->sk_state));
-- 
2.25.1