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Secretly Monopolizing the CPU Without Being Root
Posted by
CmdrTaco
on Wed Jul 11, 2007 09:32 AM
from the because-you-shouldn't dept.
from the because-you-shouldn't dept.
An anonymous reader writes "This year's
Usenix security symposium
includes a
paper
that implements a "cheat" utility, which allows any non-privileged user to
run his/her program, e.g., like so 'cheat 99% program'
thereby insuring that the programs would get 99% of the CPU
cycles, regardless of the presence of any other applications in the
system, and in some cases (like Linux), in a way that keeps the program
invisible from CPU monitoring tools (like 'top'). The utility exclusively
uses standard interfaces and can be trivially implemented by any
beginner non-privileged programmer. Recent efforts to improve the
support for multimedia applications make systems more susceptible to
the attack.
All prevalent operating systems but Mac OS X are vulnerable, though by
this kerneltrap story,
it appears that the new CFS Linux scheduler attempts to address the
problem that were raised by the paper."
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A Useful Tool (Score:5, Funny)
Re:A Useful Tool (Score:4, Informative)
Parent
Re:A Useful Tool (Score:5, Funny)
Parent
Re:A Useful Tool (Score:4, Insightful)
Much easier to just renice your root shell automatically at login
Parent
Re: (Score:3, Insightful)
Google-cache article (Score:3, Informative)
Re:Google-cache article (Score:5, Informative)
it works by avoiding running during the exact moment of a clock tick (which would be the moment when CPU usage per-process is checked). to start running immediately after a clock tick is (apparently) easy, but to stop before the next tick is harder. the paper suggests using some kind of get_cycles assembly instruction to count how many CPU cycles there are per clock tick, and use that number to gauge when the next clock tick is going to occur by counting how many CPU cycles have elapsed.
Parent
Re:Google-cache article (Score:4, Funny)
it works by avoiding running during the exact moment of a clock tick (which would be the moment when CPU usage...
--Uhm... (looks at watch...) Say, I really don't have time for wordy summaries... could you maybe cut this down into about 10 words or less? Hurry it up! I ain't got all day!
Parent
Re:Google-cache article (Score:5, Funny)
Parent
Re: (Score:3, Insightful)
gnome (Score:3, Funny)
What the?! (Score:5, Funny)
#include
int main(int argc, char *argv[])
{
while (1) {}
return 0;
}
Re: (Score:2)
Re: (Score:3, Informative)
Re: (Score:3, Informative)
Per-user process limits (Score:3, Insightful)
Re: (Score:3, Insightful)
I guess they just put on a nproc limit on each user. It's just a trivial security measure against simple fork bombs. Assuming your Linux system uses PAM (most modern distros do), take a look at
Old news (Score:4, Informative)
If you check the linux kernel mailing list for Vassili Karpov, you should find test cases that demonstrate this behavior and tools for monitoring actual CPU usage for a variety of platforms, though I notice no mention of any of that in the paper.
See http://www.boblycat.org/~malc/apc/ [boblycat.org] for the tool and 'invisible CPU hog' test case.
Re:Old news (Score:5, Informative)
Parent
ok (Score:3, Interesting)
Yes, I'm kidding. Please don't post a long reply explaining how renice differs from this cheat thing. It isn't necessary.
Talk about a fair share scheduler ! (Score:5, Insightful)
In my days (yes, I'm an old fart) - the schedulers had basic principles :
- Voluntary yielding led you to get accounted for the time you spent running.
- You could stay in the interactive queue for only a certain amount of time. After some amount of time had passed (a few secs) you were either bumped to non-interactive if you were running (with longer time slices but lower priority) or removed off the scheduler list for good (if the time spent there was idling). They had a special 'idle but interactive' (not eligible for dispatching) queue for that.
- Scheduling a new task restarted a new time slice
That particular scheduler even had a 3 queue system so that if you got accidentally bumped into the non-interactive queue or if your process was semi-interactive you had a better chance of gaining interactive status again. And they had a 'really' not interactive queue for those CPU hogging processes.
Of course this requires the hardware to have a precise timing feature (something with a granularity that is finer than the process interleaving time slice time and ideally in the magnitude of instruction execution). And this scheduler wasn't using time sampling and time quantums.. (but something more like the OSX timer on demand paradigm).
--Ivan
Re: (Score:3, Interesting)
--Ivan
How It Works (Score:5, Informative)
This is accomplished by sleeping for a fixed amount in between OS clock ticks. The timeline looks like this:
Back at NYIT we hacked the "nice" command... (Score:3, Funny)
We changed nice so that whenever this particular user ran it, it lowered his priority by exactly as much as he was attempting to raise it.
He stopped coming to work soon after that. I suppose he had the last laugh though -- NYIT continued to pay him for another six months.
