staging: ncpfs: memory corruption in ncp_read_kernel()

BugLink: http://bugs.launchpad.net/bugs/1764973

commit 4c41aa24 upstream.

If the server is malicious then *bytes_read could be larger than the
size of the "target" buffer.  It would lead to memory corruption when we
do the memcpy().
Reported-by: Dr Silvio Cesare of InfoSect &lt;Silvio Cesare <silvio.cesare@gmail.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

fs/ncpfs/ncplib_kernel.c

@@ -980,6 +980,10 @@ ncp_read_kernel(struct ncp_server *server, const char *file_id,
 		goto out;
 	}
 	*bytes_read = ncp_reply_be16(server, 0);
+	if (*bytes_read > to_read) {
+		result = -EINVAL;
+		goto out;
+	}
 	source = ncp_reply_data(server, 2 + (offset & 1));
 
 	memcpy(target, source, *bytes_read);