EditorsAbout the SiteComes vs. MicrosoftUsing This Web SiteSite ArchivesCredibility IndexOOXMLOpenDocumentPatentsNovellNews DigestSite NewsRSS

10.16.13

Linux Backdoors Revisited (New Revelations and Old Revelations)

Posted in GNU/Linux, Kernel, Security at 10:43 am by Dr. Roy Schestowitz

Claude Elwood Shannon, the man who introduced entropy

Claude Elwood Shannon

Summary: An anonymous backdooring attempt against Linux goes a decade back, but a randomisation problem in today’s Linux also seems possible (subverting encryption)

Jonathan Allen wrote this article about an incident mentioned also by Freedom to Tinker. Slashdot’s summary goes like this, documenting news from one decade ago:

“Ed Felton writes about an incident, in 2003, in which someone tried to backdoor the Linux kernel. Back in 2003 Linux used BitKeeper to store the master copy of the Linux source code. If a developer wanted to propose a modification to the Linux code, they would submit their proposed change, and it would go through an organized approval process to decide whether the change would be accepted into the master code. But some people didn’t like BitKeeper, so a second copy of the source code was kept in CVS. On November 5, 2003, Larry McAvoy noticed that there was a code change in the CVS copy that did not have a pointer to a record of approval. Investigation showed that the change had never been approved and, stranger yet, that this change did not appear in the primary BitKeeper repository at all. Further investigation determined that someone had apparently broken in electronically to the CVS server and inserted a small change to wait4: ‘if ((options == (__WCLONE|__WALL)) && (current->uid = 0)) …’ A casual reading makes it look like innocuous error-checking code, but a careful reader would notice that, near the end of the first line, it said ‘= 0′ rather than ‘== 0′ so the effect of this code is to give root privileges to any piece of software that called wait4 in a particular way that is supposed to be invalid. In other words it’s a classic backdoor. We don’t know who it was that made the attempt—and we probably never will. But the attempt didn’t work, because the Linux team was careful enough to notice that that this code was in the CVS repository without having gone through the normal approval process. ‘Could this have been an NSA attack? Maybe. But there were many others who had the skill and motivation to carry out this attack,’ writes Felton. ‘Unless somebody confesses, or a smoking-gun document turns up, we’ll never know.’”

Backdoors in Linux are a subject for jokes in Torvalds' mind, but given the above we should take this subject very seriously. In any system, for example, having no mechanism for randomness (like in some embedded devices) typically means that strong encryption (with high entropy) is not possible. Given new alleged “insecurities in the Linux /dev/random,” as Bruce Schneier put it, Linux backdoors seem possible again. David Benfell said:

I’m guessing Schneier knows what the fuck he’s talking about. If it is the same vulnerability, then Torvalds’ defense is that the vulnerable source of entropy is only one of many. But if I read Schneier correctly, the result was still too predictable.

“On the other hand,” says Benfell, “here’s Theodore T’so from the comments:”

So I’m the maintainer for Linux’s /dev/random driver. I’ve only had a chance to look at the paper very quickly, and I will at it more closely when I have more time, but what the authors of this paper seem to be worried about is not even close to the top of my list in terms of things I’m worried about.

First of all, the paper is incorrect in some minor details; the most significant error is its (untrue) claim that we stop gathering entropy when the entropy estimate for a given entropy pool is “full”. Before July 2012, we went into a trickle mode where we only took in 1 in 096 values. Since then, the main way that we gather entropy, which is via add_interrupt_randomness(), has no such limit. This means that we will continue to collect entropy even if the input pool is apparently “full”.

This is critical, because *secondly* their hypothetical attacks presume certain input distributions which have an incorrect entropy estimate —| that is, either zero actual entropy but a high entropy estimate, or a high entropy, but a low entropy estimate. There has been no attempt by the paper’s authors to determine whether the entropy gathered by Linux meets either of their hypothetical models, and in fact in the “Linux Pseudorandom Number Generator Revisited”[1], the analysis showed that our entropy estimator was actually pretty good, given the real-life inputs that we are able to obtain from an actual running Linux system.