Thad
Linux 2.6.21 is probably immune too (Score:5, Informative)
According to the paper, the reason Mac OS X is not vulnerable is that it uses one-shot timers scheduled for exactly when the next event needs to occur, rather than periodic "ticks" with a fixed interval between them. The "tickless idle" feature introduced in Linux 2.6.21 (currently only on x86, I believe) takes the same approach, and very possibly makes Linux immune too.
(Ironically, immediately after discussing OSX's ticklessness, the paper mentions that "the Linux 2.6.16 kernel source tree contains 8,997 occurrences of the tick frequency HZ macro, spanning 3,199 files", to illustrate how difficult it is to take a tick-based kernel and make it tickless. But those kernel hackers went and did it anyway.)
The tickless feature isn't yet implemented on all architectures that Linux supports, though. I think AMD64 support for it is supposed to come in 2.6.23, along with the new CFS scheduler.
Re:Linux 2.6.21 is probably immune: RDTSC? (Score:3, Informative)
Fixed recently in Linux (Score:5, Informative)
The CFS additionally removes the interactivity boost in favor of giving interactive tasks no extra time but rather just quick access to their available time, which is what they really benefit from.
Summary and Questions (Score:5, Informative)
Most OSes (Linux, Solaris, Windows but not Mac OS X) are tick-based. This means that the kernel is called from hardware periodically (this is the "HZ" value you set in the Linux kernel). Some of them (Linux) simply check which process is running at each tick and compute statistics based on that ("sample-based statistics"). This means that the process running when the tick happens is billed for the entire period of the tick.
Since ticks are typically "long" (typically 1-10 ms on Linux) more than one process may run during this period. In other words, using this approach leads to inaccuracies in the process billing. If all programs "play by the rules" this works quite well on average though.
Next thing: the classic schedulers typically maintain some sort of "priority" value for each process, which decreases whenever the process is running and increases when it's not. This means that a process runs for some time, its priority decreases, and then another process (which hasn't been running for some time) takes over.
You can exploit that by always sleeping when a tick happens and running only in-between ticks. This makes the kernel thinks that your process is never running and give it a high priority. So, when your process wakes up just after a tick happened, it will have a higher priority than most other processes and be given the CPU. If it goes to sleep again just before the next tick, its priority will not be decreased. Your process will (almost) always run when it wants to and the kernel will think that it's (almost) never running and keep its priority high. You win!
Another aspect is that modern kernels (at least Linux and Windows) distinguish between "interactive" (e.g. media players) and "non-interactive" processes. They do so by looking how many times a process goes to sleep voluntarily. An interactive program (such as a media player) will have many voluntary sleeps (e.g. inbetween displaying frames) while a non-interactive program (e.g. a compiler or some number crunching program) will likely never go to sleep voluntarily. The scheduler gives the interactive programs an additional priority boost.
Since the cheating programs go to sleep very often (at every tick) the kernel thinks they're "very interactive", which makes the situation worse.
Some of the analyzed OSes - even if tick-based - do not use sample-based statistics in the kernel but they do use sample-based statistics for scheduling decisions. So the kernel sees that a process is taking more CPU than it should but it will still keep on scheduling it.
Mac OS X is not affected because it has a tickless kernel (e.g. without periodic interrupts). Because of that sample-based statistics don't work and it has to use accurate statistics, which make it unaffected by the bug.
This bug can be exploited to (at least)
- get more CPU than you're supposed to
- hinder other programs in their normal work
- hide malicious programs (such as rootkits) which do work in the background
Here's a list with the OSes (this USED TO BE a nicely formatted table, but the darned Slashdot "lameness filter" forced me to remove much of the nice lines and the "ecode" tag collapses whitespace).
OS, Process statistics, Scheduler decisions, Interactive/non-interactive decision, Affected
Linux, sample, sample, yes, yes
Solaris, accurate, sample, ?, yes
FreeBSD 4BSD, ?, sample, no?, yes
FreeBSD ULE, ?, sample, yes, yes
Windows, accurate, sample, yes, yes
Mac OS X, accurate, accurate, not needed?, yes
I guess that Mac OS X doesn't need a interactive/non-interactive distinction because of its different (tickless) approach. I assume that interactive applications can (implicitly or explicitly) can be recognized as such in a different way. Does anyone have more information on that?
How does tickless Linux compare? What abo
Re:Summary and Questions (Score:4, Informative)
Vista changed to counting actual CPU cycle count register. The goal was to prevent process starvation in high I/O situations, but it also addresses this issue as well.
http://www.microsoft.com/technet/technetmag/issue
Parent
Clever but what loss? (Score:3, Insightful)
A very simple patch is to issue RDTSC instructions at process restart and blocking syscall to count the cycles actually used. That way the extensive tick-code doesn't need to be modified.
