分手后S朋友 发表于 2016-11-30 02:02:20
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Security practices and philosophyNo way to thoroughly lock down a systemThe need for extended access controlsExtended access controls are too complex
Firstly, I would to apologize for, and clarify the title of this article. I wanted to use a title which would hold attention and encourage discussion while remaining true to the argument I make. I certainly don’t mean to imply that OpenBSD is a horribly insecure operating system – it isn’t. I do however need to highlight that OpenBSD is quite far removed from a secure operating system, and will attempt to justify this position below.
To start, we must clarify at a bare minimum what a secure operating system can be considered to be. Generally, this would be taken to mean an operating system that was designed with security in mind, and provides various methods and tools to implement security polices and limits on the system. This definition cannot be applied to OpenBSD as OpenBSD was not designed with security in mind and provides no real way to lock down and limit a system above standard UNIX permissions, which are insufficient.
Despite this OpenBSD is widely regarded as being one of the most secure operating systems currently available. The OpenBSD approach to security is primarily focused on writing quality code, with the aim being to eliminate vulnerabilities in source code. To this end, the OpenBSD team has been quite successful, with the base system having had very few vulnerabilities in "a heck of a long time". While this approach is commendable, it is fundamentally flawed when compared to the approach taken by various extended access control frameworks.
The extended access control frameworks that I refer to are generally implementations of MAC, RBAC, TE or some combination or variation of these basic models. There are many different implementations, generally written for Linux due to its suitability as a testing platform. The most popular implementations are summarized below.
There are other simpler implementations such as SMACK and Tomoyo which are officially in the Linux kernel, as well as implementations for other platforms such as TrustedBSD and Trusted Solaris. Each of these access control frameworks provide for additional security to be setup when compared to what can be done with OpenBSD by default.
- SELinux is based on the FLASK architecture, is developed primarily by the NSA, and ships with some Linux distributions by default, such as Debian and Fedora. SELinux implements a form of MAC known as Domain and Type Enforcement.
- RSBAC is developed by German developer Dr. Amon Ott, and is an implementation of the GFAC architecture. RSBAC provides many models to choose from such as MAC, RBAC and an extensive ACL model. RSBAC ships with the Hardened Gentoo distribution.
- GRSecurity is not primarily an access control framework, but a collection of security enhancements to the Linux kernel, such as JAIL support, PID randomization and similar things, as well as having an ACL and RBAC implementation.
- AppArmor is a simple yet powerful MAC implementation, which relies on pathnames to enforce policies. Relying on pathnames is a weaker approach than that used by the above frameworks; however this is considered acceptable because it is easier to use. AppArmor ships with and is enabled is versions of Ubuntu and OpenSUSE.
Secure by default
OpenBSD is widely touted as being ‘secure by default’, something often mentioned by OpenBSD advocates as an example of the security focused approach the OpenBSD project takes. Secure by default refers to the fact that the base system has been audited and considered to be free of vulnerabilities, and that only the minimal services are running by default. This approach has worked well; indeed, leading to ‘Only two remote holes in the default install, in a heck of a long time!’. This is a common sense approach, and a secure default configuration should be expected of all operating systems upon an initial install.
An argument often made by proponents of OpenBSD is the extensive code auditing performed on the base system to make sure no vulnerabilities are present. The goal is to produce quality code as most vulnerabilities are caused by errors in the source code. This a noble approach, and it has worked well for the OpenBSD project, with the base system having considerably less vulnerabilities than many other operating systems.
Used as an indicator to gauge the security of OpenBSD however, it is worthless. The reason being is that as soon as a service is enabled or software from the ports tree installed, it is no longer the default install and the possibility of introduced vulnerabilities is equal to any other platform. Much like software certified against the common criteria, as soon as an external variable is introduced the certification, or in this case the claim can no longer be considered relevant.
It is important to note also that onlythe base system is audited. The OpenBSD ports tree is not audited, and much of the software available in the ports tree is several releases behind current versions, meaning that there is a strong possibility that software will be obtained from outside of the ports tree. Given that a default install of OpenBSD has all network services are disabled by default, it is very likely that software will be installed or a service enabled if the server is going to be used to actually provide any kind of service.
Since the majority of attacks are not against the base system but against software operating at a higher level actively listening over the network, it is likely that if an OpenBSD machine were attacked, it would be through such software. This is where OpenBSD falls down, as it provides no means to protect from damage in the event of a successful attack.
Providing a default secure configuration is an important practice, and one that is employed by the majority of operating systems these days. OpenBSD followed this practice in the early part of the last decade when most other operating systems did not bother, and for that the OpenBSD team should be praised. While it is a good practice it is specious at best to take this as a measure of the actual security OpenBSD provides.
