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Hyper-Threading, Linus Torvalds vs. Colin Percival
OutsideIn writes "The recent Hyper-Threading vulnerability
announcement has generated a fair amount of discussion since it was released. KernelTrap
has an interesting article quoting Linux creator Linus Torvalds who recently compared the vulnerability to similar issues with early SMP
and direct-mapped caches suggesting, "it doesn't seem all that worrying in real life." Colin Percival,
who published a recent paper on the vulnerability,
strongly disagreed with Linus' assessment saying, "it is at
times like this that Linux really suffers from having a single dictator in charge; when Linus doesn't understand a problem,
he won't fix it, even if all the cryptographers in the world are standing against him.""
Then somebody else will.
A dictator who has made his domain open-source, thereby giving everybody free reign to change and make distinct copies of it?
Come on.
you found an obscure and difficult to exploit vulnerability. Now quit trying to make out the world is doomed and trolling on Linus to keep the spotlight on youself.
Do not try to read the dupe, thats impossible. Instead, only try to realize the truth
What truth?
There is no dupe
or, to put it in Pratchett's words:
He doesn't administer a reign of terror, just the occasional light shower.
The all powerful Dvorak said linux had no leaders...
Hmmm witty sig or funny sig? Maybe elitest techy sig!
Now that I think of it I've never seen Castro and Linus in the same room....and Linus always seems to be smoking fine cigars...and open source software is practically communism anyway...it all makes sense now!
Everyone that disagrees with me is a paid shill
The kernel developers don't seem to agree on the right way to fix this, whether at the kernel level or in userspace. However, it may affect the performance of the kernel if it's done in kernelspace, and it is impractical to have everyone rewrite their userland software, as someone else pointed out. The "patch" which is available for FreeBSD to fix this problem only disables hyperthreading and does not provide a real fix.
"Backups are for wimps. Real men upload their data to an FTP site and have everyone else mirror it." -- Linus Torvalds
It's along the same lines of the "if all you got is a hammer" problem. If you've spent a lot of time working on something, it's obviously important to you. That doesn't mean that it's important to everyone else. This may well be a significant flaw from the crytographer's perspective, but then again, they study crypto a lot and have a vested interest in it.
As someone pointed out, yay for linux being free. As one or two above pointed out, someone who does care with the knowledge will write a patch. It'll get implemented as an option in the code, and if shown to be unobtrusive enough, may even get turned on by default.
-- Who is the bigger fool? The fool or the fool who follows him? --
The most common alternative model is community development, where a - usually but not always elected - committee of developer 'elders' steer the project. Apache and Mozilla would be good examples of the latter.
I appreciate some people have heard about this comment first today, people are joining the Free Software and Open Source communities all the time, but it kind of surprises me that so many are criticising Colin for this without anyone explaining this.
You are not alone. This is not normal. None of this is normal.
Scene: A wispy cloud scuds across the sunny blue sky. Not much happening, and the cloud is hardly even black.
Chicken Little: The sky is falling! The Sky is falling!
The Penguin DictatorNo, not really. It might fall, but it's very, very unlikely. So calm down!
Chicken Little: I strongly disagree. The sky is falling! And because you do not understand the problem we're all going to die!
The Penguin Dictator:Listen here. It's almost certainly not going to fall, and I need to worry about real problems!
Chicken Little: (Runs screaming to the nearest coffeehouse with free wireless, where he types incessently:) The sky is falling! The Sky is falling! Tell Slashdot! Tell Tom's Hardware! Tell Cnet! Tell Linux Business News!
The Penguin Dictator: Sigh. (And then he gets back to work. He looks up at the audience) They just do not get it, do they?
The Windows Dark Lord: (Rubs hands together) Excellent, MOST excellent. (Yelling) Bring me my marketing minion!
Marketing Minion: (being drug in by a bald guy yelling at him) Yes, O Master!?
The Windows Dark Lord:Tell all the peasants that the sky is raining huge chunks of fire and dung! Tell everyone, tell them now! And have our independent consultants work on this day and night, night and day! Make sure that they independently tell everyone that they can easily avoid falminf chunks of sky dung if they stand behind our Windows! And RAISE the price!
Some Guy At Some House In Some City Somewhere: "Wow, that was easy. Let me send this up to the Penguin Dictator. No sky ever fell, and that cloud is easily blown away. Nothing happening here, move along."
The Penguin Dictator "Well that was easy. Include this patch in the next day's weather update!" Marketing Minion: Press Release!!! Millions killed by falling flaming sky chunks of burning dung with brain eating worms who eat children!!! Run for your lives!!!!
Laura Didio, munching a do-nut"If you would hide behind Windows, the sky would stop falling! Your children would be safer and the world a better place." (looks at stoick ticker, says to self) 'Excellent. MOST excellent. Bring me a donut!'
The Penguin Dictator "Sigh. Why didn't I just keep Sky 0.7a for myself? Why the bother, wy the bother?"
EPILOGUE: No one was ever hurt by the piece of sky that never fell, and Chicken Little kept looking upward for another cloud to rant about.
The End.
Nice ad hominem attack. Attack the argument, not the person.
RMS is more like the tribal elder reminding you of your ideals - especially during those times when you consider putting them aside because they seem impossible to live up to.
The point of this debatte is that the Intel implementation sucks, it allows you to spy a lot on processes running on the virtual CPU. Sure, there are better alternatives than disabling HTT like the suggestions of Colin to only schedule threads of the same program on the virtual CPU. Actually, that is something you want to do anyway or otherwise you can seriously loose speed and drop under the performance of a processor with HTT disabled.
