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. Links 27/7/2014: KDE 4.14 Beta 3, KDE 4.14 Beta 3 Released

    Links for the day



  2. Apple and Microsoft Are Proprietary Software Companies and the Media Should Stop Openwashing Them

    New examples where proprietary software giants are characterised as FOSS-embracing and FOSS-friendly by gullible or dishonest 'journalists'



  3. Bloomberg's Microsoft Propaganda

    Bloomberg delivers 'damage control' and PR ahead of the layoffs announcement; Microsoft uses Nokia to hide it and Bloomberg helps Microsoft by radically modifying headlines



  4. Frequency of Browser Back Doors in Microsoft Windows is Doubling

    The vulnerabilities which Microsoft tells the NSA about (before these are patched) are significantly growing in terms of their numbers



  5. FUD Entities Entering the FOSS World

    Symantec enters the AllSeen Alliance and Sonatype is once again trying to claim great insecurity in FOSS due to software licensing



  6. Groklaw Back in the Wake of ODF in the UK?





  7. Links 26/7/2014: New Wine, Chromebooks Strong Sales

    Links for the day



  8. Links 25/7/2014: GOG With GNU/Linux, Ubuntu 14.04.1 LTS

    Links for the day



  9. Links 24/7/2014: Oracle Linux 7; Fedora Delays

    Links for the day



  10. Valerie Strauss Explains Why Gates Foundation's Lobbying for 'Common Core' (Privatisation) is a Swindle That Makes Microsoft Richer

    Continued criticism of the Gates Foundation's lobbying and masquerading, with more journalists brave enough to highlight the corruption



  11. USPTO Officially Sets New Guidelines to Limit Scope of Software Patents in the United States

    Even patent lawyers finally acknowledge that the incentive to file software patent applications has been reduced, as the scope of patents on software has been noticeably narrowed and they are harder to acquire, let alone enforce in a courtroom



  12. UK Government Adopts OpenDocument Format (ODF) and Microsoft Already Attacks the Government Over It, Showing Absolutely No Commitment to Open Standards

    Only "Microsoft as the standard" is the 'standard' Microsoft is willing to accept, as its response to the Cabinet Office's judgment reveals



  13. Microsoft Layoffs of 2014

    Another quick look at Microsoft's horrible state of affairs and why it has virtually nothing to do with Nokia



  14. Links 22/7/2014: Linux 3.16 RC 6, New UberStudent

    Links for the day



  15. Links 20/7/2014: Jolla in India, Mega Censored in Italy

    Links for the day



  16. Longtime Mono Booster Joins Microsoft-linked Xamarin

    Jo Shields almost joins Microsoft, settling instead for its proxy, Xamarin



  17. Linux Foundation Welcomes Patent Aggressor Red Bend Software

    The Linux Foundation's AllSeen Alliance welcomes as a member a company that uses software patents to sue Free/Open Source software



  18. Matt Levy From Patent Progress (and CCIA) Does Not Really Want Patent Progress

    Matthew ('Matt') Levy moved into a foe of patent progress last year, but he still runs a site calls Patent Progress, in which he diverts all attention to patent trolls (as large corporations such as Microsoft like to do)



  19. Attacking FOSS by Ignoring/Overlooking Issues With Proprietary Software

    The biasing strategy which continues to be used to demonise Free/Open Source software (FOSS) along with some new examples



  20. Links 19/7/2014: CRUX 3.1 is Out, CyanogenMod Competes With Google Now

    Links for the day



  21. Microsoft's Massive Layoffs Go Far Beyond Nokia; Nokia's Android Phones Axed by Microsoft's Elop

    Microsoft's rapid demise and permanent exit from Nokia's last remaining Linux platform (after Microsoft had killed two more)



  22. Patents on Software Already Being Invalidated in Courts Owing to SCOTUS Ruling on 'Abstract' Patents

    The Federal Circuit Appeals Court has just "invalidated a software patent for being overly abstract," says a patents expert



  23. OpenSUSE 'Community' is Crumbling, AttachMSFT Killed SUSE's Potential (Except as Microsoft Tax)

    Not much too see in the land of SUSE and Attachmate, or formerly the company known as Novell



  24. Links 18/7/2014: Slackware Turns 21, Spotify Switches to Ubuntu

    Links for the day



  25. Links 16/7/2014: Manjaro 0.8.10 Third Update, SIA Migrates to Red Hat

    Links for the day



  26. Microsoft's Latest Round of Massive/Bulk/Large-scale Layoffs

    Microsoft boosters are preparing 'damage control' pieces ahead of massive layoffs at Microsoft



  27. Secrecy Allows British Government to be Manipulated by Microsoft for Spyware Behind Closed Doors

    Dependence on malicious software from NSA ally Microsoft is highly dependent, at least in Britain, on government secrecy and vain refusal to comply with Freedom of Information (FOI) requests



  28. Software Patent Applications Already Being Rejected in the US Owing to SCOTUS Ruling, Some Patent Lawyers Are Fuming

    Good news on the software patents front as the USPTO starts rejecting software patent applications, based on patent lawyers' words



  29. Links 15/7/2014: New Plasma, Google Announces Project Zero

    Links for the day



  30. Interest in Free Software Coverage and 9 Months With Tux Machines

    Thoughts about the level of interest in Free/Open Source software (FOSS) and growth of at least some sites that focus on GNU/Linux


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