Lab Exercise oob1

This is a lab exercise on developing secure software. For more information, see the introduction to the labs.

Task

Please change the code below to fix this simplified version of the "Heartbleed" bug in OpenSSL, an example of a buffer overread.

Background

In almost all programming languages, if program attempts to read or write outside of a buffer, the default is always either an attempt to resize the buffer or an error of some kind (e.g., by raising an exception). That's because it's extremely easy to accidentally attempt to read or write outside of a buffer.

However, C and C++ are different. C++ has evolved to become somewhat safer (e.g., through smart pointers), In C and C++, attempting to do actions like read or write outside a buffer is in many cases undefined behavior, and when undefined behavior occurs, anything is allowed to happen without any kind of protection. In practice, what often happens is a read or write (respectively) of other data. There are proposals to improve C++ memory safety, but currently many C++ built-in constructs (like arrays) are not memory safe, so we will treat the two languages together here.

The 2014 revelation of the Heartbleed vulnerability (CVE-2014-0160) is an example of a buffer overread vulnerability. Heartbleed was a vulnerability in OpenSSL, a widely-used toolkit written in C that implements the cryptographic protocol Secure Sockets Layer (SSL) and its successor the Transport Layer Security (TLS). Heartbleed affected a huge number of popular websites, including Google, YouTube, Yahoo!, Pinterest, Blogspot, Instagram, Tumblr, Reddit, Netflix, Stack Overflow, Slate, GitHub, Yelp, Etsy, the U.S. Postal Service (USPS), Blogger, Dropbox, Wikipedia, and the Washington Post. See here for more.

Task Information

We're going re-create the fix in OpenSSF for the Heartbleed vulnerability by modifying function dtls1_process_heartbeat.

At this point in the code, the construct s->s3->rrec.length indicates how many bytes are available. If we don't check for the maximum sizes, we could easily cause reading beyond a buffer. Modify the code below in two places to fix this.

First, modify the code so that if the minimum length of a response (1 + 2 + 16) is more than the length claimed by s->s3->rrec.length, then return 0 without sending a heartbeat, This will prevent trying to create a heartbeat when there's not enough room to create a heartbeat at all.

Second, modify the code so that if the minimum length of a response with a payload (1 + 2 + payload + 16) is more than the total length for a response given in s->s3->rrec.length then again return 0 without sending a heartbeat.

This will prevent trying to create a heartbeat when there's not enough room to create one and there was a payload to return as a heartbeat.

Note that this is not terribly difficult to fix. The code we add is short. The problem is that reading and writing buffers is extremely common, but by default such accesses are unsafe in C and C++. In practice it is difficult to always check all ranges in all possible cases, which is why memory safety vulnerabilities are so common in programs written in C or C++.

Use the “hint” and “give up” buttons if necessary.

Interactive Lab ()

int
dtls1_process_heartbeat(SSL *s)
  {
  unsigned char *p = &s->s3->rrec.data[0], *pl;
  unsigned short hbtype;
  unsigned int payload;

  // ... Some details omitted here

  hbtype = *p++;
  n2s(p, payload);

  // ... Later on there will be a memory copy ("memcpy")
  // to copy the payload data into a new buffer. If we
  // had not checked, it would not be long enough:
  //   memcpy(bp, pl, payload);


This lab was developed by David A. Wheeler at The Linux Foundation.

Source: This example was extracted from OpenSSL file ssl/d1_both.c commit 731f431497f4; you can see its full header by expanding details here. 
/* ssl/d1_both.c */
/* 
 * DTLS implementation written by Nagendra Modadugu
 * (nagendra@cs.stanford.edu) for the OpenSSL project 2005.  
 */
/* ====================================================================
 * Copyright (c) 1998-2005 The OpenSSL Project.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer. 
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. All advertising materials mentioning features or use of this
 *    software must display the following acknowledgment:
 *    "This product includes software developed by the OpenSSL Project
 *    for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
 *
 * 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
 *    endorse or promote products derived from this software without
 *    prior written permission. For written permission, please contact
 *    openssl-core@openssl.org.
 *
 * 5. Products derived from this software may not be called "OpenSSL"
 *    nor may "OpenSSL" appear in their names without prior written
 *    permission of the OpenSSL Project.
 *
 * 6. Redistributions of any form whatsoever must retain the following
 *    acknowledgment:
 *    "This product includes software developed by the OpenSSL Project
 *    for use in the OpenSSL Toolkit (http://www.openssl.org/)"
 *
 * THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
 * EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE OpenSSL PROJECT OR
 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 * ====================================================================
 *
 * This product includes cryptographic software written by Eric Young
 * (eay@cryptsoft.com).  This product includes software written by Tim
 * Hudson (tjh@cryptsoft.com).
 *
 */
/* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
 * All rights reserved.
 *
 * This package is an SSL implementation written
 * by Eric Young (eay@cryptsoft.com).
 * The implementation was written so as to conform with Netscapes SSL.
 * 
 * This library is free for commercial and non-commercial use as long as
 * the following conditions are aheared to.  The following conditions
 * apply to all code found in this distribution, be it the RC4, RSA,
 * lhash, DES, etc., code; not just the SSL code.  The SSL documentation
 * included with this distribution is covered by the same copyright terms
 * except that the holder is Tim Hudson (tjh@cryptsoft.com).
 * 
 * Copyright remains Eric Young's, and as such any Copyright notices in
 * the code are not to be removed.
 * If this package is used in a product, Eric Young should be given attribution
 * as the author of the parts of the library used.
 * This can be in the form of a textual message at program startup or
 * in documentation (online or textual) provided with the package.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *    "This product includes cryptographic software written by
 *     Eric Young (eay@cryptsoft.com)"
 *    The word 'cryptographic' can be left out if the rouines from the library
 *    being used are not cryptographic related :-).
 * 4. If you include any Windows specific code (or a derivative thereof) from 
 *    the apps directory (application code) you must include an acknowledgement:
 *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
 * 
 * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * 
 * The licence and distribution terms for any publically available version or
 * derivative of this code cannot be changed.  i.e. this code cannot simply be
 * copied and put under another distribution licence
 * [including the GNU Public Licence.]
 */