Speaking of paranoia, it is often not a bad thing to have, many big security problems can be avoided. Oh, I forget to patch the Linux box next door.
... or in any other general-purpose operating system on a general-purpose computer. PCs are fundamentally insecure. There are a dozen ways to spy on cryptographic operations done in them, ranging from trojans, to hardware side-channel attacks, and dozens more to get copies of keys that they store. This is just one particular attack that may permit an attacker who can't get a trojan running with sufficient privileges to spy on operations directly to obtain some key bits. But if the attacker can't do that, there are lots of other ways to get the keys. General-purpose computers are simply not trustworthy.
If security is important, you do your crypto in a secure crypto module, like the FIPS 140-2 Level 4 IBM 4758 or the Level 3 Luna SA. Or, you use a general-purpose computer with special-purpose, very simple software and then provide strict physical access control to the machine and very limited network access -- often through a serial link using a custom protocol rather than via a real network. Or you could theoretically use a general-purpose machine with a TCPA chip with a regular, general-purpose operating system that has been modified to make use of the TCPA chip and with keys tightly bound to a well-defined system software configuration. But only if you have good physical security. In many situations it's still better to use a FIPS 140-2 Level 3 or Level 4 device.
IMO, the existence of weaknesses like this in Linux, and the fact that they're widely known, is a *good* thing, because it helps convince people not to trust that which is inherently untrustworthy. We need more publicity of similar problems in Windows (and there are lots of them).
Note to ACs: I usually delete AC replies without reading them. If you want to talk to me, log in.
Why should this be fixed in the Kernel?
This appears to be an application bug, not a kernel one. The kernel never claims to completely isolate processes from one another; though there are memory protections, there are loads of ways that processes can observer each other's actions. This is just a particularly high-resolution one.
The real "bug" here, IMO, is that openSSL believes that no other process can observe anything about its secret computations. Timing attacks against RSA have been known for some time, particularly with regard to modular exponentiation.
It wouldn't be too hard to make RSA encryption take the same amount of time no matter what code path is used, and to make its memory access patterns uncorrelated with the keys (perhaps by using randomization during allocation). They should do this--the fact that their application leaks information has nothing to do with the processor it's running on; it's just that HT makes it particularly easy to measure that information. This would have a performance penalty, and I think the OpenBSD folks are too obsessed with performance, and that's why they've not done this. The performance obsession is a serious problem in the Unix world, and software systems in general.
If implementing openSSL effectively means adding special kernel support for things like constant-length timeslices or cache invalidation between context switches, that's fine. But this is not a bug in the kenel unless the kernel purports to enforce total separation between processes, which it certainly does not.
No just compare every character in the input everytime, rather than short-cut when you know it wont match.
He ran the bloddy "exploit" well over 1000 time to retrieve 30% of an RSA key.
Did you read a paper other than the one I read? I ran the exploit once, taking under one second, and I retrieved enough information to factor the RSA modulus N.
Tarsnap: Online backups for the truly paranoid
This issue exists with AES implementations as well (search for AES SBOX cache timing on google). The AES vulnerability is, if anything, more worrying in a way. It can be made to be constant-time, but at a serious cost in performance.
Here's another option: since this vulnerability depends on using the L1/L2 cache states to ferret out information, remove that from consideration. When processing an RSA (etc) key, have the code temporarily lock out context switches, AND have it take a fixed period of time to compute the result (or portion of the result), followed by flushing the cache. No data is left in the cache to analyze. The execution time of the code is fixed, so no dat leaks there. You don't have to make the algorithm a constant-time calculation if you can pad it at the end to the maximum (or close enough to the theoretical maximum that there's no true information leakage in practice). This helps avoid the potentially very slow algorithms that are truely constant-time. And flushing the cache at the end of each (portion of the) calculation removes that as a leak. (You need to flush it before waiting for the allocated time to elapse, note, and include max flush time in your calculation, and if possible factor into your calculation possible interrupt effects.)
A pain? yes. Requires extensive mods to crypto algorithm implementation? Yes (though perhaps not to the core calculation.) Requires OS support? Almost certainly. Requires HW support? Would be helpful but probably not required. Loss of performance? Yes, though far lower than disabling HT I imagine in normal cases (when not decrypting/encrypting).
Also, some of the restrictions above can probably be eased if the crypto algorithm is carefully designed and matched to the hardware.
Disclaimer: I am NOT a crypto geek! I have worked in processor and cache design, though.
Potential performance problems are things you should defer on until proper profiling can be done (unless they're total show stoppers). Security and correctness are things you cannot ignore except in extreme cases. Security is particularly important to nail down, because it can result in your customers losing data (even data not pertaining to your app), which is the first no-no of software.
Application software has four priorities, in this order:
- Safety (shouldn't destroy data)
- Correctness (do what it says it does)
- Security (don't do anything else)
- Performance (do it fast)
YMMV, of course, sometimes correctness falls below security, and occasionally performance goes above correctness in some mathmatical functions (if doing it correctly would take a decade and doing a close approximation would take a day, obviously you want the approximation and then a heuristic). In this case, I'd say proper fix is to disable hyperthreading by default, and make sure the user is aware of the hardware bug/consequence of using HT when they decide to turn it on. You need to let the user decide if they're willing to accept the security risk or not.The Linux Kernel Developers may decide otherwise, but that's how I'd call it if it was in my shop. It's a hardware problem and the software fix is not obvious.
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