Bonum Certa Men Certa

Linux Backdoors Revisited (New Revelations and Old Revelations)

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.

Recent Techrights' Posts

The Goal of Coopetition Assumes You're Friends
it will never work with Microsoft
Seductive Mirage or Allure of Complex, Proprietary Coffee Machines (or Similar White Elephants)
Software is a lot like those things
Hate Mail From Anonymous Cowards
if this persists, we'll need to escalate
Informal Open Letter to the Lawyer of the Microsofters (on Who's Funding the SLAPPs Against Techrights)
Whenever I ask about the funding they try to change the subject and act all aggressive
Microsoft Lunduke is Just Provoking People for Provocation's Sake
Be forewarned and remember where this guy came from: Microsoft
 
The Register MS (Microsoft) or The Register AI (Slop)?
What a slopfest!
Is Red Hat About to Give the Boot to GNOME People Who Helped Microsoft 'Secure' (Monopolised) Boot?
It was always a dumb idea to play along with Microsoft's hardware mischief
Sales of Windows on PCs (Windows Licences) Go Down
Microsoft has a big problem in its hands
The Hype That Microsoft and The Register MS (Among Others) Promote Helps Stage DDoS Attacks on Free Software Sites
Microsoft is, to put it bluntly, pure evil
Links 09/08/2025: Putin Allegedly to Visit Alaska (Which He Deems Part of Russia), Mike Tyson Sued for Copyright Infringement
Links for the day
Slopwatch: Linux Journal, LinuxSecurity, and Google News With Its Slopfarms of Choice
SEO spam, made with LLMs
Follow the Money: The Register MS Gets Paid to Promote "Hey Hi" Ponzi Scheme/Hype, Some Fake 'Articles' Might Be Composed by LLMs Already
paid to promote slop
Gemini Links 09/08/2025: Rethinking Aliases and Posting on Gopher vs. the Web
Links for the day
Links 09/08/2025: Apollo 13 Astronaut Jim Lovell Dies, Slop Future Bleak
Links for the day
After Shutting Down Studios, Divisions, Applications (e.g. Skype) Microsoft is Also Shutting Down 'Apps'
Cuts all around as layoffs persist this month, Microsoft tries to get many people to resign, and debt skyrockets
Most of Geminispace Can Probably Fit on a CD-ROM or a DVD (the Textual Part)
If one excludes very large capsules and ones that contain non-textual contenty
Eventually UEFI 'Secure Boot' Will be Dropped (Users Will Demand Its Removal and Boycott Its Pushers)
we expect OEMs will just listen to users
The Register MS: We Know Slop is a Bubble and Mindless Hype, But We Get Paid to Participate
Call out the culprits
There Are Probably Over a Million Pages in Geminispace
there are two many limitations which merit a mention when it comes to assessing magnitude
Besieged by Plagiarists Who Play With LLMs and Image Fusions
We really need to exercise or use our collective voice to oppose Serial Sloppers
Over at Tux Machines...
GNU/Linux news for the past day
IRC Proceedings: Friday, August 08, 2025
IRC logs for Friday, August 08, 2025
Gemini Links 09/08/2025: Water Painting and Political Violence
Links for the day
Slopwatch: LLM Sloppers in Google News, LinuxSecurity, and More
they also perpetuate some falsehoods as the LLMs lack any comprehension
Links 08/08/2025: China King of Plastics and US Dictator Plans to Meet Russian Dictator
Links for the day
Gemini Links 08/08/2025: Cracking a Family Member's Password and Overdose of Slop
Links for the day
Red Hat's Latest Talent Hunt, Day Ahead of Mass Layoffs, is Yet Another Microsoft Executive
Red Hat will apparently commence mass layoffs early this coming Monday
Links 08/08/2025: "Quit Facebook" and High Cost of Microsoft/Windows Shown Again ("BlackSuit")
Links for the day
Good Morning, Readers of The Register MS
Things The Register MS could (but does not) cover this morning