[1]http://eprint.iacr.org/2012/251.pdf

The main thing which I am much more worried about is that on various embedded systems, which do not have a fine-grained clock, and which is reading from flash which has a much more deterministic timing for their operations, is that when userspace tries to generate long-term public keys immediately after the machine is taken out of the box and plugged in, that there isn’t a sufficient amount of entropy, and since most userspace applications use /dev/urandom since they don’t want to block, that they end up with keys that aren’t very random. We had some really serious problems with this, which was written up in the “Mining Your Ps and Qs: Detection of Widespread Weak Keys in Network Devices” [2]paper, and the changes made in July 2012 were specifically designed to address these worries.

[2]https://www.factorable.net/paper.html

However, it may be that on certain systems, in particular ARM and MIPS based systems, where a long-term public key is generated very shortly after the first power-on, that there’s enough randomness that the techniques used in [2]would not find any problems, but that might be not enough randomness to prevent our friends in Fort Meade from being able to brute force guess the possible public-private key pairs.

Speaking more generally, I’m a bit dubious about academic analysis which are primarily worried about recovering from the exposure of the state of the random pool. In practice, if the bad guy can grab the state of random pool, they probably have enough privileged access that they can do many more entertaining things, such as grabbing the user’s passphrase or their long-term private key. Trying to preserve the amount of entropy in the pool, and making sure that we can extract as much uncertainty from the system as possible, are much higher priority things to worry about.

That’s not to say that I might not make changes to /dev/random in reaction to academic analysis; I’ve made changes in reaction to [2], and I have changes queued for the next major kernel release up to make some changes to address concerns raised in [1]. However, protection against artificially constructed attacks is not the only thing which I am worried about. Things like making sure we have adequate entropy collection on all platforms, especially embedded ones, and adding some conservatism just in case SHA isn’t a perfect random function are some of the other things which I am trying to balance as we make changes to /dev/random.

T’so, who is the former CTO of the Linux Foundation, at least acknowledges the possibility that there is a real issue here.

Share this post: These icons link to social bookmarking sites where readers can share and discover new web pages.
  • Digg
  • del.icio.us
  • Reddit
  • co.mments
  • DZone
  • email
  • Google Bookmarks
  • LinkedIn
  • NewsVine
  • Print
  • Technorati
  • TwitThis
  • Facebook

If you liked this post, consider subscribing to the RSS feed or join us now at the IRC channels.

Pages that cross-reference this one

What Else is New


  1. Microsoft's Media Attack on Free Software and GNU/Linux

    Brainwash war is still being waged by Microsoft and its friends to convince people that Windows is universally dominant and that Microsoft is now part of the Free software world



  2. Microsoft Accounting Practices After Fire Again, After Previous Abuses and Book-Cooking

    After the infamous IRS brawl comes another confrontation between Microsoft and the SEC, which is unhappy with Microsoft for seemingly cooking the books again



  3. Links 26/1/2015: Debian 8.0 “Jessie” RC1, Linux Kernel 3.19 RC6

    Links for the day



  4. Links 25/1/2015: Android Wear 5.0, Tizen in Bangladesh

    Links for the day



  5. IRC Proceedings: January 11th, 2015 – January 24th, 2015

    Many IRC logs



  6. Links 24/1/2015: Zenwalk Linux Reviewed, Netrunner 14.1 Released

    Links for the day



  7. The Latest 'Microsoft is Open Source' Propaganda a Parade of Lies

    Microsoft myth makers continue their assault on what is objectively true and try to tell the public that Microsoft is a friend of "Open Source"



  8. Apple -- Like Microsoft -- Not Interested in the Security of Its Operating Systems

    Apple neglected to patch known security flaws in Mac OS X for no less than three months and only did something about that vector of intrusion when the public found out about it



  9. As Battistelli Breaks the Rules and Topić Silences Staff, New European Parliament Petition for Tackling the EPO's Abuses is Needed

    The neglected (by EPO) Article 4a of the European Patent Convention (EPC) and the European Parliament petition/complaint against the EPO's crooked management