Why is this new? (Score:3, Insightful)
I remember seeing this done on the VAX/VMS mainframe back in 1987. In that environment, it simply meant that you kept track of your timeslice and voluntarily gave it up before the scheduler took it away from you. That meant you got put at the top of the run queue, and unless someone else was doing the same thing, you were the next program to run. Voila... 99% CPU for you!
Of course, ordinary users were given a limited amount of CPU time (as well as connect time, disk space, etc), so for the ordinary student, this just meant they used it up in a day or two instead of having a whole month. But then again, for class accounts, they could usually beg for more.
Under unix variants, one could do the same by implementing cpu quotas at the user level. I've seen network packet quotas, and I'm sure someone out there has done cpu quotas along the same lines.
Re:What does this mean? (Score:5, Informative)
Parent
Re:What does this mean? (Score:5, Funny)
If you reply, do so only to what I explicitly wrote. If I didn't write it, don't assume or infer it.
You gun-toting marxist redneck zealot astroturfers make me sick!
Parent
Inevitable reply (Score:5, Funny)
Parent
Re:Inevitable reply (Score:5, Funny)
Parent
Re: (Score:3, Interesting)
We already have that. They're called McAfee Automatic Update and Windows Automatic Update.
God dammit, I hate those things. I turn on my office computer in the morning, and just let it sit for ten minutes because it's otherwise useless. (I turned-off Windows Automatic Update, but McAfee was more than happy to fill its shoes in needless resource hogging.)
Re:What does this mean? (Score:5, Insightful)
Parent
Re:What does this mean? (Score:5, Interesting)
This comment has been retracted by its poster
-nB
Parent
Re:What does this mean? (Score:5, Insightful)
Parent
Re:What does this mean? (Score:4, Interesting)
Parent
Re: (Score:3, Interesting)
That isn't likely to happen without a change in attitude due to both starting furthur behind and progressing more slowly. The current malware situation looks like bad SF and a morality tale of what happens when you allow really stupid things to happen (eg. letting arbitrary code embedded in images run - hopefully that person was dismissed from Microsoft).
Re: (Score:3, Insightful)
Of course not. It shows that OS research work is likely to be done on a Unix of some sort where the source code is available for anaylsis
TFA points out that Windows is just as vulnerable to these cheats as BSD, Linux and Solaris. The cheat works by releasing the CPU just before the end of a time tic
Re: (Score:3, Informative)
Re:The "sue" command (Score:5, Funny)
Parent
Here's the difference (Score:3, Informative)
What 'saved' the Mac OS was its different use of timing triggers. "All" other OS'es use one common steadily ticking clock as a dealer of time slots. This allows the cheat to "skip to the start of the line (queue)" every time it's had its turn.
OTOH, the Mac uses a stack of alarms set to specific points in the future, and polled in order as they occur. So the difference on Mac OS is that there's no skipping the queue, it's rather "there is no queue, we'll call you when it's your turn
Re: (Score:3, Interesting)
If you just want to DoS the box as a local user (which is all this lets you do, from a security standpoint), then there are much easier ways of doing this on OS X via the VM subsystem.
You're missing the point here. Because the CPU accounting is off it's possible to do a QoS attack on a box rather than a DoS, that's virtually impossible to detect as the end user. From his or her standpoint, the system will be sluggish, but because of the way the attack works various random processes will seem to be taking up all that extra slack so that most likely no one process will appear to be hogging the CPU.
There's also the possibility when combined with a worm or rootkit, as well as a bot net to
Re: (Score:3, Interesting)
I guess I'm not hip, but what exactly is the difference between a QoS attack and a DoS attack? I mean severly degrading the quality of the service potentially up to the point of denying it *is* a DoS attack.
A DoS attack is an extreme form of QoS. If you perform a QoS attack on someone their performance is reduced, but the system is still usable, where as in a DoS the goal is to make the system totally unusable. In some ways a QoS is even more effective than a DoS because it's more subtle and causes more frustration. If for instance a website gets DoSed the owner is upset and will try to get someone to investigate and shutdown if possible whoever is DoSing them, and the users simply cannot connect to the serv
Re: (Score:3, Funny)
I'll prove you wrong as soon as that stupid spinning beach ball of death lets me do something.
Re:First announced exploit.. (Score:5, Funny)
Do you think this might be related to that incident where thousands of English teachers all burst into flames moments after the first SMS-enabled phone was released?
Parent
Re: (Score:3, Informative)
Not exactly. This is a technique that will, in prinicple, work with any scheduler that prioritizes tasks on the basis of time ticks previously used by the task. That turns out to be most of them. The technique does not require being an I/O driver, other special task, or having unusual user priviliges.
So yes, it IS news.