Why Gemini Protocol Has a Bright Future
Maybe Gemini Protocol's promise becomes more appealing as the Web turns to slop and bloat
It's a Lot Easier to Participate in the Unethical System Than to Oppose Injustices in It
Going after powerful and high-budget interests is never easy
Microsofters Filed Two SLAPPs Against Us, Now They Cannot Keep Up With Judges' Orders
For over 4 months already their facilitator in London has been under investigation by British authorities because of what's being done to my wife and I
Censorship Regarding Red Hat Layoffs
Talk about this? They'd rather not.
Struggling to Cut Costs, Microsoft Continues Shutting Down and Cancelling Stuff This Month
There are August layoffs at Microsoft
Over at Tux Machines...
GNU/Linux news for the past day
IRC Proceedings: Thursday, August 07, 2025
IRC logs for Thursday, August 07, 2025
Fake 'Linux' Articles, Written by Bots to Take Traffic Away From Real Articles
LLM slop helps replace information with junk or misinformation
When Google's Googlebombing of "Gemini" Was Not Enough; They Now Also Googlebomb "Gemini Space"?
We know GAFAM not only worries about Gemini Protocol but also attempts to 'infiltrate' Geminispace
The Register MS Promotes Microsoft Slop, Assumes All Readers Use Microsoft Windows
Microsoft really dominates the site
Gemini Links 08/08/2025: KDE/Qt Development and What's Missing From "Retro"
Links for the day
Links 07/08/2025: US Punishes India Instead of Russia, Attacks Law Firms to Prevent Scrutiny
Links for the day
Read Us in Geminispace as Well
it's definitely a lot simpler than using a Web browser
Once a Site About BSD and GNU/Linux, and After Months of Silence, LinuxBSDos.com Comes Back Only as a Slopfarm
very frustrating
Links 07/08/2025: Hardware Wars, Mass Recall of Colgate Total Clean Mint, More Microsoft Holes Found
Links for the day
Gemini Links 07/08/2025: "Right To Manage" and LoRa Analysis
Links for the day
For the First Time in a Month OSI's "OpenSource.org" Blogs and It's Basically a Microsoft Blog Post (Microsoft Controls OSI)
For the first time in a month OSI writes something and it is Microsoft propaganda composed by a Microsoft-salaried operative
Microsoft, Already Borrowing 3 Billion Dollars a Month, is Trying to Cause Many People to Resign
MSN (i.e. Microsoft) and others openly admit it
GAFAM 'Says' is Front Page "News"
The point of journalism is to check and assess facts, not parrot what people and companies merely claim
Links 07/08/2025: Apple Makes False Promises, More Trouble for Microsoft
Links for the day
OSS Didn't Always Mean Open Source Software
"oligarchs all the way down"
The Register MS Does More Microsoft Sez or GitHub Sez (Says) Pieces
60 minutes ago
They Want Activists to Just Barely Walk and Eat, Not Do Activism Anymore
It's sort of like the ending of '1984'
Quit Perpetuating the Narrative of Gemini Protocol 'Dying' (It's False)
The "whisper campaign" against Gemini Protocol
Criticising Social Control Media in Social Control Media
Many people are quitting Social Control Media (fewer of them announce this in public)
Non-Free JavaScript Programs in Banks Aren't Even the Biggest Problem
Technology was supposed to make life easier; in practice, however, for most of us the opposite effect can be observed
Slopfarms Are Typically Fake News
Slopfarms typically relay falsehoods
Gemini Links 06/08/2025: Replacing a Pocket Watch and Buying in Bulk
Links for the day
IBM is Obliterating Fedora
"Fedora releases were shipping with an increasing number of bugs on launch day even while I was using it for a several year stretch."
Over at Tux Machines...
GNU/Linux news for the past day
IRC Proceedings: Wednesday, August 06, 2025
IRC logs for Wednesday, August 06, 2025