It should also be noted that the OpenBSD team uses a different definition of security vulnerability, limited to vulnerabilities that are allow for remote arbitrary code to execute. While most people may consider a DOS attack or local privilege escalation problems to be vulnerabilities, the OpenBSD team disagrees. If we use a more generally accepted definition of security vulnerability, OpenBSD suddenly has a far greater number than two remote holes in the default install a heck of a long time.
Security practices and philosophy
The OpenBSD team seems very reluctant to actually admit security problems and work towards fixing them. One such example is this CoreSecurity advisoryfrom 2007. Instead of working and testing to see the extent of the damage that could be caused by a particular vulnerability, they prefer to dismiss and assume arbitrary code execution is impossible until pushed by Core releasing proof of concept code to show otherwise. This is similar to behavior observed by many corporations. Unfortunately this seems to be typical behavior rather than an exception going by the various mailing list threads when a vulnerability is reported.
OpenBSD was never designed with security in mind. OpenBSD was started when Theo de Raadt left the NetBSD project, with the goal of providing complete access to the source repositories. The focus on security came at a later stage, along with the “secure by default” slogan. As noted above, a secure operating system is not synonymous with a lack of vulnerabilities, and certainly not with a lack of vulnerabilities limited to the base install. This should be contrasted with the various extended access control frameworks, which despite being patches to an existing project, were designed from the ground up with a focus on security.
OpenBSD by itself contains a feature set similar in comparison to the GRSecurity patch for Linux without the ACL or RBAC implementation. GRSecurity and the Openwall project actually pioneered many of the protections that occurred later in OpenBSD such as Executable Space Protection, chroot restrictions, PID randomization and attempts to prevent race conditions. OpenBSD is often credited with pioneering many advances in security when this is not the case. OpenBSD tends to add protections much later, and only when absolutely necessary as they continue to erroneously believe that eliminating vulnerabilities in the base system is sufficient.
It is also odd that for a project that claims to be focused on security, sendmail is still their MTA of choice and BIND is still their DNS server of choice. Sendmail and BIND are old, and they both have atrocious security records. To look through OpenBSD’s security history, many of the vulnerabilities can be attributed to BIND or Sendmail. Why would anyone choose these programs for a security focused operating system, when far more secure alternatives designed from the ground up to be secure are available? Examples might include Exim or Postfix and MaraDNS or NSD.
It is interesting to compare OpenBSD to its cousin, FreeBSD. While FreeBSD does not claim to have a focus on security, it is in fact a far more secure operating system than OpenBSD due to its implementation of the TrustedBSD projects work. FreeBSD implements proper access control lists event auditing, extended file system attributes, fine-grained capabilities and mandatory access controls which allow for a system to be completely locked down and access controlled as necessary to protect against users or break in attempts.
Despite the TrustedBSD codebase being open and available for OpenBSD to implement or improve, they reject it simply because they consider it to be too complex and unnecessary. Even if the OpenBSD team did not want to implement extended access controls they could implement proper auditing through the OpenBSD project, which they still reject as unnecessary.
It is no wonder then that when governments or organizations look for a secure operating system, they look to systems that have proper access control lists and auditing, something OpenBSD is not concerned about. A good example of this is China choosing FreeBSD as the base of their secure operating system, as OpenBSD was considered insufficient to meet the criteria.
The library calls strlcpy and strlcat should also be mentioned here. These library calls were developed by Todd Miller and Theo de Raadt as a way to eliminate buffer overflows by ensuring strings are always null terminated. However this approach is controversial, and can actually result in further problems and security vulnerabilities than they solve. While they may have their place, they should certainly not be relied on, and doing so shows a poor understanding of computer security.
No way to thoroughly lock down a system
This is the main problem with OpenBSD, and what prevents it from being able to be considered a secure system. No matter how quality the codebase or how free of vulnerabilities, there is no sufficient way to restrict access other than with standard UNIX permissions. OpenBSD team leader Theo de Raadt has openly stated that he is against anything more powerful such as MAC being implemented which is a shame. There is no good reason to avoid implementing extended access controls when the greater security and control they provide is irrefutable.
OpenBSD does offer some basic protections to protect a running system, namely the chroot functionality, chflags and securelevels. The chroot implementation is a secure version much improved over the standard UNIX chroot, but still far lacking when compared to a proper jail implementation such as that provided by FreeBSD. The consensus among OpenBSD developers and community is that you can achieve the same result using chroot and systrace. Which means they rely on a third party tool to implement a secure design that is present by default in FreeBSD, NetBSD and numerous other unices.