  10. Links 23/1/2015: Red Hat on IBM Power, Meizu Leaks With Ubuntu

    Links for the day



  11. Links 23/1/2015: Plasma 5.2, Manjaro 0.9-pre1

    Links for the day



  12. Microsoft is Dying Due to Free Software, Tries to Infect GNU/Linux With .NET and to Infect Moodle in Schools With Microsoft Office and OOXML Lock-in

    'Free' drugs (a proprietary software analogy) the new strategy of Microsoft in its latest battle against Free software, especially in schools where choice is a rarity (if not an impossibility), with the premeditated intention of forming dependency/addiction among young people



  13. Microsoft Symptoms of a Dying Company: More Boosters Depart, Back Doors Revealed, Microsoft's Outlook Cracked

    Bad news for Microsoft shortly before the marketing extravaganza served to cover much of it up



  14. The Collapse of European Patent Office Management Culminates With Resignations

    No blood is spilled, but even the management of the EPO is falling apart as the Director of Internal Communication is said to have just resigned



  15. New LCA Talk: Open Invention Network's Deb Nicholson on Software Patents and Patent Trolls

    Deb Nicholson's LCA talk is now publicly accessible



  16. Links 22/1/2015: GNU/Linux Sysadmin Opportunities, TraceFS Introduced

    Links for the day



  17. Links 21/1/2015: Andrew Tridgell, Torvalds Being Baited

    Links for the day



  18. Vesna Stilin Renews Her Fight for Justice in Željko Topić Case (EPO VP)

    Željko Topić's abuses continue to cloud the legitimacy of the European Patent Office, in which he is a Vice-President



  19. Failure of the EPO Can Derail the Trojan Horse of Software Patents and Patent Trolls

    Dazzled by his endless pursuit of infinite money and power, Battistelli pushes for expansion of patent scope (geographically too), but he won't have it without a challenge



  20. Links 20/1/2015: Linux 3.19 RC5, 30 Years of FSF

    Links for the day



  21. Translations of Member of the European Parliament Complaining About European Patent Office (EPO)

    French, German, Dutch, and English translations of the article from Dennis De Jong



  22. Microsoft, the Back Doors Company, is Gradually Dying and Trying to Embrace the Competition

    The world is leaving Microsoft's common carrier (Windows) behind, so Microsoft, which is shrinking, tries to conquer Free software and GNU/Linux



  23. Battistelli's Latest Propaganda War Tries to Convince EPO Staff That Željko Topić's Many Criminal Charges Don't Exist

    Battistelli's right-hand man, Željko Topić, is now facing real danger of prosecution and possibly arrest in his home country, so Battistelli rushes to defend this thug's reputation



  24. Links 18/1/2015: Sailfish OS RoadMap, ownCloud Turns 5

    Links for the day



  25. Strategy of Litigation With Patents Has Collapsed Since SCOTUS Ruling in Alice v. CLS Bank

    The latest figures from Lex Machina show a massive decrease (-18%) in patent litigation last month; lawyers look for ways to spin the data in their favour



  26. Patent Lawyers Can't Help Rewriting Alice v. CLS Bank History

    The league of patent lawyers -- people who profit at the expense of software producers -- keeps brainwashing the public about the patentability of software (both the rationale and the potential)



  27. Myths and Hype About Patents

    Distortion of history and fabricated reports about patents in the corporate media leave many people confused and ultimately unable to make rational judgment



  28. Large Corporations, Including Microsoft Allies, Call for Abolition of Software Patents

    The calls for ending all patents on software are getting louder and patents as a whole are de-emphasised as a business strategy



  29. Links 17/1/2015: Lennart Poettering in Headlines, Mageia 5 Beta 2

    Links for the day



  30. Links 16/1/2015: Chapeau 21, Tails 1.2.3

    Links for the day


CoPilotCo

RSS 64x64RSS Feed: subscribe to the RSS feed for regular updates

Home iconSite Wiki: You can improve this site by helping the extension of the site's content

Home iconSite Home: Background about the site and some key features in the front page

Chat iconIRC Channel: Come and chat with us in real time

CoPilotCo

Recent Posts