back to article Thought you'd addressed those data-leaking Spectre holes on Linux? Guess again. The patches aren't perfect

Linux kernel developers are fixing up a trio of weaknesses in the open-source project – after a Google engineer reported that defenses implemented to stop speculative-execution snooping do not work as intended. In three posts marked urgent to the Linux kernel mailing list on Tuesday, Anthony Steinhauser points out problems …

  1. Anonymous Coward
    Anonymous Coward

    OY! 'Nuff o' this shite!

    Hypersupermultiubergigathreading this and teraultrahypersmeggingthreading that -- fek it all fer a lark. Bin 'em all an' go back to a trusty Acorn.

    1. Bo Lox

      Re: OY! 'Nuff o' this shite!

      I prefer abacuses and punch cards, far safer.

      1. Anonymous Coward
        Anonymous Coward

        Re: OY! 'Nuff o' this shite!

        Abacuses and Punch Cards?!? You were lucky. We had to count on't fingers.

        1. seven of five

          Re: OY! 'Nuff o' this shite!

          Posh. We din't use t have fingrs...

          1. DJV Silver badge

            Re: OY! 'Nuff o' this shite!

            Aye, back in't primordial soup we could only count up to 1, and that was on a good day wi' no fog.

  2. Henry Wertz 1 Gold badge

    Remove high accuracy timers?

    Is it possible to just remove nanosecond-accuracy timing sources from Linux?

    When they found (much to everyone's surprise and alarm) that both Firefox and Chrome had fast enough Javascript (JIT compiler compiling the Javascript into native code) to allow these timing effects INSIDE THE BROWSER, Firefox and Chrome simply removed access to nanosecond-accurate timers; they Javascript high-accuracy timer calls are now simply rounded off so they have like 1ms (1/000th of a second) accuracy. Could the same be applied to the Linux user-space itself? Maybe 1/10,000th of a second accuracy (since things like ping show 0.1ms timings)... there's a lot of wiggle room between that and the nanosecond (billionths of a second!) accuracy timers that exist now.

    Is there a technical problem with this, like instruction counter or something that's hard to trap? Otherwise, this'd let most mitigations be left off with no ill side effects.

    1. sofaspud

      Re: Remove high accuracy timers?

      The short answer is no.

      Browser makers are able to do that because that level of precision was never part of the design spec and exposes more problems than having it available solves. An actual, functional OS (rather than a JavaScript sandbox) is designed to allow access to that level of accuracy and there's no telling how many crucial bits of code out there rely on it -- everything from sensor monitoring to file access logging to cryptography and beyond may rely on timers that precise, and rounding off could break entire industries.

      Nor can you simply enforce any sort of useful gatekeeping around it. You can't say that "only trusted code" gets access to the timers, because malware doesn't respect your rules as-is and there's no effective way to sign all the possible valid apps at this late stage. If it had been baked in from day one, maybe -- but even then it would probably be more hassle than its worth, as high-accuracy timers are not themselves the problem, they're just a means of exploiting it.

      1. KSM-AZ

        Re: Remove high accuracy timers?

        I'll bite. Specifically what apps use nano-second timers. Explotation of these 'cpu' flaws is esoteric lab research at best. It's easier just to phish passwords. I'm still not seeing how one could realistically exploit this stuff. You gotta know the workload, know the process, and sniff aound petabytes of data until you match something. All the POC's involved running two processes in a controlled environment, or sniffing around with a known workload passing thru at a known time, I mean if I got all that, I'm pretty sure I already have you pwned.

        There has not been a single ransomware tied back to meltdown or spectre. If resticting access to precision timers is feasable, and takes the bulk of the risk away, it beats crippling performance for day-to-day worloads. I know it's *possible* someone could inject a stored procedure on my sql server, that daisy chains something to exploit meltdown. My only question would be why? I'm already f*cked. If it makes you sleep better by all means have fun.

        1. Flocke Kroes Silver badge

          Re: Remove high accuracy timers?

          Years ago some AMD hardware had defective High Precision Event Timer hardware which had to be disabled to keep operating systems happy. This often became company policy which remained in place long after all the defective hardware was retired. Clearly plenty of people were unaware of any problems caused by disabling perfectly good HPETs.

          There is a wide variety of timer hardware and well made software should ask the OS what is available, what each is capable of and use the most appropriate for the task. There are piles of web pages about the effect of re-enabling HPETs on various games. Occasionally someone noticed a clear improvement. Lots noticed about one extra frame per second and some did not spot any measurable change. To me this looks like many games are able to cope with whatever hardware they find with just the odd combination of hardware and software benefiting from a specific extra time source.

