Meltdown

17 Articles

Spectre And Meltdown: How Cache Works

January 15, 2018 by 15 Comments

The year so far has been filled with news of Spectre and Meltdown. These exploits take advantage of features like speculative execution, and memory access timing. What they have in common is the fact that all modern processors use cache to access memory faster. We’ve all heard of cache, but what exactly is it, and how does it allow our computers to run faster?

In the simplest terms, cache is a fast memory. Computers have two storage systems: primary storage (RAM) and secondary storage (Hard Disk, SSD). From the processor’s point of view, loading data or instructions from RAM is slow — the CPU has to wait and do nothing for 100 cycles or more while the data is loaded. Loading from disk is even slower; millions of cycles are wasted. Cache is a small amount of very fast memory which is used to hold commonly accessed data and instructions. This means the processor only has to wait for the cache to be loaded once. After that, the data is accessible with no waiting.

A common (though aging) analogy for cache uses books to represent data: If you needed a specific book to look up an important piece of information, you would first check the books on your desk (cache memory). If your book isn’t there, you’d then go to the books on your shelves (RAM). If that search turned up empty, you’d head over to the local library (Hard Drive) and check out the book. Once back home, you would keep the book on your desk for quick reference — not immediately return it to the library shelves. This is how cache reading works.

Continue reading “Spectre And Meltdown: How Cache Works” →

Meltdown Code Proves Concept

January 11, 2018 by 46 Comments

If you’ve read about Meltdown, you might have thought, “how likely is that to actually happen?” You can more easily judge for yourself by looking at the code available on GitHub. The Linux software is just proof of concept, but it both shows what could happen and — in a way — illustrates some of the difficulties in making this work. There are also two videos in the repository that show spying on password input and dumping physical memory.

The interesting thing is that there are a lot of things that will stop the demos from working. For example a slow CPU, a CPU without out-of-order execution, or an imprecise high-resolution timer. This is apparently especially problematic in virtual machines.

Continue reading “Meltdown Code Proves Concept” →

Spectre And Meltdown: Attackers Always Have The Advantage

January 10, 2018 by 45 Comments

While the whole industry is scrambling on Spectre, Meltdown focused most of the spotlight on Intel and there is no shortage of outrage in Internet comments. Like many great discoveries, this one is obvious with the power of hindsight. So much so that the spectrum of reactions have spanned an extreme range. From “It’s so obvious, Intel engineers must be idiots” to “It’s so obvious, Intel engineers must have known! They kept it from us in a conspiracy with the NSA!”

We won’t try to sway those who choose to believe in a conspiracy that’s simultaneously secret and obvious to everyone. However, as evidence of non-obviousness, some very smart people got remarkably close to the Meltdown effect last summer, without getting it all the way. [Trammel Hudson] did some digging and found a paper from the early 1990s (PDF) that warns of the dangers of fetching info into the cache that might cross priviledge boundaries, but it wasn’t weaponized until recently. In short, these are old vulnerabilities, but exploiting them was hard enough that it took twenty years to do it.

Building a new CPU is the work of a large team over several years. But they weren’t all working on the same thing for all that time. Any single feature would have been the work of a small team of engineers over a period of months. During development they fixed many problems we’ll never see. But at the end of the day, they are only human. They can be 99.9% perfect and that won’t be good enough, because once hardware is released into the world: it is open season on that 0.1% the team missed.

The odds are stacked in the attacker’s favor. The team on defense has a handful of people working a few months to protect against all known and yet-to-be discovered attacks. It is a tough match against the attackers coming afterwards: there are a lot more of them, they’re continually refining the state of the art, they have twenty years to work on a problem if they need to, and they only need to find a single flaw to win. In that light, exploits like Spectre and Meltdown will probably always be with us.

Let’s look at some factors that paved the way to Intel’s current embarrassing situation.

Continue reading “Spectre And Meltdown: Attackers Always Have The Advantage” →

Getting A Handle On Meltdown Update Impact, Stay Tuned For Spectre

January 9, 2018 by 31 Comments

When news broke on Meltdown and Spectre ahead of the original disclosure plan, word spread like wildfire and it was hard to separate fact from speculation. One commonly repeated claim was that the fix would slow down computers by up to 30% for some workloads. A report released by Microsoft today says that “average users” with post-2015 hardware won’t notice the difference. Without getting into specific numbers, they mention that they expect folks running pre-2015 hardware to experience noticeable slowdowns with the patches applied.

The impact from Meltdown updates are easier to categorize: they slow down the transition from an user’s application level code to system level kernel code. The good news: such transitions were already a performance killjoy before Meltdown came along. There exists an extensive collection of tools (design patterns, libraries, and APIs) to help software developers reduce the number of user-kernel transitions.

Performance sensitive code that were already written to minimize kernel transitions will suffer very little from Meltdown updates. This includes most games and mainstream applications. The updates will have a greater impact on the minority of applications that frequently jump between kernel and user worlds. Antivirus software (with their own problems) have reasons to do so, and probably will end up causing most of the slowdowns seen by normal users.