Securelevels are an interesting concepts and they do help with security somewhat. Securelevels can only be increased not decreased on a running system. The higher levels prevent writing to /dev/mem and /dev/kmem, removing file immutable flags, loading kernel modules and changing pf rules. These all help to restrict what an attacker can do, but do absolutely nothing to prevent reading or changing database records, obtaining user info, running malicious programs etc. These protections do absolutely nothing to stop information leakage. Making files immutable or appendable only is a poor option when contrasted with the ability to prevent reading and writing/appending to only specific users or processes.
The OpenBSD project and community had access to a tool for policy enforcement named systrace. Systrace is a third party tool developed by Niels Provos, and has never been embraced by the OpenBSD team. Systrace lacks the versatility of a proper MAC implementation, and had similar weaknesses to AppArmor since it relies on pathnames for enforcement. Systrace is a form of system call interposition, which has been shown to be insecure.
The only software even close to a MAC implementation is rejected by the OpenBSD team, and is insecure. Despite this, systrace is still maintained and offered/recommended by the community as the preferred way to sandbox and restrict applications. Given this obvious deficit, it would seem even more prudent for OpenBSD to make use of the TrustedBSD project.
This is the main reason why OpenBSD is unable to offer a secure environment in the event an attacker is successful. Instead of implementing a form of extended access controls and ensuring the system is secure even in the event of a successful attack, they prefer to remove as many vulnerabilities as possible. This approach is naïve at best and arrogant at worst.
The need for extended access controls
The main argument against OpenBSD is that it provides very limited access controls. OpenBSD attempts to remove the source of vulnerabilities by producing quality code, and has such faith in this approach that very little is provided to deal with a situation when a machine is exploited, and root access obtained. Perhaps inevitably. It is this lack of access controls and protection mechanisms that prevent OpenBSD from being the secure system it is often credited as being.
It is also the reason the aforementioned frameworks such as SELinux and RSBAC have an inherent security advantage over any OpenBSD machine. Due to the use of some sort of MAC, RBAC, TE or other advanced access control used by these frameworks, a level of control is possible above that in traditional DAC systems. With a traditional DAC system, the user has complete ownership over their files and processes, and the ability to change permissions at their discretion. This leads to many security concerns, and is the reason most attacks can be successful at all.
When a computer is hacked regardless of if it is due to a drive by download targeting an insecure browser on a user’s computer or a targeted attack exploiting a server process, the malicious process or user will inherit the access of the browser or process that was attacked. The prevalence of the DAC architecture throughout most operating systems is still the primary cause of many security issues today. With many server processes still running as a privileged user this is a large concern.
It is also something that is hard to fix without changing to a different design paradigm. Many of the technologies that were developed to help prevent attacks such as privilege separation; executable space protection and process ID randomization help, but are not sufficient for a majority of cases. This is why the need for an extended access control framework is present. With the use of something like SELinux or RSBAC, the significance of individual user accounts or processes as an attack vector is decreased.
With these systems every single aspect of your system can be controlled to a fine grained level. Every file, directory, device, process, user, network connection etc can be controlled independently allowing for extremely fine grained policies to be defined. This is something that simply is not possible with current DAC systems which include OpenBSD .
As an example of what is possible with extended access controls, it a web server process running as root could be set to only have append access(as opposed to general write access available in a DAC system) to specific files in a specific directory, and to only have read access to specific files in a specific directory. If some files need to execute, then that file itself (or the interpreter if a script) can be restricted in a similar way. This alone would prevent web site defacement and arbitrary code execution in a great many cases.
On present systems using DAC if a targeted attack is successful and access to the root account is gained, there is nothing the attacker cannot do. Run their own malicious executables, alter files etc. This is why OpenBSD is necessarily less secure than any system making use of advanced access control frameworks, and also why OpenBSD is not a secure system. While OpenBSD has many innovative technologies that make it harder for an attacker to gain access, it does not provide any way to sufficiently protect a system from an attacker who has gained access.
It is possible for example to restrict something like perl or python with extended access controls. On OpenBSD if a user or an attacker has access to perl or python, then they can run whichever scripts they like. With extended access controls, it is possible to restrict only certain scripts to have access to an interpreter (and additionally make those scripts immutable), and prevent the interpreter from running at all unless called by those specific scripts. There is no equivalent fine grained granularity on OpenBSD.