          "less /proc/timer_list" shows that lots of timers are being used but it is not obvious what they are being used for and how much precision they really need. An off switch could cause noticeable problems but a reduced precision knob might be harmless and useful.

          1. sofaspud

            Re: Remove high accuracy timers?

            Now that I could go for -- a setting you can dial in per timer, so if you bork something in your environment -- kill a critical app by mistake -- you can dial it back to the required precision without having to revert the OS to a previous version.

            Probably a kernel compile switch. Dynamic settings can potentially be buggered with.

          2. Claptrap314 Silver badge

            Re: Remove high accuracy timers?

            Hand-tuning individual timers from outside the process is at best blindfolded whack-a-mole. Whenever you think of a mitigation, assume that the attacker knows about it, and then ask yourself what an attacker could to to bypass it.

        2. sofaspud

          Re: Remove high accuracy timers?

          Pff, like I know *specifically* what apps would be affected. I was pointing out the generic class of problem and as I noted, it's impossible to know with certainty what *would* be affected; all we can be certain of is that a lot of widely-divergent code paths *could* be affected because people (rightly or wrongly) use high-precision timers all over the place, for a wide variety of reasons.

          As far as "my sql server" goes: you're not wrong, but you're not thinking about it in the right terms if that's your stance. If your server is physically distinct, on your own network, you control all access, etc, then sure, fine, you're right -- this sort of attack isn't really a problem for you and if it becomes one you're already fucked because they already have access to your hardware to pull it off -- and as we all know, if they have access to the hardware, you're fucked.

          But that's not the target profile.

          This sort of attack is the sort that opens you up to loss because some idiot running alongside your instance in the datacenter wasn't careful and the attacker escaped *their* sandbox, likely using some other exploit, and is now running their own process at the hypervisor layer, and what they're after is the encryption keys (for example) so they can peer into anybody's process space at will. You're probably not being targeted at all. It's a shotgun approach.

          So datacenter operators are understandably worried about it. People who operate in the cloud *should* be worried about it, though I don't recommend losing sleep at the moment. Chip manufacturers are sweating bullets about it because they know just how bad it could really be, even if your (bold!) statement about "no single ransomware tied back to meltdown or spectre" holds true. But dedicated-instance operators? Meh. As you note, there are better ways to get you, if that's your setup.

          1. Claptrap314 Silver badge

            Re: Remove high accuracy timers?

            And to be clear, even in the datacenter, if you are renting machines, you are essentially in the clear. In fact, I interviewed with one company that is doing just that--and running without Spectre mitigations and their eye-watering costs.

            And, by "in the clear", I mean that anyone hitting you with Spectre has already compromised you to the point they don't need to.

        3. Anonymous Coward
          Anonymous Coward

          Re: TL;DR get your filty paws offa my high accuracy timers

          Profilers for one, accurate timing quite useful there.

          The basic attacks work on the principle of measuring latency of access to cache lines, it's not really about the precision of the timer.

          Side note, crap timers can be made more accurate, with more iterations.

          As an example, I dont have a <30ns timer on this machine, and other stuff is happening, like me waffling on elreg, so if I run my loop, a eleventy-billion times, averaging out the measurements, I will coverage on a very precise measurement.

    2. Naich

      Re: Remove high accuracy timers?

      Ha! Nanosecond timers? When I wrote my JS script for a space invaders game back in 2004, the timers in a browser couldn't be guaranteed to resolve less than 28mS. You youngsters and your fancy nanosecond timers...

      1. heyrick Silver badge

        Re: Remove high accuracy timers?


        Your fancy timers.

        The 8254 interrupted 18.2 times per second...

      2. MarkET

        Re: Remove high accuracy timers?

        28mS, you were blessed. We had to rely on sundial. Didn't even think Intel's hardware datasheets were accurate to the nanosecond...especially not for memory or cache access?

        1. Robert Grant

          Re: Remove high accuracy timers?

          Sundial? Luxury! When we were building the Pyramids the interrupt frequency was whenever the architect strolled over to stop us and tell us we were doing it wrong.

  3. Herby

    Reminds me of....

    ...Back to the Future. Change the time-space continuum, and bad (biff) things happen. Got to go back and correct them before things change the wrong way.

    Of course, meanwhile in Redmond......

    1. sabroni Silver badge

      Re: Of course, meanwhile in Redmond......

      Of course, meanwhile in Redmond what? They've also had to roll out patches for this to Azure?

  4. RM Myers


    Oh what a tangled web we weave, when first we practice to deceive guess which path execution will take.

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