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


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.


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.


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. Links 24/10/2016: Linux 4.9 RC2

    Links for the day

  2. Battistelli Plans to Expand the Social [sic] 'Study' (Then 'Conference') Propaganda Until Next Month, Under the 'Workshop' Umbrella

    Milking his shameless propaganda (paid-for 'studies'), Battistelli wants to rewrite the record by all means possible, then pretend that EPO staff participates in it

  3. EPO and EUIPO Join Hands to Release Propaganda (for European Media to Parrot) Some Time Tomorrow

    EPO and EUIPO in collaboration for the promotion of the notion that they are both necessary (and reinforced speculations about growing overlap between them)

  4. UPC Preparatory Committee Puts the Brakes on UPC Amid Brexit and Growing Uncertainty

    The Unified Patent Court (UPC) preparatory committee recognises that the UPC isn't going anywhere (any time soon) and false job advertisements -- or advertisements for jobs that will never exist -- are withdrawn

  5. Updates Regarding EPO and BoAC: Unrest and Injustice Carry on

    Some of the latest information which is publicly and privately available to us, in particular regarding the case of a suspended judge which represents unprecedented erosion of the appeal boards' independence (and hence lack of justice in the Organisation)

  6. EPO and the “Iberian Connection”: Patricia García-Escudero Márquez - Battistelli's Pet Chinchilla on the Boards of Appeal Committee?

    Why the Boards of Appeal Committee has begun showing prominent signs that it is anything but independent and capable of standing up to Battistelli (or his circle at the Office, which includes the “Iberian Connection")

  7. Links 23/10/2016: Alcatel's New Android Smartphones, Another Honorary Doctorate for Stallman

    Links for the day

  8. Open Letter Exposing the Farce Which Was Battistelli's 'Social Conference' Coinciding With Further (New) Attacks on EPO Staff Representatives

    A detailed letter reveals legitimate concerns expressed by staff representatives at the EPO ahead of the so-called Social Conference, in which we have highlighted severe factual flaws

  9. Translation of Latest Rant From French MP Philip Cordery About Benoît Battistelli's Abuses at the EPO

    Philip Cordery crosses horns with Benoît Battistelli, who has become a source of embarrassment for France with his autocratic tendencies and misguided policies that rapidly ruin the European Patent Office (EPO)

  10. Battistelli-Commissioned PwC ‘Study’: Leaked Document Shows PwC's Dishonesty and Misrepresentation of EPO Staff

    An in-depth analysis (but not comprehensive, just preliminary) of the so-called 'study' from PwC, which basically did what it was paid for (pay to say)

  11. Links 22/10/2016: Deus Ex for GNU/Linux, Global DDoS (DNS)

    Links for the day

  12. Battistelli-Commissioned PwC ‘Study’: Survey Comparison Shows Serious Deterioration and Efforts by PwC to Disguise the Truth

    The latest output from PwC turns out to be even worse than initially thought, indicating that not only did it find a degradation in the EPO but also attempted to hide/obscure it

  13. EPO Teaser - The "Iberian Connection" - Some Photos of García-Escudero and His Royal/Government Connections

    A look at the undeniably close connections between Mr. García-Escudero and the most powerful people in Spain

  14. Disruption to Site's Service

    A technical note about why Techrights has not been publishing many articles recently

  15. Links 21/10/2016: MPV 0.21, Mad Max for GNU/Linux

    Links for the day

  16. EPO Caricature: Battistelli's High Five

    Another cartoon about the sad state of the EPO

  17. Battistelli Ruins Not Only the EPO But Also the Whole of Europe By Ushering in Software Patents That Patent Trolls Love So Much

    Battistelli's bad leadership at the EPO threatens to bring to Europe all the ills and menaces of the patent system in the United States

  18. EPO Spokesman Lies to IP Watch in Order to Save Face and Save the King (Battistelli)

    Rewriting history (revisionism) regarding Battistelli and what was demanded amidst abusive behaviour from him

  19. Unitary Patent (UPC) is Dead, But 'Managing IP' and Selfish Patent Law Firms Still Try to Resurrect It

    The latest attempts to shore up the Unitary (or Unified) Patent Court and who's behind it other than the usual suspects

  20. Links 20/10/2016: Linux 4.10 Preview, ONF and ON.Labs to Merge

    Links for the day

  21. Battistelli-Commissioned PwC 'Study': The Raw Outcome Shows Distortion of the Facts at the EPO's Notorious 'Social Conference'

    Results of the Staff Survey carried out by PwC, in order to provide some propaganda for Battistelli's expensive Social Conference

  22. Addendum: EPO's Alberto Casado Cerviño, WIPO's Francis Gurry, and EUIPO's Archambeau

    Photos taken as part of an IP event which took place in Riga (Latvia) in March 2015

  23. Worrisome Connections Between EPO VP2 Alberto Casado Cerviño and Patricia García-Escudero Márquez

    Exploring the potential conflicts of interests implicating the EPO's Boards of Appeal Committee

  24. Site's Infrastructure Under Attack and Upgrades Ahead of Major New Publications

    Protections for the Web site have been improved and capacity increased in order to avoid or at least prepare for another week of abusive/spam traffic

  25. Team Battistelli's Conspiracy Theory: SUEPO is Behind Everything, EPO Management is Trying to Tell the Media

    Attempts to blame SUEPO, the staff union of the EPO, even though SUEPO has nothing to do with articles that are critical of the EPO while many thousands of EPO employees are disgruntled

  26. Links 19/10/2016: Canonical Livepatch Service, Plasma Plans

    Links for the day

  27. The 'Sarah Sharps' of Microsoft: Not the Kind of Scandal the Media Cares Enough to Write About

    Another example of the large (industrial) scale of sexual discrimination at Microsoft -- a company that tries to advertise itself as diverse or tolerant and stigmatise Free/Open Source software (FOSS) as intolerant and/or not diverse

  28. EPO Caricature: EQE Questions

    The latest EPO cartoon, this time about European qualifying examination (EQE)

  29. The Long History or Seeds of Control by Fear and Punishment at the EPO

    The latest hogwash from Team Battistelli (Pinocchio), the latest instance of software patents promotion by EPO Principal Director, and an old (decade-old) nugget of information from the Forum for Principal Directors

  30. Subject of the European Patent Office's Abuses Raised in European Parliament by Ulrike Müller (ALDE)

    A local copy of a bunch of questions asked less than a month ago by Ulrike Müller at the European Parliament, regarding the unacceptable state of affairs at the European Patent Office (EPO)


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


Recent Posts