Another way in which extended access controls can help is to protect against users. Even on a desktop system there is a significant security advantage. At the moment most malware requires or tries to obtain root privileges to do damage or propagate. What most people don’t realize is that even malware running as a normal user can do significant damage as it has complete access to a users files under the current DAC model. With some form of MAC, if a user decided to demonstrate the dancing pigs problem and run an untrusted piece of malware, it could be restricted from having any access to a users files or being able to make network connections.
Even windows implements a form of MAC – Mandatory Integrity Controls. While not terribly powerful, and not used for much at the moment, it still provides increased protection and allows for more security than an OpenBSD box can provide. If even Microsoft can understand the need and significance of these technologies after their track record, why is OpenBSD the only project still vehemently rejecting this technology?
Extended access controls are too complex
Some people are of the view that extended access controls are simply added complexity, which increases the scope for vulnerabilities without providing any meaningful additional security. This position makes little sense. While it is true that adding extended access controls increases complexity, the meaningful increase in security cannot be denied. There are plenty of examples of exploits being contained due to such technology…exploits that would have allowed full access to the system if OpenBSD had been the targeted platform.
It has also been said such systems only serve to shift the attack point. Instead of attacking a particular piece of software, they simply have to attack the access control framework itself. This is simply a myth. While the frameworks themselves can be attacked and even possibly exploited, the increase in security far outweighs any risk. The extended access control framework can be extensively audited and made secure while allowing policies to be enforced. Having one relatively small section of code that is easily maintained and audited and responsible for maintaining security is not a decrease in security, but an increase.
Ideally, a proper extended access control framework would also be formally verified, as I believe is the case with SELinux and RSBAC, based on the FLASK and GFAC architectures respectively. This basically means that these systems have been mathematically proven to meet all of their specifications, making it extremely unlikely that it will be possible for the systems to fail in an unexpected way and be vulnerable to attack.
In almost 10 years, there have been no vulnerabilities reported for these major systems that allowed the framework to be bypassed. The times when there has been a problem, it has been due to poor policy. The example everybody likes to mention is the cheddar bay exploitthat Brad Spender(author of GRSecurity) made public in July 2007. It’s true that this exploit allowed for disabling SELinux, but this was due to a stupid policythat allowed 0 to be mmaped for the purposes of allowing WINE to work. Only the RHEL derived distributions were affected. This is not a valid example of the framework being vulnerable, and it certainly does nothing to discredit the technology as a whole.
Due to limitations of certain hardware platforms, it is possible that with the right kernel level vulnerability, an extended access control framework could be subverted. These cases however are quite rare, and with the use of technologies like PaX they become even more unlikely to succeed. In fact, as of writing this article, I am not aware of example of an extended access control being able to be successfully subverted to the contrary. There are however, examples of extended access controls successfully protecting against certain kernel vulnerabilities such as SELinux preventinga /proc vulnerability that could lead to local root.
Some of these frameworks have been criticized for being too complex, in particular SELinux. While I don’t think this is entirely justified, as the SELinux team has made great progress with making this easier with tools such as setroubleshoot and learning mode, I can understand it may be a valid concern. Even so it only applies to a specific implementation. RSBAC, which is just as powerful as SELinux has far clearer error messages and is much easier to craft a policy for. Other implementations such as that of GRSecurity are far simpler yet again. The point here is that the technology is powerful and should be embraced as the added security advantaged is undeniable.
If complexity and user unfriendliness was the main concern the OpenBSD team had then they could still embrace the idea while making the implementation simple to use and understand Instead, they flat-out reject the idea, believing antiquated Unix permissions are more than enough. Unfortunate in this day and age this is no longer the case. Security should not be grafted on, it should be integrated into the main development process. This does not mean patching in protections for specific attacks along the way which is the approach favored by the OpenBSD team. The OpenBSD approach has resulted in a very impressive and stable fortress built upon sand.
While the implementation of various policy frameworks will mature and grow as needed, OpenBSD will remain stale. With a refusal to implement options for properly restricting users or a system in the event an attacker does gain access, the OpenBSD system will be considered a less reliable and trustworthy platform for use as a server or user operating system.
Extended access control frameworks should not be considered a perfect solution, or the be all and end all of security. There are still many situations where they are insufficient such as large applications that necessarily require wide ranging access to properly function. Even so, the level of control these frameworks provide are the best tools we have to secure systems as best we can.
It is interesting to note that even with Linux not really caring about security and having a non disclosure policy, things still end up being more secure than OpenBSD because of the presence of extended access controls. Being able to restrict access in such a powerful way which reinforces that simply trying to eliminate all bugs at the code level while noble, is an inferior approach.
