1938: Meltdown and Spectre
|Meltdown and Spectre|
Title text: New zero-day vulnerability: In addition to rowhammer, it turns out lots of servers are vulnerable to regular hammers, too.
| This explanation may be incomplete or incorrect: Missing comment on Cueballs suggestion that he should install updates and Ponytail humoring him... else seems almost complete.|
If you can address this issue, please edit the page! Thanks.
This comic was inspired by the Meltdown and Spectre bugs in certain processors. These vulnerabilites were disclosed to the public the week of this comic. The bugs made big news because they broke the "walls" between programs executing concurrently on the same computer, in some circumstances allowing malware to steal secrets from normal, bug-free programs.
Speculative execution is a technique used to speed up computer processors. Processors handle instructions in a series of steps, like an assembly line. The processor works on several successive instructions, each at a different stage in the assembly line. It may start speculatively executing instructions that follow a particular result of a decision before the logic that makes that decision is finished. Once the decision is made, it keeps results from the selected path, and discards unnecessary processing. This allows it to keep doing useful work while some slower decision is made.
The Trolley Problem has been mentioned before by Randall; see the end of this paragraph. It is a thought experiment where an out-of-control trolley is heading to a switch which you control. Leaving the switch as-is will cause it to kill multiple people (typically five) stuck on the tracks, but switching the track will cause it to kill only one person, who would not have died if the switch was left untouched; this creates the ethical dilemma of passively causing multiple deaths versus actively causing one. The Trolley Problem has gained significant memetic traction, helped in no small part by its frequent inclusion in “introduction to philosophy” type courses. Also, the problem has seen revitalized interest with the emergence of autonomous cars, which may be faced with what are essentially such choices in emergency situations. The Trolley problem was mentioned a month before this comic in the last milestone on this list in 1925: Self-Driving Car Milestones; see more there regarding why this problem might have resurfaced in xkcd. Three year before that comic, a comic was even named after the problem: 1455: Trolley Problem.
The "quantum" aspect of this is that in some interpretations of quantum mechanics, quantum-level particles can be viewed as taking every possible path at once, with the result being the sum of all of them, a result popularized by the common interpretation of Schrödinger's cat, where the cat is both dead and alive until some event results in one of the states being selected.
Meltdown and Spectre result from flawed implementations of speculative execution, where the effects of speculatively executed instructions are not completely discarded. The phantom trolley driving through walls is an analogy for the computer instructions being able to access protected areas. This may also be a reference to quantum tunnelling.
In many cases, contrary to what the comic implies, both paths are not taken simultaneously during speculative execution. A branch predictor may be used to select the most likely path, and the effects should be completely erased if the predicted path is incorrect. Both branch prediction and taking both paths (known as eager evaluation) are considered speculative execution and are affected by these bugs.
The Row Hammer problem had been known for many years before this cartoon. A common form of computer memory is constructed from tiny capacitors organized in a two-dimensional grid of rows and columns. Capacitors store charge to represent information. By applying a pattern of memory access that rapidly changes a row of capacitors, you can cause charge to overflow to nearby rows and incorrectly change their states.
Ponytail mentions that we especially suck at building "shared computers" because Rowhammer, Spectre, and Meltdown all break down the security divisions built between programs and between users. A hacker running a separate program in a separate account shouldn't be able to access your secrets or change the behavior of your program, but these problems allow them to. This is particularly dangerous for time-sharing, servers, and the cloud, where different programs, websites, or even companies can be sharing the same hardware.
Cueball took her explanation literally, and came to the conclusion that the cloud "is full of phantom trolleys armed with hammers". Instead of correcting him, Ponytail decides to accept his interpretation (perhaps because she found this idea to be kinda cool).
The title text humorously states that as well as row hammer, computer servers also can be harmed by regular hammers. A zero-day vulnerability is an attack that takes advantage of a vulnerability that hasn't been published yet, and so is not patched in any vulnerable system. This would imply that the Rowhammer vulnerability is what inspired someone to try taking a hammer to a server. One might "patch" a server against this attack by plating it with stronger metal. (The fact that compromising IT systems is sometimes easier done physically than logically is also mentioned in 538: Security.)
Although this is clearly not part of the series Code Quality, it might be the same two characters, with Ponytail again displaying a much better understanding of computers than Cueball with his Computer Problems.
- [Zoom out with Cueball and Ponytail walking to the right on the ground.]
- Cueball: The Meltdown and Spectre exploits use "speculative execution?" What's that?
- Ponytail: You know the trolley problem? Well, for a while now, CPUs have basically been sending trolleys down both paths, quantum-style, while awaiting your choice. Then the unneeded "phantom" trolley disappears.
- [Zoom in on only Ponytail who has turned towards Cueball off-panel left.]
- Ponytail: The phantom trolley isn't supposed to touch anyone. But it turns out you can still use it to do stuff.
- Ponytail: And it can drive through walls.
- [Cueball and Ponytail, lifting both her palms up, are standing, facing each other.]
- Cueball: That sounds bad.
- Ponytail: Honestly, I've been assuming we were doomed ever since I learned about Rowhammer.
- [In a frame-less panel they continue talking, both with their arms down.]
- Cueball: What's that?
- Ponytail: If you toggle a row of memory cells on and off really fast, you can use electrical interference to flip nearby bits and—
- Cueball: Do we just suck at...computers?
- Ponytail: Yup. Especially shared ones.
- [Zoom out again as they resume walking to the right on the ground. Cueball is lifting his smartphone up and looks at the screen.]
- Cueball: So you're saying the cloud is full of phantom trolleys armed with hammers.
- Ponytail: ...Yes, that is exactly right.
- Cueball: Okay. I'll, uh... install updates?
- Ponytail: Good idea.
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