Bluetooth: af_bluetooth: Fix Use-After-Free in bt_sock_recvmsg

This can cause a race with bt_sock_ioctl() because
bt_sock_recvmsg() gets the skb from sk->sk_receive_queue
and then frees it without holding lock_sock.
A use-after-free for a skb occurs with the following flow.
```
bt_sock_recvmsg() -> skb_recv_datagram() -> skb_free_datagram()
bt_sock_ioctl() -> skb_peek()
```
Add lock_sock to bt_sock_recvmsg() to fix this issue.

Cc: stable@vger.kernel.org
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Hyunwoo Kim <v4bel@theori.io>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

net/bluetooth/af_bluetooth.c

@@ -309,11 +309,14 @@ int bt_sock_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 	if (flags & MSG_OOB)
 		return -EOPNOTSUPP;
 
+	lock_sock(sk);
+
 	skb = skb_recv_datagram(sk, flags, &err);
 	if (!skb) {
 		if (sk->sk_shutdown & RCV_SHUTDOWN)
-			return 0;
+			err = 0;
 
+		release_sock(sk);
 		return err;
 	}
 
@@ -343,6 +346,8 @@ int bt_sock_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 
 	skb_free_datagram(sk, skb);
 
+	release_sock(sk);
+
 	if (flags & MSG_TRUNC)
 		copied = skblen;