As much as I am disappointed with the fix silently without disclosure approach to security the Linux kernel project has taken since Greg K-H took over, and having to rely on sites like xorl.wordpress.com to learn about security problems that were fixed, Linux is the only real project making progress with testing and improving extended access control frameworks. With continued development and support the implementations will become easier to use and the problems eradicated until such technology is common, as it should be.
OpenBSD cannot be considered a secure system until it makes some effort towards facilitating locking down a system with more than the standard UNIX permissions model which has been shown to be insufficient, and stop discounting the possibility that a system will be secure because all bugs have been removed. While well intentioned and accurate to a small extent, it is ultimately meaningless if even just one vulnerability is present.The OpenBSD team consists of highly skilled programmers who have an interest in security and have shown excellent skill at auditing code and identifying and fixing vulnerabilities in software. Unfortunately, they have shown no interest in extending OpenBSD to implement extended access controls as almost all other operating systems have done, leaving their system inherently more vulnerable in the event of a successful intrusion. The OpenBSD serve a useful role in the community, similar to dedicate security analysts or advisors, and for this they should be celebrated.
Note: I am aware that many people use OpenBSD for nothing more than a router, and for this it indeed ideal. For the use of a router, extended access controls would not provide much benefit. I wrote this argument however because many people seem convinced that OpenBSD has suerior security in all instances and including as a network server or user operating system. I became tired of reading these comments and people simply dismissing extended access controls as too complex and not providing any real security.
Update 1 – January 23rd 2010
- SELinux – http://www.nsa.gov/selinux
- RSBAC – http://www.rsbac.org
- GRSecurity – http://www.grsecurity.net
- AppArmor – http://developer.novell.com/wiki/index.php/Apparmor_FAQ
- The TrustedBSD Project – http://www.trustedbsd.org
- Core Security OpenBSD Advisory – http://www.coresecurity.com/content/open-bsd-advisorie
- Marc Espie talking about security complexity and calling MAC security theater- http://thread.gmane.org/gmane.os.openbsd.misc/129217/focus=129371
- Theo de Raadt stating that MAC should not be included in OpenBSD – http://www.eweek.com/index2.php?option=content&task=view&id=30680&pop=1&hide_ads=1&page=0&hide_js=1
- An older similar argument on the OpenBSD misc mailing list – http://kerneltrap.org/OpenBSD/SELinux_vs_OpenBSDs_Default_Security
- A simple argument now out of date, that makes a similar argument without going into detail – http://www.seifried.org/security/os/20011107-openbsd-linux.html
- Traps and Pitfalls: Practical Problems in System Call Interposition Based Security Tools – http://www.stanford.edu/~talg/papers/traps/abstract.html
- Exploiting Concurrency Vulnerabilities in System Call Wrappers – http://www.watson.org/~robert/2007woot/20070806-woot-concurrency.pdf
- Bob Beck talking about systrace – http://thread.gmane.org/gmane.os.openbsd.misc/160797
- China chooses FreeBSD as basis for secure OS – http://blogs.techrepublic.com.com/security/?p=1682
- An example of SELinux preventing an exploit on RHEL 5 – https://rhn.redhat.com/errata/RHSA-2007-0960.html
- Dan Walsh talking about SELinux successfully mitigating vulnerabilities – http://danwalsh.livejournal.com/10131.html
- The start of the thread where Brad Spender’s Cheddar Bay exploit is introduced and discussed – http://thread.gmane.org/gmane.comp.security.dailydave/3905
- Details on the SELinux policy that allowed for the Cheddar Bay exploit – http://eparis.livejournal.com/606.html
- SELinux preventing a kernel vulnerability from succeeding – http://lwn.net/Articles/191954/
- A second example of a vulnerability that SELinux prevented, due to the users not having the required socket access- http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2003-0127
- A Phrack article detailing the ways current security frameworks can be exploited, and how to prevent against this – http://www.phrack.com/issues.html?issue=66&id=15
- A primer on OpenVMS security, a highly secure OS designed with security in mind at every level – http://www.blacksheepnetworks.com/security/resources/openvms/
- Presentation introducing Strlcpy and strlcat – http://www.usenix.org/events/usenix99/millert.html
- Start of a mailing list thread where strlcpy and strlcat are discussed and criticized – http://sources.redhat.com/ml/libc-alpha/2000-08/msg00052.html
I have updated the article to talk about the benefit of formal verification, and address the possibility of an EACL framework being bypassed with a kernel vulnerability.
Update 1 – April 23rd 2010
I have updated the article to reflect the correct status of the Openwall project (i.e. not abandoned), thanks to a comment byVincent.
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