Servers, with their extensive disk and networking IO — and thus kernel usage — are going to have a much worse time, even as seen through Microsoft’s rosy spectacles. So much so that Microsoft is recommending that admins “balance the security versus performance tradeoff for your environment”.

The impact from Spectre updates are harder to pin down. Speculative execution and caching are too important in modern CPUs to “just” turn off. The fixes will be more complex and we’ll have to wait for them to roll out (bumps and all) before we have a better picture.

The effects might end up being negligible as some tech titans are currently saying, and that probably will fit your experience, unless you’re running a server farm. But even if they’re wrong, you’ll still be comfortably faster than an Intel 486 or a Raspberry Pi.

Do any of you have numbers yet?

[via The Verge]

 

Raspberry Pi Ain’t Afraid Of No Spectre And Will Not Meltdown

January 9, 2018 by 43 Comments

While there’s broad agreement that Meltdown and Spectre attacks are really bad news at a fundamental level, there is disagreement on its immediate practical impact in the real world. Despite reassurance that no attacks have been detected in the wild and there’s time to roll out the full spectrum of mitigation, some want to find protection right now. If you’re interested in an usable and easy to set up modern desktop that’s free of Meltdown or Spectre threats, a Raspberry Pi can provide the immunity you seek.

[Eben Upton] explained the side channel attacks using fragments of Python for illustration, which was an enlightening read independent of the Raspberry Pi pitch. While these ARM cores perform speculative instruction fetches, they don’t speculatively execute them or modify the cache. Under the current circumstances, that makes all the difference in the world.

A clever security researcher may yet find a way to exploit speculative fetches in the future, and claiming that Raspberry Pi has superior security would be a stretch. The platform has its own set of security problems, but today Meltdown/Spectre is not among them. And that just might be enough to sway some decisions.

If you need to stay in the x86 world, look over what it’d take to to rewind back to an Intel 486.

Thanks to [D00med] for sharing the link in a comment to our overview article.

Speculative Execution Was A Troublemaker For Xbox 360

January 8, 2018 by 28 Comments

Part of why people can’t stop talking about Meltdown/Spectre is the fact that all the individual pieces have been sitting in plain sight for a long time. When everyone saw how it all came together last week, many people (and not even necessarily security focused people) smacked themselves on the forehead: “Why didn’t I see that earlier?” Speculative execution has caused headaches going way back. [Bruce Dawson] tells one such story he experienced back in 2005. (Warning: ads on page may autoplay video.)

It’s centered around Xbox 360’s custom PowerPC processor. Among the customization on this chip was the addition of an instruction designed to improve memory performance. This instruction was a hack that violated some memory consistency guarantees held by the basic design, so they knew up front it had to be used very carefully. Even worse: debugging problems in this area were a pain. When memory consistency goes wrong, the code visible in the debugger might not be the actual code that crashed.

Since we’re talking about the dark side of speculative execution, you can already guess how the story ends: no matter how carefully it was used, the special instruction continued to cause problems when speculatively executed outside the constrained conditions. Extensive testing proved that instructions that were not being executed were causing crashes. That feels more like superstition than engineering. As far as he can recall, it ended up being more trouble than it was worth and was never used in any shipped Xbox 360 titles.

[Main image source: AnandTech article on Xbox 360 hardware]

Go Retro To Build A Spectre And Meltdown-Proof X86 Desktop

January 7, 2018 by 95 Comments

[Yeo Kheng Meng] had a question: what is the oldest x86 processor that is still supported by a modern Linux kernel? Furthermore, is it actually possible to use modern software with this processor? It’s a question that surely involves experimentation, staring into the bluescreen abyss of BIOS configurations, and compiling your own kernel. Considering Linux dropped support for the 386 in 2012, the obvious answer is a 486. This supposition was tested, and the results are fantastic. You can, indeed, install a modern Linux on an ancient desktop.

This project got its start last month at a Super Silly Hackathon where [Yeo] and [Hui Jing] installed Damn Small Linux on an ancient IBM PS/1 desktop of 1993 vintage. The hardware consists of an AMD 486 clone running at 133MHz, 64 MB of RAM, a 48x IDE CDROM drive (wow!), a floppy emulator, a Sound Blaster, 10Mbps Ethernet card, and a CompactFlash to IDE adapter. By any account, this is a pimped-out rig for 1993 that would have cost more than a car at the time. The hardware works, but can you run a modern Linux kernel on it?

[Yeo] decided to install the Gentoo x86 minimal installation, but sanity and time constraints meant compiling a kernel on a 486 wasn’t happening. That was done on a modern Thinkpad after partitioning all the drives, verifying all the compilation parameters, and configuring the kernel itself. The bootloader is LILO (Grub2 didn’t work), but for the most part, this is entirely modern software running on a 25-year-old machine. The step-by-step instructions for becoming a /g/entooman on a 486 are available on GitHub.

The entire (boring) boot process can be seen in the video below. One interesting application of this build is that the 486 does not support out-of-order execution, making this completely safe from Meltdown and Spectre attacks. It’s an impressive retrocomputing achievement that right now could not be more timely.

Continue reading “Go Retro To Build A Spectre And Meltdown-Proof X86 Desktop” →